Anatomical terms of location
Encyclopedia
Standard anatomical terms of location are designations employed in science that deal with the anatomy
of animal
s to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation. They are universal terms that may be readily understood by zoologists
who speak any language.
While these terms are standardized within specific fields of biology
, they can differ dramatically from one discipline to another. Differences in terminology remain a problem that, to some extent, still separates the fields of zoological anatomy (sometimes called zootomy
) and human (medical) anatomy
(sometimes called androtomy
).
The Craniata
(vertebrates) share a substantial heritage of common structure, allowing much of the same terminology to be used for all of them. It is necessary for this terminology to be based on the anatomy of the animal in a standard way to avoid ambiguities such as might occur if a word such as "top" were used, which might designate the head
of a human
but the left or right side of a flounder
. Most animals, furthermore, are capable of moving relative to their environment. So while "up" might refer to the direction of a standing human's head, the same term ("up") might be used to refer to the direction of the belly of a supine
human. It is also necessary to employ some specific anatomical knowledge in order to apply the terminology unambiguously: For example, while the ears would be superior to (above) the shoulders in a human, this fails when describing the armadillo
, where the shoulders are above the ears. Thus, in veterinary
terminology, the ears would be cranial to (i.e., "toward the head from") the shoulders in the armadillo, the dog
, the kangaroo
, or any other vertebrate, including the human. Likewise, while the belly is considered anterior to (in front of) the back in humans, this terminology fails for the flounder, the armadillo, and the dog. In veterinary terms, the belly would be ventral ("toward the abdomen") in all vertebrates.
In human anatomy, as will be explained below, all naming is based on positions relative to the body in a standing (standard anatomical) position with arms at the side and palms facing forward (thumbs out). While the universal vertebrate terminology used in veterinary medicine would work in human medicine, the human terms are thought to be too well established to change
.
For invertebrate
s, locational terminology becomes more complicated, as many species are not bilaterally symmetrical. In these species, terminology depends on the type of symmetry present (if any).
Thus, standardized anatomical (and zootomical) terms of location have been developed, usually based on Latin
words, to enable all biological and medical scientists to precisely delineate and communicate information about animal (including human) bodies and their component organs.
s can change orientation with respect to their environment, and because appendage
s (arms, legs, tentacle
s, etc.) can change position with respect to the main body, it is important that positional descriptive terms refer to the organism
when it is in its standard anatomical position.
Thus, all descriptions are with respect to the organism in its standard anatomical position, even when the organism in question has appendages in another position. For example, see Fig. 9, where the tentacles are curved, and therefore not in anatomical position. However, a straight position is assumed when describing the proximo-distal axis. This helps avoid confusion in terminology when referring to the same organism in different postures.
, standard anatomical position is rigidly defined for human anatomy. As with other vertebrates, the human body is standing erect and at rest. Unlike the situation in other vertebrates, the limbs are placed in positions reminiscent of the supine position
imposed on cadaver
s during autopsy. Therefore, the body has its feet together (or slightly separated), and its arms are rotated outward so that the palms
are forward, and the thumb
s are pointed away from the body (forearms supine). As well, the arms are usually moved slightly out from the body, so that the hands do not touch the sides. The positions of the limbs (and the arms in particular) have important implications for directional terms in those appendages. The penis
in males is also erect in the anatomical position, hence the dorsal surface of the penis is actually anterior in the flaccid state.
has been agreed by international convention to be the Frankfurt plane
, a position in which the lower margins of the orbits
, the orbitales, and the upper margins of the ear canal
s, the poria, all lie in the same horizontal plane. This is a good approximation to the position in which the skull would be if the subject were standing upright and facing forward normally.
s (including humans) have the same basic body plan (or bauplan
)—they are bilaterally symmetrical. That is, they have mirror-image left and right halves if divided down the centre. For these reasons, the basic directional terms can be considered to be those used in vertebrates. By extension, the same terms are used for many other (invertebrate
) organisms as well.
are chosen. By definition, each pair of opposite points defines an axis
. In a bilaterally-symmetrical organism, there are 6 polar opposite points, giving three axes that intersect at right angles—the x, y, and z axes familiar from three-dimensional geometry.
The most obvious end-points are the "nose" and "tail" (see Fig. 2). In terms of anatomy, the nose is referred to as the anterior end (Latin
ante; before). In organisms like vertebrates, that have distinct heads, the anterior end is sometimes referred to as the rostral end (Latin rostrum; beak), the cranial end (Greek kranion; skull), or the cephalic end (Greek
kephalē; head). For reasons of broader applicability, especially in organisms without distinct heads (many invertebrate
s), "anterior" is usually preferred.
The polar opposite to the anterior end is the posterior end (Latin
post; after). Another term for posterior is caudal (Latin caudum; tail, though in humans this refers to the feet i.e. inferior rather than posterior) — a term that strictly applies only to vertebrate
s, and therefore less preferred, except in veterinary medicine where these terms are standard.
By drawing a line connecting these two points, we define the anteroposterior axis (sometimes written antero-posterior). Caudal and Posterior (back end) are often used interchangeably. In veterinary medicine, caudo-cranial is preferred between head and tail, and rostro-caudal between nose and neck. Less-used synonyms would be rostrocaudal or cephalocaudal axes (see Table 1). For brevity, the term anteroposterior is often abbreviated to read AP (or A-P) axis. As well as defining the anteroposterior axis, the terms "anterior" and "posterior" also define relative positions along the axis. Thus, in the fish in Fig. 2, the gill openings are posterior relative to the eyes, but anterior to the tail.
(Latin dorsum; back) and the ventral end (Latin venter; abdomen
), respectively. By connecting the outermost points the dorsoventral axis is formed (sometimes hyphenated: dorso-ventral). This is commonly abbreviated to DV (or D-V) axis. The DV axis, by definition, is perpendicular (at right angles to) the AP axis at all times (see below).
As with anteroposterior, the terms "dorsal" and "ventral" are also used to describe relative positions along the dorsoventral axis. Thus, the pectoral fins are dorsal to the anal fin, but ventral to the dorsal fin
in Fig. 2. (Note that these fins are not aligned anteroposteriorly, either — the dorsal fin being posterior to the pectoral, and anterior to the anal fins, respectively.)
definition, must be at right angles to both the AP and the DV axes. The left side and right side of the organism are the outermost points between the two "sides" of the organism. When connected, these points form the left-right axis (commonly abbreviated to LR (or L-R) axis. In Latin, this is called the dextro-sinistral (or, more uncommonly, the sinistro-dextral) axis, from dexter (right) and sinister (left). It is important to note that the "left" and "right" sides are the sides of the organism, and not those of the observer.
"Left-right" is typically used in English and some other languages.
As with the other directions, the terms can be used as relative terms, to describe locations along the left-right axis. Thus, in Fig. 2 the dorsal fin
is right of the left pectoral fin, but is left of the right eye
. However, as left and right sides are mirror images, usage like this tends to be somewhat confusing, as structures are duplicated on both sides (i.e., above there is both a right eye and a left eye, forcing one to specify which is used as a reference).
To counter this clumsiness of usage, the directional term lateral (Latin
lateralis; "to the side") is used as a modifier for both sides, yielding the left lateral and right lateral sides. As an opposite to lateral, the term median (Latin
medius; "middle") is used to define a point in the centre of the organism (where the left-right axis intersects the midsagittal plane —see below), and the term medial means "toward the median plane". Thus, rather than "left-right" axis and its inherent clumsiness of usage, the term mediolateral (also sometimes hyphenated medio-lateral) axis is frequently used. Sometimes this is abbreviated to ML (or M-L) axis. In proper usage, the ML axis is a half-axis; in practice, its usage is less clumsy and less linguistically biased than "left-right". The terms may still be used relatively to describe locations along the LR axis. Thus, in Fig. 2 the gills are medial to the operculum
, but lateral to the heart
.
The usage "mediolateral" is strictly used to describe relative position along the left-right axis, to avoid confusion with the terms "superficial" and "deep" (see below).
Together, the AP, DV and LR (or ML) axes allow for precise three-dimensional descriptions of location within any bilaterally-symmetrical organism, whether vertebrate
or invertebrate
. In practice, the terms can cause some confusion when, unlike the fish shown in Fig. 2, the organism in question is not strictly linear in form, which includes most tetrapods (see Figs. 3 and 4). For example, the AP axis in Fig. 3 does not appear to be at right angles to the DV axis. Rather, it is a depiction of the approximate average AP axis, when all body segments are included.
When considering any one segment, the dorsoventral axis is perpendicular to the AP axis. Thus, in Fig. 4, the DV axis of the tail would run from the "back" of the tail (posterior end of the trunk), to the "underside" of the tail (near the legs) — nearly parallel to the AP axis of the main body.
As a general rule of thumb, if the body is included in consideration, the AP axis of the main body would be used, as would the DV and ML axes perpendicular to it. However, if considering only one segment, the AP axis would shift to reflect the axes shown in Fig. 4, with the DV and ML axes shifting correspondingly. In alternative manner, to avoid confusion, AP, DV, and ML terms are used strictly in relation to the main body, and the terms proximal and distal are used for body segments such as the head, neck, and tail (see below).
To avoid this confusion, in veterinary medicine, the terms anterior, posterior, superior, and inferior are in general avoided except for certain structures within the head. By using the terms cranial, caudal, dorsal and ventral, all tetrapod organisms (including bipeds) can be described uniformly.
In humans, the directions "rostral" and "caudal" often become confused with anterior and posterior, or superior and inferior. The difference between the two is most easily visualized when looking at the head, as can be seen in the image to the right. From the most caudal of positions in the nervous system (of a person) to a nearby, rostral area, it is equally accurate to say the area in question is rostral as to say it is superior. However, in the frontal lobes of the telencephalon
, to say an area is rostral to a nearby area is equivalent to saying it is anterior (or ventral). Those two lines lie on planes perpendicular to one another. This occurs, as becomes clear in the diagram, due to the intuitive yet curious curving "C" shape of rostrocaudal directionality when discussing the human brain.
proximus; nearest) describes where the appendage joins the body, and the term distal (Latin distare; to stand away from) is used for the point furthest from the point of attachment to the body. Since appendage
s often move independently of (and therefore change position with respect to) the main body, these separate directional terms are used when describing them.
As noted above, the standard AP, DV and ML directional axes, can cause some confusion when describing parts of the body that can change position (move) relative to the main body. This is particularly true when considering appendage
s. "Appendages" would include vertebrate
fins
(see Fig. 2) and limbs
(see Figs. 3 and 4), but properly apply to any structure that extends (and can at least potentially move separately) from the main body. Thus, "appendage" would also include such structures as external ears
(pinnae) and hair
(in mammals), feathers (in birds) and scales
(fish
, reptiles and birds). As well, varieties of tentacles or other projections from the body in invertebrate
s and the male in many vertebrates and some invertebrates, would be included.
By connecting the two points, the proximodistal (sometimes hyphenated to proximo-distal) axis is created. (The abbreviation AB axis is occasionally, but not commonly, used.) As before, the terms "proximal" and "distal" can be used as relative terms to indicate where structures lie along the proximodistal axis. Thus, the "elbow" is proximal to the hoof, but distal to the "shoulder" in Figs. 3 and 4.
Choosing terms for the other two axes perpendicular to the proximodistal axis could be variable, as they would also depend on the position of the limb. For that reason, when considering any organism, the other two axes are considered to be relative to the appendage when in standard anatomical position. This is roughly defined for all organisms, as in the normal position when at rest and not moving. For tetrapod
vertebrate
s, this includes the caveat that they are standing erect and not lying down. Thus, the fish in Fig. 2, and the horse in Figs. 3 and 4 are in standard anatomical position. (Special considerations with respect to limb position are applied in human anatomy—see below).
Dentistry uses the same terminology, as well as
present in invertebrates presents a difficult problem when attempting to apply standard directional terms. Depending on the organism, some terms are taken by analogy from the vertebrate terms, and appropriate novel terms are applied, as necessary. In all cases, the usage of terms is dependent on the bauplan
of the organism.
organisms (Fig. 5a), directional terms are meaningless, since the shape of the organism is changeable, and no fixed axes are present. Similarly, in organisms that are spherical in shape (Fig. 5b), there is nothing to distinguish one line through the centre of the organism from another. An infinite number of triads of mutually perpendicular axes could be defined, but any such choice of axes would be functionally and practically indistinguishable from all others, and therefore would be useless. In such organisms, only the terms superficial and deep hold any descriptive meaning.
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s, where they appear essentially two-dimensional. In some cases a third axis can be defined, particularly where a non-terminal cytostome
or other unique structure is present.
Some elongated protists have distinctive ends of the body. In such organisms, the end with a mouth (or equivalent structure, such as the cytostome
in Paramecium
or Stentor
), or the end that usually points in the direction of the organism's locomotion
(such as the end with the flagellum
in Euglena
), is normally designated as the anterior end. The opposite end then becomes the posterior end, and by connecting them, an anteroposterior axis is formed. Properly, this terminology would apply only to an organism that is always planktonic (not normally attached to a surface, as in Fig. 7 top), although the term can also be applied to one that is sessile
(normally attached to a surface, as in Fig. 7, bottom, and Fig. 8).
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Organisms that are attached to a substrate
, such as sponges (Fig. 8), or some animal-like protists also have distinctive ends. The part of the organism attached to the substrate is usually referred to as the basal end (Latin
basis; support or foundation), whereas the end furthest from the attachment is referred to as the apical end (Latin apex; peak, tip). Thus, by joining the two ends, an apical-basal (or basal-apical) axis is formed (see Fig. 8). Transverse axes may be defined indifferently in any direction perpendicular to this axis, as there is no symmetry present.
—primarily jellyfish, sea anemones and corals
and the comb jellies. Adult echinoderms (sea stars (starfish), sea urchins, and sea cucumbers and others) are also included, since they are pentaradial (i.e. they have fivefold discrete rotational symmetry
). Echinoderm larva
e are not included, since they are bilaterally symmetrical.
Unlike spherical and asymmetrical organisms, radially-symmetrical animals always have one distinctive axis.
Cnidaria
ns have an incomplete digestive system, meaning that one end of the organism has a mouth, and the opposite end has no opening from the gut (coelenteron). For this reason, the end of the organism with the mouth is referred to as the oral end (Latin
oris; mouth), and the opposite surface is the aboral end (Latin ab-; prefix meaning "away from"). Thus, by joining the polar opposite oral and aboral ends, an oral-aboral axis is formed (Fig. 9).
As with vertebrates, appendage
s that move independently of the body (tentacles in cnidaria
ns and comb jellies), have a definite proximodistal axis (Fig. 8). Unlike vertebrates, cnidarians (jellyfish, sea anemones, corals) have no other distinctive axes, and multiple radial axes are possible (Fig. 10).
It is noteworthy that some "biradially-symmetrical" comb jellies have distinct "tentacular" and "pharyngeal" axes, and are thus anatomically equivalent to bilaterally-symmetrical
animals. As well, adult echinoderms (starfish, sea urchins, sea cucumbers) are pentaradial, and have only five symmetrical radial axes (unlike the multiple axes in cnidarians).
Lateral, dorsal, and ventral have no meaning in such organisms, and all can be replaced by the generic term peripheral (Latin peri-; around; see Table 2). Medial can be used, but in the case of radiates indicates the central point of these organisms, rather than a central axis (as in vertebrates). Thus, as there are many possible radial axes, there are multiple medio-peripheral (half-) axes (Fig. 10).
legs and pedipalp
s. Prolateral refers to the surface of a leg that is closest to the anterior end of an arachnid's body. Retrolateral refers to the surface of a leg that is closest to the posterior end of an arachnid's body.
. However, for historical and other reasons, standard human directional terminology has several differences from that used for other bilaterally-symmetrical organisms.
For almost all vertebrates, including almost all bipeds, these axes all provide a consistent reference for anatomical positions across species—with the inferior/superior axis being roughly the same as the dorsal/ventral axis, and therefore redundant. Humans, however, have the rare property of having a torso oriented perpendicular to their direction of forward motion—while their head orientation remains consistent with other vertebrates on this axis. This makes the dorsal/ventral axis on humans redundant with the anterior/posterior axis, and the inferior/superior axis necessary. Because of this difference with humans, the anterior/posterior and inferior/superior axes are inconsistent between humans and other vertebrates in torso anatomy but consistent in head anatomy. As all three of these axes are used in the naming of anatomical structures, and most human anatomical structures are shared by other animals, these differences can lead to considerable confusion. For example, in the naming of brain structures, the non-human context of the dorsal/ventral axis was used. Therefore, in human anatomy, "dorsal" can refer to two different (perpendicular) directions—the posterior direction in the context of the torso, and the superior direction in the context of the brain. Ironically, the "dorsal" direction in the human brain, besides being perpendicular to the "dorsal" direction in the human torso, is actually the opposite direction of what might be inferred from the literal Latin meaning of "toward the spine".
While it would be possible to introduce a system of axes that is completely consistent between humans and other vertebrates by having two separate pairs of axes, one used exclusively for the head (e.g. anterior/posterior and inferior/superior) and the other exclusively for the torso (e.g. dorsal/ventral and caudal("toward the tail")/rostral("toward the beak")), doing so would require the renaming of very many anatomical structures.
For a quick comparison of equivalent terminology used in vertebrate and human anatomy, see Table 3 (below).
superior: "above"), while the feet are referred to as the inferior end (Latin inferior: "below"). Thus, the axis formed by joining the two is the superior-inferior axis.
As with other vertebrate terminology, there are synonymous terms for superior and inferior (Table 3). The terms cranial and cephalic are often encountered. "Cranial", as a reference to the skull, is fairly commonly used, whereas "cephalic" is uncommonly used. The term "rostral" is rarely used in human anatomy, referring more to the front of the face than the superior aspect of the organism. This term is more applicable in organisms with longer heads, such as equids. Similarly, the term caudal is occasionally used in human anatomy, and the cranio-caudal axis is occasionally encountered. Generally, this usage would be used with respect to only the head and the main body (trunk), and not when considering the limbs.
As with vertebrate directional terms, superior and inferior can be used in a relative sense in humans, but can not be uniformly applied to other organisms with varying normal anatomical positions. For example, the shoulders
are superior to the navel
, but inferior to the eyes in humans. In any tetrapod, the shoulders are cranial to the belly, but caudal to the eyes.
"Anterior" and "posterior" can also be used as relative terms. Thus, the eye
s are posterior to the nose
, but anterior to the back of the head in humans.
However, in the horse
, for example, the eyes are caudal to the nose, and rostral to the back of the head.
In standard anatomical position, the palms of the hands point anteriorly. Thus, anterior can be used to describe the palm
of the hand, and posterior can be used to describe the back of the hand and arm.
However, presumably for improved clarity, the directional term palmar (Latin
palma; palm of the hand) is usually used for the anterior of the hand, and dorsal is used to describe the back of the hand. Thus, by connecting the extremes, dorsopalmar axis is formed. Most commonly, "dorsopalmar" is used when describing the hand, although it is sometimes applied to the arm as a whole (see Fig. 12).
For the third axis, the mediolateral axis suffices, although if referring to the limb alone, "medial" may refer to the centre of the arm itself.
are termed proximal, as opposed to those closest to the anus
, which are termed distal. The terms oral "of the mouth" and aboral "away from the mouth" are also used.
is attached to the bladder
.
In the limbs of most animals, the terms cranial and caudal are used in the regions proximal to the carpus (the wrist
, in the forelimb) and the tarsus (the ankle
in the hindlimb). Objects and surfaces closer to or facing toward the head are cranial; those facing away or farther from the head are caudal.
Distal to the carpal joint, the term dorsal replaces cranial and palmar replaces caudal. Similarly, distal to the tarsal joint the term dorsal replaces cranial and plantar replaces caudal. For example, the top of a dog
's paw
is its dorsal surface; the underside, either the palmar (on the forelimb) or the plantar (on the hindlimb) surface.
The sides of the forearm are named after its bones: Structures closer to the radius
are radial, structures closer to the ulna
are ulnar, and structures relating to both bones are referred to as radioulnar. Similarly, in the lower leg, structures near the tibia
(shinbone) are tibial and structures near the fibula are fibular (or peroneal).
Volar (sometimes used as a synonym for "palmar") refers to the underside, for both the palm and the sole (plantar), as in volar pads on the underside of hands, fingers, feet and toes.
The terms valgus and varus
are used to refer to angulation of the distal part of a limb at a joint. For example, at the elbow
joint, in the anatomical position, the forearm and the upper arm do not lie in a straight line, but the forearm is angulated laterally with respect to the upper arm by about 5–10°. The forearm is said to be "in valgus". Angulation at a joint may be normal (as in the elbow) or abnormal.
For post-embryo
nic humans a coronal plane is vertical and a transverse plane is horizontal, but for embryos and quadrupeds a coronal plane is horizontal and a transverse plane is vertical.
When describing anatomical motion, these planes describe the axis along which an action is performed. So by moving through the transverse plane, movement travels from head to toe. For example, if a person jumped directly up and then down, their body would be moving through the transverse plane in the coronal and sagittal planes.
Some of these terms come from Latin. Sagittal means "like an arrow", a reference to the position of the spine that naturally divides the body into right and left equal halves, the exact meaning of the term "midsagittal", or to the shape of the sagittal suture, which defines the sagittal plane and is shaped like an arrow.
A longitudinal plane is any plane perpendicular to the transverse plane. The coronal plane and the sagittal plane
are examples of longitudinal planes.
techniques such as sonography
, CT scans, MRI scans
, or PET scans
. One imagines a human in the anatomical position, and an X-Y-Z coordinate system
with the Z-axis going from front to back, the X-axis going from left to right, and the Y-axis going from up to down. The Z-axis axis is always forward (Tait-Bryan angles) and the right-hand rule
applies.
The axes and the sagittal plane are the same for bipeds and quadrupeds, but the orientation of the coronal and transverse planes switch. The axes on particular pieces of equipment may or may not correspond to axes of the body, especially since the body and the equipment may be in different relative orientations.
Occasionally, in medicine, abdominal
organs may be described with reference to the trans-pyloric plane, which is a transverse plane passing through the pylorus
.
of animals, particularly rodent
s used in neuroscience
research, a simplistic convention has been to name the sections of the brain according to the homologous human sections. Hence, what is technically a transverse (orthogonal) section with respect to the body length axis of a rat (dividing anterior from posterior) may often be referred to in rat neuroanatomical coordinates as a coronal section, and likewise a coronal section with respect to the body (i.e. dividing ventral from dorsal) in a rat brain is referred to as transverse. This preserves the comparison with the human brain, whose length axis in rough approximation is rotated with respect to the body axis by 90 degrees in the ventral direction. It implies that the planes of the brain are not necessarily the same as those of the body.
However, the situation is more complex, since comparative embryology shows that the length axis of the neural tube (the primordium of the brain) has three internal bending points, namely two ventral bendings at the cervical and cephalic flexures (cervical flexure roughly between the medulla oblongata and the spinal cord, and cephalic flexure between the diencephalon and the midbrain), and a dorsal (pontine or rhombic) flexure at the midst of the hindbrain, behind the cerebellum. The latter flexure mainly appears in mammals and sauropsids (reptiles and birds), whereas the other two, and principally the cephalic flexure, appear in all vertebrates (the sum of the cervical and cephalic ventral flexures is the cause of the 90 degree angle mentioned above in humans between body axis and brain axis). This more realistic concept of the longitudinal structure of vertebrate brains implies that any section plane, except the sagittal plane, will intersect variably different parts of the same brain as the section series proceeds across it (relativity of actual sections with regard to topological morphological status in the ideal unbent neural tube). Any precise description of a brain section plane therefore has to make reference to the anteroposterior part of the brain to which the description refers (e.g., transverse to the midbrain, or horizontal to the diencephalon). A necessary note of caution is that modern embryologic orthodoxy indicates that the brain's true length axis finishes rostrally somewhere in the hypothalamus where basal and alar zones interconnect from left to right across the median line; therefore, the axis does not enter the telencephalic area, although various authors, both recent and classic, have assumed a telencephalic end of the axis. The causal argument for this lies in the end of the axial mesoderm -mainly the notochord, but also the prechordal plate- under the hypothalamus. Early inductive effects of the axial mesoderm upon the overlying neural ectoderm is the mechanism that establishes the length dimension upon the brain primordium, jointly with establishing what is ventral in the brain (close to the axial mesoderm) in contrast with what is dorsal (distant from the axial mesoderm). Apart of the lack of a causal argument for introducing the axis in the telencephalon, there is the obvious difficulty that there is a pair of telencephalic vesicles, so that a bifid axis is actually implied in these outdated versions.
, made to landmarks that are on the skin or visible underneath. As with planes, lines and points are imaginary. Examples include:
In addition, reference may be made to structures at specific levels of the spine
(e.g. the 4th cervical vertebra, abbreviated "C4"), or the rib cage (e.g., the 5th intercostal space
).
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
of animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s to avoid ambiguities that might otherwise arise. They are not language-specific, and thus require no translation. They are universal terms that may be readily understood by zoologists
Zoology
Zoology |zoölogy]]), is the branch of biology that relates to the animal kingdom, including the structure, embryology, evolution, classification, habits, and distribution of all animals, both living and extinct...
who speak any language.
While these terms are standardized within specific fields of biology
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
, they can differ dramatically from one discipline to another. Differences in terminology remain a problem that, to some extent, still separates the fields of zoological anatomy (sometimes called zootomy
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
) and human (medical) anatomy
Human anatomy
Human anatomy is primarily the scientific study of the morphology of the human body. Anatomy is subdivided into gross anatomy and microscopic anatomy. Gross anatomy is the study of anatomical structures that can be seen by the naked eye...
(sometimes called androtomy
Androtomy
Androtomy is the dissection of the human body. Another term for it is anthropotomy "dissection of a human". Androtomy is to be distinguished from zoötomy "dissection of an animal"....
).
The Craniata
Craniata
Craniata is a proposed clade of chordate animals that contains the Myxini , Petromyzontida , and Gnathostomata as living representatives...
(vertebrates) share a substantial heritage of common structure, allowing much of the same terminology to be used for all of them. It is necessary for this terminology to be based on the anatomy of the animal in a standard way to avoid ambiguities such as might occur if a word such as "top" were used, which might designate the head
Head
In anatomy, the head of an animal is the rostral part that usually comprises the brain, eyes, ears, nose and mouth . Some very simple animals may not have a head, but many bilaterally symmetric forms do....
of a human
Human
Humans are the only living species in the Homo genus...
but the left or right side of a flounder
Flounder
The flounder is an ocean-dwelling flatfish species that is found in coastal lagoons and estuaries of the Northern Atlantic and Pacific Oceans.-Taxonomy:There are a number of geographical and taxonomical species to which flounder belong.*Western Atlantic...
. Most animals, furthermore, are capable of moving relative to their environment. So while "up" might refer to the direction of a standing human's head, the same term ("up") might be used to refer to the direction of the belly of a supine
Supine position
The supine position is a position of the body: lying down with the face up, as opposed to the prone position, which is face down, sometimes with the hands behind the head or neck. When used in surgical procedures, it allows access to the peritoneal, thoracic and pericardial regions; as well as the...
human. It is also necessary to employ some specific anatomical knowledge in order to apply the terminology unambiguously: For example, while the ears would be superior to (above) the shoulders in a human, this fails when describing the armadillo
Armadillo
Armadillos are New World placental mammals, known for having a leathery armor shell. Dasypodidae is the only surviving family in the order Cingulata, part of the superorder Xenarthra along with the anteaters and sloths. The word armadillo is Spanish for "little armored one"...
, where the shoulders are above the ears. Thus, in veterinary
Veterinary medicine
Veterinary Medicine is the branch of science that deals with the prevention, diagnosis and treatment of disease, disorder and injury in non-human animals...
terminology, the ears would be cranial to (i.e., "toward the head from") the shoulders in the armadillo, the dog
Dog
The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...
, the kangaroo
Kangaroo
A kangaroo is a marsupial from the family Macropodidae . In common use the term is used to describe the largest species from this family, especially those of the genus Macropus, Red Kangaroo, Antilopine Kangaroo, Eastern Grey Kangaroo and Western Grey Kangaroo. Kangaroos are endemic to the country...
, or any other vertebrate, including the human. Likewise, while the belly is considered anterior to (in front of) the back in humans, this terminology fails for the flounder, the armadillo, and the dog. In veterinary terms, the belly would be ventral ("toward the abdomen") in all vertebrates.
In human anatomy, as will be explained below, all naming is based on positions relative to the body in a standing (standard anatomical) position with arms at the side and palms facing forward (thumbs out). While the universal vertebrate terminology used in veterinary medicine would work in human medicine, the human terms are thought to be too well established to change
Status quo bias
The status quo bias is a cognitive bias for the status quo; in other words, people tend not to change an established behavior unless the incentive to change is compelling...
.
For invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s, locational terminology becomes more complicated, as many species are not bilaterally symmetrical. In these species, terminology depends on the type of symmetry present (if any).
Thus, standardized anatomical (and zootomical) terms of location have been developed, usually based on Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
words, to enable all biological and medical scientists to precisely delineate and communicate information about animal (including human) bodies and their component organs.
Standard anatomical position
Because animalAnimal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s can change orientation with respect to their environment, and because appendage
Appendage
In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body . It is a general term that covers any of the homologous body parts that may extend from a body segment...
s (arms, legs, tentacle
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...
s, etc.) can change position with respect to the main body, it is important that positional descriptive terms refer to the organism
Organism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
when it is in its standard anatomical position.
Thus, all descriptions are with respect to the organism in its standard anatomical position, even when the organism in question has appendages in another position. For example, see Fig. 9, where the tentacles are curved, and therefore not in anatomical position. However, a straight position is assumed when describing the proximo-distal axis. This helps avoid confusion in terminology when referring to the same organism in different postures.
Medical (human) anatomy
Unlike the situation in zootomyAnatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...
, standard anatomical position is rigidly defined for human anatomy. As with other vertebrates, the human body is standing erect and at rest. Unlike the situation in other vertebrates, the limbs are placed in positions reminiscent of the supine position
Supine position
The supine position is a position of the body: lying down with the face up, as opposed to the prone position, which is face down, sometimes with the hands behind the head or neck. When used in surgical procedures, it allows access to the peritoneal, thoracic and pericardial regions; as well as the...
imposed on cadaver
Cadaver
A cadaver is a dead human body.Cadaver may also refer to:* Cadaver tomb, tomb featuring an effigy in the form of a decomposing body* Cadaver , a video game* cadaver A command-line WebDAV client for Unix....
s during autopsy. Therefore, the body has its feet together (or slightly separated), and its arms are rotated outward so that the palms
Hand
A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...
are forward, and the thumb
Thumb
The thumb is the first digit of the hand. When a person is standing in the medical anatomical position , the thumb is the lateral-most digit...
s are pointed away from the body (forearms supine). As well, the arms are usually moved slightly out from the body, so that the hands do not touch the sides. The positions of the limbs (and the arms in particular) have important implications for directional terms in those appendages. The penis
Penis
The penis is a biological feature of male animals including both vertebrates and invertebrates...
in males is also erect in the anatomical position, hence the dorsal surface of the penis is actually anterior in the flaccid state.
Skull
In humans, the anatomical position of the skullHuman skull
The human skull is a bony structure, skeleton, that is in the human head and which supports the structures of the face and forms a cavity for the brain.In humans, the adult skull is normally made up of 22 bones...
has been agreed by international convention to be the Frankfurt plane
Frankfurt plane
The Frankfurt plane was established at the World Congress on Anthropology in Frankfurt, Germany in 1884, and decreed as the anatomical position of the human skull...
, a position in which the lower margins of the orbits
Orbit (anatomy)
In anatomy, the orbit is the cavity or socket of the skull in which the eye and its appendages are situated. "Orbit" can refer to the bony socket, or it can also be used to imply the contents...
, the orbitales, and the upper margins of the ear canal
Ear canal
The ear canal , is a tube running from the outer ear to the middle ear. The human ear canal extends from the pinna to the eardrum and is about 35 mm in length and 5 to 10 mm in diameter....
s, the poria, all lie in the same horizontal plane. This is a good approximation to the position in which the skull would be if the subject were standing upright and facing forward normally.
Directional terms
Ultimately, the bodies we are most familiar with are vertebrate bodies similar to our own. All vertebrateVertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s (including humans) have the same basic body plan (or bauplan
Body plan
A body plan is the blueprint for the way the body of an organism is laid out. An organism's symmetry, its number of body segments and number of limbs are all aspects of its body plan...
)—they are bilaterally symmetrical. That is, they have mirror-image left and right halves if divided down the centre. For these reasons, the basic directional terms can be considered to be those used in vertebrates. By extension, the same terms are used for many other (invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
) organisms as well.
Vertebrate directional terms
To begin with, distinct, polar-opposite ends of the organismOrganism
In biology, an organism is any contiguous living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homoeostasis as a stable whole.An organism may either be unicellular or, as in the case of humans, comprise...
are chosen. By definition, each pair of opposite points defines an axis
Rotation around a fixed axis
Rotation around a fixed axis is a special case of rotational motion. The fixed axis hypothesis exclude the possibility of a moving axis, and cannot describe such phenomena as wobbling or precession. According to Euler's rotation theorem, simultaneous rotation around more than one axis at the same...
. In a bilaterally-symmetrical organism, there are 6 polar opposite points, giving three axes that intersect at right angles—the x, y, and z axes familiar from three-dimensional geometry.
Anterior and posterior
The most obvious end-points are the "nose" and "tail" (see Fig. 2). In terms of anatomy, the nose is referred to as the anterior end (Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
ante; before). In organisms like vertebrates, that have distinct heads, the anterior end is sometimes referred to as the rostral end (Latin rostrum; beak), the cranial end (Greek kranion; skull), or the cephalic end (Greek
Greek language
Greek is an independent branch of the Indo-European family of languages. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. Its writing system has been the Greek alphabet for the majority of its history;...
kephalē; head). For reasons of broader applicability, especially in organisms without distinct heads (many invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s), "anterior" is usually preferred.
The polar opposite to the anterior end is the posterior end (Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
post; after). Another term for posterior is caudal (Latin caudum; tail, though in humans this refers to the feet i.e. inferior rather than posterior) — a term that strictly applies only to vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s, and therefore less preferred, except in veterinary medicine where these terms are standard.
By drawing a line connecting these two points, we define the anteroposterior axis (sometimes written antero-posterior). Caudal and Posterior (back end) are often used interchangeably. In veterinary medicine, caudo-cranial is preferred between head and tail, and rostro-caudal between nose and neck. Less-used synonyms would be rostrocaudal or cephalocaudal axes (see Table 1). For brevity, the term anteroposterior is often abbreviated to read AP (or A-P) axis. As well as defining the anteroposterior axis, the terms "anterior" and "posterior" also define relative positions along the axis. Thus, in the fish in Fig. 2, the gill openings are posterior relative to the eyes, but anterior to the tail.
| Directional term | Defined Axis | Synonyms | Axis runs... |
|---|---|---|---|
| Anterior | Anteroposterior | Rostrocaudal1, Craniocaudal1, Cephalocaudal2 | ...from head end to opposite end of body or tail. |
| Posterior | |||
| Dorsal | Dorsoventral | — | ...from spinal column (back) to belly (front). |
| Ventral | |||
| Left (lateral) | Left-right | Dextro-sinister2, Sinistro-dexter2 | ...from left to right sides of body. |
| Right (lateral) | |||
| Medial | Mediolateral3 | — | ...from centre of organism to one or other side. |
| Left or right (lateral) | |||
| Proximal | Proximodistal | — | ...from tip of an appendage Appendage In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body . It is a general term that covers any of the homologous body parts that may extend from a body segment... (distal) to where it joins the body (proximal). |
| Distal | |||
| Notes: (1) Fairly common usage. (2) Uncommon usage. (3) Equivalent to one-half of the left-right axis. (The terms "intermediate", "ipsilateral", "contralateral", "superficial" and "deep", while indicating directions, are relative terms and thus do not properly define fixed anatomical axes. Also, while the "rostrocaudal" and anteroposterior directionality are equivalent in a significant portion of the human body, they are different directions in other parts of the body.) |
|||
Dorsal and ventral
The next most obvious end-points are the back and belly. These are termed the dorsal endDorsum (biology)
In anatomy, the dorsum is the upper side of animals that typically run, fly, or swim in a horizontal position, and the back side of animals that walk upright. In vertebrates the dorsum contains the backbone. The term dorsal refers to anatomical structures that are either situated toward or grow...
(Latin dorsum; back) and the ventral end (Latin venter; abdomen
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
), respectively. By connecting the outermost points the dorsoventral axis is formed (sometimes hyphenated: dorso-ventral). This is commonly abbreviated to DV (or D-V) axis. The DV axis, by definition, is perpendicular (at right angles to) the AP axis at all times (see below).
As with anteroposterior, the terms "dorsal" and "ventral" are also used to describe relative positions along the dorsoventral axis. Thus, the pectoral fins are dorsal to the anal fin, but ventral to the dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
in Fig. 2. (Note that these fins are not aligned anteroposteriorly, either — the dorsal fin being posterior to the pectoral, and anterior to the anal fins, respectively.)
Left and right (lateral), and medial
The last axis, by geometricGeometry
Geometry arose as the field of knowledge dealing with spatial relationships. Geometry was one of the two fields of pre-modern mathematics, the other being the study of numbers ....
definition, must be at right angles to both the AP and the DV axes. The left side and right side of the organism are the outermost points between the two "sides" of the organism. When connected, these points form the left-right axis (commonly abbreviated to LR (or L-R) axis. In Latin, this is called the dextro-sinistral (or, more uncommonly, the sinistro-dextral) axis, from dexter (right) and sinister (left). It is important to note that the "left" and "right" sides are the sides of the organism, and not those of the observer.
"Left-right" is typically used in English and some other languages.
As with the other directions, the terms can be used as relative terms, to describe locations along the left-right axis. Thus, in Fig. 2 the dorsal fin
Dorsal fin
A dorsal fin is a fin located on the backs of various unrelated marine and freshwater vertebrates, including most fishes, marine mammals , and the ichthyosaurs...
is right of the left pectoral fin, but is left of the right eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...
. However, as left and right sides are mirror images, usage like this tends to be somewhat confusing, as structures are duplicated on both sides (i.e., above there is both a right eye and a left eye, forcing one to specify which is used as a reference).
To counter this clumsiness of usage, the directional term lateral (Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
lateralis; "to the side") is used as a modifier for both sides, yielding the left lateral and right lateral sides. As an opposite to lateral, the term median (Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
medius; "middle") is used to define a point in the centre of the organism (where the left-right axis intersects the midsagittal plane —see below), and the term medial means "toward the median plane". Thus, rather than "left-right" axis and its inherent clumsiness of usage, the term mediolateral (also sometimes hyphenated medio-lateral) axis is frequently used. Sometimes this is abbreviated to ML (or M-L) axis. In proper usage, the ML axis is a half-axis; in practice, its usage is less clumsy and less linguistically biased than "left-right". The terms may still be used relatively to describe locations along the LR axis. Thus, in Fig. 2 the gills are medial to the operculum
Operculum (fish)
The operculum of a bony fish is the hard bony flap covering and protecting the gills. In most fish, the rear edge of the operculum roughly marks the division between the head and the body....
, but lateral to the heart
Heart
The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...
.
The usage "mediolateral" is strictly used to describe relative position along the left-right axis, to avoid confusion with the terms "superficial" and "deep" (see below).
Sources of confusion
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
or invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
. In practice, the terms can cause some confusion when, unlike the fish shown in Fig. 2, the organism in question is not strictly linear in form, which includes most tetrapods (see Figs. 3 and 4). For example, the AP axis in Fig. 3 does not appear to be at right angles to the DV axis. Rather, it is a depiction of the approximate average AP axis, when all body segments are included.
As a general rule of thumb, if the body is included in consideration, the AP axis of the main body would be used, as would the DV and ML axes perpendicular to it. However, if considering only one segment, the AP axis would shift to reflect the axes shown in Fig. 4, with the DV and ML axes shifting correspondingly. In alternative manner, to avoid confusion, AP, DV, and ML terms are used strictly in relation to the main body, and the terms proximal and distal are used for body segments such as the head, neck, and tail (see below).
To avoid this confusion, in veterinary medicine, the terms anterior, posterior, superior, and inferior are in general avoided except for certain structures within the head. By using the terms cranial, caudal, dorsal and ventral, all tetrapod organisms (including bipeds) can be described uniformly.
In humans, the directions "rostral" and "caudal" often become confused with anterior and posterior, or superior and inferior. The difference between the two is most easily visualized when looking at the head, as can be seen in the image to the right. From the most caudal of positions in the nervous system (of a person) to a nearby, rostral area, it is equally accurate to say the area in question is rostral as to say it is superior. However, in the frontal lobes of the telencephalon
Telencephalon
The cerebrum or telencephalon, together with the diencephalon, constitutes the forebrain. The cerebrum is the most anterior region of the vertebrate central nervous system. Telencephalon refers to the embryonic structure, from which the mature cerebrum develops...
, to say an area is rostral to a nearby area is equivalent to saying it is anterior (or ventral). Those two lines lie on planes perpendicular to one another. This occurs, as becomes clear in the diagram, due to the intuitive yet curious curving "C" shape of rostrocaudal directionality when discussing the human brain.
Proximal and distal
The term proximal (LatinLatin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
proximus; nearest) describes where the appendage joins the body, and the term distal (Latin distare; to stand away from) is used for the point furthest from the point of attachment to the body. Since appendage
Appendage
In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body . It is a general term that covers any of the homologous body parts that may extend from a body segment...
s often move independently of (and therefore change position with respect to) the main body, these separate directional terms are used when describing them.
As noted above, the standard AP, DV and ML directional axes, can cause some confusion when describing parts of the body that can change position (move) relative to the main body. This is particularly true when considering appendage
Appendage
In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body . It is a general term that covers any of the homologous body parts that may extend from a body segment...
s. "Appendages" would include vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
fins
FINS
FINS is a network protocol used by Omron PLCs, over different physical networks like Ethernet, Controller Link, DeviceNet and RS-232C....
(see Fig. 2) and limbs
Limb (anatomy)
A limb is a jointed, or prehensile , appendage of the human or other animal body....
(see Figs. 3 and 4), but properly apply to any structure that extends (and can at least potentially move separately) from the main body. Thus, "appendage" would also include such structures as external ears
EARS
EARS may refer to:* Electoral software* Electronic Arts, Redwood Shores campus.* Emirates Amateur Radio Society...
(pinnae) and hair
Hair
Hair is a filamentous biomaterial, that grows from follicles found in the dermis. Found exclusively in mammals, hair is one of the defining characteristics of the mammalian class....
(in mammals), feathers (in birds) and scales
Scale (zoology)
In most biological nomenclature, a scale is a small rigid plate that grows out of an animal's skin to provide protection. In lepidopteran species, scales are plates on the surface of the insect wing, and provide coloration...
(fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
, reptiles and birds). As well, varieties of tentacles or other projections from the body in invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s and the male in many vertebrates and some invertebrates, would be included.
By connecting the two points, the proximodistal (sometimes hyphenated to proximo-distal) axis is created. (The abbreviation AB axis is occasionally, but not commonly, used.) As before, the terms "proximal" and "distal" can be used as relative terms to indicate where structures lie along the proximodistal axis. Thus, the "elbow" is proximal to the hoof, but distal to the "shoulder" in Figs. 3 and 4.
Choosing terms for the other two axes perpendicular to the proximodistal axis could be variable, as they would also depend on the position of the limb. For that reason, when considering any organism, the other two axes are considered to be relative to the appendage when in standard anatomical position. This is roughly defined for all organisms, as in the normal position when at rest and not moving. For tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
vertebrate
Vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...
s, this includes the caveat that they are standing erect and not lying down. Thus, the fish in Fig. 2, and the horse in Figs. 3 and 4 are in standard anatomical position. (Special considerations with respect to limb position are applied in human anatomy—see below).
Ostial and distal
Similar to appendages that branch out from the body, the directions of blood vessels may be labeled with the terms Ostial (referring to the Ostium or opening where the vessel branches off) and distal (the extreme end away from the branch point).Teeth
Teeth may be aligned with their main axis identical to that of the jaw, but they can also be rotated. In such a case using terms like "anterior" or "lateral" can be confusing. Therefore, a special set of terms exists, used mainly in palaeontology.- lingual ([Latin] lingua; tongue): on the side that faces the tongue.
- labial ([Latin] labium; lip): facing the lips
- buccal ([Latin] bucca; cheek): on the side that faces the cheek.
- mesial: toward the mandibular symphysis (where lower jaws meet)
- distal: away from the mandibular symphysis
Dentistry uses the same terminology, as well as
- vestibular ([Portugese] vestibulo; entrance): the side away from the tongue (includes labial and buccal).
Other directional terms
In addition to the three primary axes (AP, DV and the ML half-axis) and the proximodistal axis of appendages, several directional terms can be used in bilaterally symmetrical animals. These terms are strictly relative, and as such do not and cannot be used to define fixed axes. These terms include:- Ipsilateral (LatinLatinLatin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
ipse; self/same + latus-eris, "side"): on the same side as another structure. Thus, the left arm is ipsilateral to the left leg. - Contralateral (Latin contra; against): on the opposite side to another structure. Thus, the left arm is contralateral to the right arm, or the right leg.
- Superficial (Latin superfacies; at the surface or face): near the outer surface of the organism. Thus, skin is superficial to the muscle layer. The opposite is "deep", or "visceral".
- Deep: further away from the surface of the organism. Thus, the muscular layer is deep to the skin, but superficial to the intestines. This is one of the few terms where the English vernacular is prevalent. The proper anglicised Latin term would be profound (Latin profundus; due to depth), but this word has other meanings in English. In other languages, the equivalent term is usually similar to "profound" (e.g. profond, meaning deep, in French).
- Intermediate (Latin intermedius; inter, between and medius, middle): between two other structures. Thus, the navelNavelThe navel is a scar on the abdomen caused when the umbilical cord is removed from a newborn baby...
is intermediate to (or intermediate between) the left arm and the contralateral (right) leg. - Visceral (Latin viscus,-eris; internal organs, flesh): associated with organs within the body's cavities. The stomach is a viscusViscusIn anatomy, a viscus is an internal organ, and viscera is the plural form. The viscera, when removed from a butchered animal, are known collectively as offal...
within the abdominal cavity, and is covered with a lining called the visceral peritoneumPeritoneumThe peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...
. - Parietal (Latin paries,-etis, "wall"): pertaining to the wall of a body cavity. The parietal peritoneum is the lining on the inside of the abdominal cavity. (Parietal can also refer specifically to the parietal boneParietal boneThe parietal bones are bones in the human skull which, when joined together, form the sides and roof of the cranium. Each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named from the Latin pariet-, wall....
of the skull or associated structures.) - Axial (Latin axis from Greek axōn "axle"): Toward the central axis of the organism or the extremity.
- Abaxial : away from the central axis of the organism or extremity
- Rostral (Latin - rostr(um), beak or nose ): situated toward the oral or nasal region, or in the case of the brain, toward the tip of the frontal lobe.
- Caudal (Latin - cauda, tail): of, at, or near the tail or the posterior end of the body. In the human case, toward the bottom of the feet (also the "tail" of the spinal cord, and body).
Invertebrate directional terms
The large variety of body shapesBody plan
A body plan is the blueprint for the way the body of an organism is laid out. An organism's symmetry, its number of body segments and number of limbs are all aspects of its body plan...
present in invertebrates presents a difficult problem when attempting to apply standard directional terms. Depending on the organism, some terms are taken by analogy from the vertebrate terms, and appropriate novel terms are applied, as necessary. In all cases, the usage of terms is dependent on the bauplan
Body plan
A body plan is the blueprint for the way the body of an organism is laid out. An organism's symmetry, its number of body segments and number of limbs are all aspects of its body plan...
of the organism.
Asymmetrical and spherical organisms
In organisms with a changeable shape, such as amoeboidAmoeboid
Amoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amœba" are often used interchangeably even by biologists, and especially refer to a creature moving by using pseudopodia. Most references to "amoebas" or "amoebae" are to amoeboids in general rather than to...
organisms (Fig. 5a), directional terms are meaningless, since the shape of the organism is changeable, and no fixed axes are present. Similarly, in organisms that are spherical in shape (Fig. 5b), there is nothing to distinguish one line through the centre of the organism from another. An infinite number of triads of mutually perpendicular axes could be defined, but any such choice of axes would be functionally and practically indistinguishable from all others, and therefore would be useless. In such organisms, only the terms superficial and deep hold any descriptive meaning.
Elongated organisms
In organisms that maintain a constant shape and have one dimension longer than the other, at least two directional terms can be used. The long or longitudinal axis is defined by points at the opposite ends of the organism. Similarly, a perpendicular transverse axis can be defined by points on opposite sides of the organism. There is typically no basis for the definition of a third axis. Usually such organisms, like that pictured in Fig. 6, are planktonic (free-swimming) protists, and are nearly always viewed on microscope slideMicroscope slide
A microscope slide is a thin flat piece of glass, typically 75 by 25 mm and about 1 mm thick, used to hold objects for examination under a microscope. Typically the object is placed or secured on the slide, and then both are inserted together in the microscope for viewing...
s, where they appear essentially two-dimensional. In some cases a third axis can be defined, particularly where a non-terminal cytostome
Cytostome
A cytostome or cell mouth is a part of a cell specialized for phagocytosis, usually in the form of a microtubule-supported funnel or groove. Food is directed into the cytostome, and sealed into vacuoles. Only certain groups of protozoa, such as the ciliates and excavates, have cytostomes. Such...
or other unique structure is present.
Elongated organisms with distinctive ends
Cytostome
A cytostome or cell mouth is a part of a cell specialized for phagocytosis, usually in the form of a microtubule-supported funnel or groove. Food is directed into the cytostome, and sealed into vacuoles. Only certain groups of protozoa, such as the ciliates and excavates, have cytostomes. Such...
in Paramecium
Paramecium
Paramecium is a group of unicellular ciliate protozoa, which are commonly studied as a representative of the ciliate group, and range from about 0.05 to 0.35 mm in length. Simple cilia cover the body, which allow the cell to move with a synchronous motion at speeds of approximately 12 body...
or Stentor
Stentor
In Greek mythology, Stentor was a herald of the Greek forces during the Trojan War. Although he is mentioned only briefly in Homer's Iliad, in which Hera takes Stentor's character to encourage the Greeks to fight, his name has been living in the term "stentorian" voice, meaning loud-voiced, for...
), or the end that usually points in the direction of the organism's locomotion
Animal locomotion
Animal locomotion, which is the act of self-propulsion by an animal, has many manifestations, including running, swimming, jumping and flying. Animals move for a variety of reasons, such as to find food, a mate, or a suitable microhabitat, and to escape predators...
(such as the end with the flagellum
Flagellum
A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and plays the dual role of locomotion and sense organ, being sensitive to chemicals and temperatures outside the cell. There are some notable differences between prokaryotic and...
in Euglena
Euglena
Euglena is a genus of unicellular flagellate protists. It is the best known and most widely studied member of the phylum Euglenozoa , a diverse group containing some 44 genera and at least 800 species. Species of Euglena are found in fresh and salt waters...
), is normally designated as the anterior end. The opposite end then becomes the posterior end, and by connecting them, an anteroposterior axis is formed. Properly, this terminology would apply only to an organism that is always planktonic (not normally attached to a surface, as in Fig. 7 top), although the term can also be applied to one that is sessile
Sessility (zoology)
In zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
(normally attached to a surface, as in Fig. 7, bottom, and Fig. 8).
Substrate (biology)
In biology a substrate is the surface a plant or animal lives upon and grows on. A substrate can include biotic or abiotic materials and animals. For example, encrusting algae that lives on a rock can be substrate for another animal that lives on top of the algae. See also substrate .-External...
, such as sponges (Fig. 8), or some animal-like protists also have distinctive ends. The part of the organism attached to the substrate is usually referred to as the basal end (Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
basis; support or foundation), whereas the end furthest from the attachment is referred to as the apical end (Latin apex; peak, tip). Thus, by joining the two ends, an apical-basal (or basal-apical) axis is formed (see Fig. 8). Transverse axes may be defined indifferently in any direction perpendicular to this axis, as there is no symmetry present.
Radially-symmetrical organisms
Radially symmetrical organisms include those in the group RadiataRadiata
The Radiata are the radially symmetric animals of the Eumetazoa subkingdom. The term Radiata has had various meanings in the history of classification...
—primarily jellyfish, sea anemones and corals
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
and the comb jellies. Adult echinoderms (sea stars (starfish), sea urchins, and sea cucumbers and others) are also included, since they are pentaradial (i.e. they have fivefold discrete rotational symmetry
Symmetry (biology)
Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry or "spherical symmetry". A small minority exhibit no symmetry .In nature and biology,...
). Echinoderm larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e are not included, since they are bilaterally symmetrical.
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
ns have an incomplete digestive system, meaning that one end of the organism has a mouth, and the opposite end has no opening from the gut (coelenteron). For this reason, the end of the organism with the mouth is referred to as the oral end (Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
oris; mouth), and the opposite surface is the aboral end (Latin ab-; prefix meaning "away from"). Thus, by joining the polar opposite oral and aboral ends, an oral-aboral axis is formed (Fig. 9).
As with vertebrates, appendage
Appendage
In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body . It is a general term that covers any of the homologous body parts that may extend from a body segment...
s that move independently of the body (tentacles in cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
ns and comb jellies), have a definite proximodistal axis (Fig. 8). Unlike vertebrates, cnidarians (jellyfish, sea anemones, corals) have no other distinctive axes, and multiple radial axes are possible (Fig. 10).
It is noteworthy that some "biradially-symmetrical" comb jellies have distinct "tentacular" and "pharyngeal" axes, and are thus anatomically equivalent to bilaterally-symmetrical
Bilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
animals. As well, adult echinoderms (starfish, sea urchins, sea cucumbers) are pentaradial, and have only five symmetrical radial axes (unlike the multiple axes in cnidarians).
Lateral, dorsal, and ventral have no meaning in such organisms, and all can be replaced by the generic term peripheral (Latin peri-; around; see Table 2). Medial can be used, but in the case of radiates indicates the central point of these organisms, rather than a central axis (as in vertebrates). Thus, as there are many possible radial axes, there are multiple medio-peripheral (half-) axes (Fig. 10).
| Table 2: Comparison of Directional Terms used in Radially-Symmetrical1 and Bilaterally-Symmetrical Animals |
|||
|---|---|---|---|
| Bilateral Bilateria The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back... Bauplan Body plan A body plan is the blueprint for the way the body of an organism is laid out. An organism's symmetry, its number of body segments and number of limbs are all aspects of its body plan... s |
Radial Bauplans | ||
| Direction | Synonyms | Direction | Synonyms |
| Anterior | Rostral, Cranial, Cephalic2 | Oral | Apical3 |
| Posterior | Caudal2 | Aboral | Basal3 |
| Dorsal | — | Peripheral4,5 | — |
| Ventral | — | Peripheral4,5 | — |
| Left (lateral) | Sinister | Peripheral4,5 | — |
| Right (lateral) | Dexter | Peripheral4,5 | — |
| Medial | — | Same6 | — |
| Proximal | — | Same | — |
| Distal | — | Same | — |
| Notes: (1) Includes both Radiates and adult Echinoderms. (2) Rarely used. (3) Only in organisms attached to a substrate Substrate (biology) In biology a substrate is the surface a plant or animal lives upon and grows on. A substrate can include biotic or abiotic materials and animals. For example, encrusting algae that lives on a rock can be substrate for another animal that lives on top of the algae. See also substrate .-External... . (4) Vertebrate equivalents are meaningless in radial animals. (5) Roughly equivalent to "superficial". (6) Roughly equivalent to "deep". |
|||
Arachnids
Two specialized terms are sometimes used for describing views of arachnidArachnid
Arachnids are a class of joint-legged invertebrate animals in the subphylum Chelicerata. All arachnids have eight legs, although in some species the front pair may convert to a sensory function. The term is derived from the Greek words , meaning "spider".Almost all extant arachnids are terrestrial...
legs and pedipalp
Pedipalp
Pedipalps , are the second pair of appendages of the prosoma in the subphylum Chelicerata. They are traditionally thought to be homologous with mandibles in Crustacea and insects, although more recent studies Pedipalps (commonly shortened to palps or palpi), are the second pair of appendages of the...
s. Prolateral refers to the surface of a leg that is closest to the anterior end of an arachnid's body. Retrolateral refers to the surface of a leg that is closest to the posterior end of an arachnid's body.
Medical (human) directional terms
As humans are bilaterally-symmetrical organisms, anatomical directions in humans can usually be correctly described using the same terms as those for vertebrates and other members of the taxonomic group BilateriaBilateria
The bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
. However, for historical and other reasons, standard human directional terminology has several differences from that used for other bilaterally-symmetrical organisms.
Why zootomy and androtomy terms differ
The terms of zootomy and androtomy came into usage at a time when all scientific communication took place in Latin. In their original Latin forms the respective meanings of "anterior" and "posterior" are in front of (or before) and behind (or after), those of "dorsal" and "ventral" are toward the spine and toward the belly, and those of "superior" and "inferior" are above and below. From these meanings it can be seen that in the most general terms the anterior/posterior axis is oriented to the direction of forward motion, the dorsal/ventral axis is oriented to the anatomy of the vertebrate torso, and the superior/inferior axis is oriented to gravity.For almost all vertebrates, including almost all bipeds, these axes all provide a consistent reference for anatomical positions across species—with the inferior/superior axis being roughly the same as the dorsal/ventral axis, and therefore redundant. Humans, however, have the rare property of having a torso oriented perpendicular to their direction of forward motion—while their head orientation remains consistent with other vertebrates on this axis. This makes the dorsal/ventral axis on humans redundant with the anterior/posterior axis, and the inferior/superior axis necessary. Because of this difference with humans, the anterior/posterior and inferior/superior axes are inconsistent between humans and other vertebrates in torso anatomy but consistent in head anatomy. As all three of these axes are used in the naming of anatomical structures, and most human anatomical structures are shared by other animals, these differences can lead to considerable confusion. For example, in the naming of brain structures, the non-human context of the dorsal/ventral axis was used. Therefore, in human anatomy, "dorsal" can refer to two different (perpendicular) directions—the posterior direction in the context of the torso, and the superior direction in the context of the brain. Ironically, the "dorsal" direction in the human brain, besides being perpendicular to the "dorsal" direction in the human torso, is actually the opposite direction of what might be inferred from the literal Latin meaning of "toward the spine".
While it would be possible to introduce a system of axes that is completely consistent between humans and other vertebrates by having two separate pairs of axes, one used exclusively for the head (e.g. anterior/posterior and inferior/superior) and the other exclusively for the torso (e.g. dorsal/ventral and caudal("toward the tail")/rostral("toward the beak")), doing so would require the renaming of very many anatomical structures.
For a quick comparison of equivalent terminology used in vertebrate and human anatomy, see Table 3 (below).
| Vertebrate Vertebrate Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds... zootomy Anatomy Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy... |
Human torso | Human head | |||
|---|---|---|---|---|---|
| Direction | Synonyms | Direction | Synonyms | Direction | Synonyms |
| Anterior | Rostral, Cranial, Cephalic1 | Superior | Same1, Up | Anterior | Front |
| Posterior | Caudal | Inferior | Caudal1, Down | Posterior | Back |
| Dorsal | — | Posterior | Dorsal, Back | Superior | Dorsal, Up |
| Ventral | — | Anterior | Ventral, Front | Inferior | Ventral, Down |
| lateral | Away from the middle | Same | — | Same | — |
| Left (lateral) | Sinister1 | Same | — | Same | — |
| Right (lateral) | Dexter1 | Same | — | Same | — |
| Medial | Middle | Same | — | Same | — |
| Proximal | Away from extremity | Same | — | Same | — |
| Distal | Toward extremity | Same | — | Same | — |
| Intermediate2 | — | Same | — | Same | — |
| Ipsilateral2 | Same side | Same | — | Same | — |
| Contralateral2 | Opposite side | Same | — | Same | — |
| Superficial2 | — | Same | — | Same | — |
| Deep2 | — | Same | — | Same | — |
| Notes: (1) Rarely used. (2) Strictly relative term, used with other locational descriptors. |
|||||
Superior and inferior
As with other vertebrates, two of the most obvious extremes are the "top" and the "bottom" of the organism. In standard anatomical position, these correspond to the head and feet, respectively in humans. The head end is referred to as the superior end (LatinLatin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
superior: "above"), while the feet are referred to as the inferior end (Latin inferior: "below"). Thus, the axis formed by joining the two is the superior-inferior axis.
As with other vertebrate terminology, there are synonymous terms for superior and inferior (Table 3). The terms cranial and cephalic are often encountered. "Cranial", as a reference to the skull, is fairly commonly used, whereas "cephalic" is uncommonly used. The term "rostral" is rarely used in human anatomy, referring more to the front of the face than the superior aspect of the organism. This term is more applicable in organisms with longer heads, such as equids. Similarly, the term caudal is occasionally used in human anatomy, and the cranio-caudal axis is occasionally encountered. Generally, this usage would be used with respect to only the head and the main body (trunk), and not when considering the limbs.
As with vertebrate directional terms, superior and inferior can be used in a relative sense in humans, but can not be uniformly applied to other organisms with varying normal anatomical positions. For example, the shoulders
Shoulders
Shoulders is drinking game that involves players competing in a fast paced game attempting to "count" to 21.-Game play:*The game begins with a player slapping his left/right shoulder, this starts the count at 1....
are superior to the navel
Navel
The navel is a scar on the abdomen caused when the umbilical cord is removed from a newborn baby...
, but inferior to the eyes in humans. In any tetrapod, the shoulders are cranial to the belly, but caudal to the eyes.
Anterior and posterior
In human anatomical usage, anterior refers to the "front" of the individual, and is synonymous with ventral, other than in the head. Similarly, posterior, refers to the "back" of the subject, and is synonymous with dorsal, other than in the head (see Table 3). The terms "dorsal" and "ventral" are used in human anatomy, but infrequently when referring to the body as a whole. The anteroposterior axis is preferred usage for describing the axis connecting the front and the back in humans."Anterior" and "posterior" can also be used as relative terms. Thus, the eye
Human eye
The human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
s are posterior to the nose
Human nose
The visible part of the human nose is the protruding part of the face that bears the nostrils. The shape of the nose is determined by the ethmoid bone and the nasal septum, which consists mostly of cartilage and which separates the nostrils...
, but anterior to the back of the head in humans.
However, in the horse
Horse
The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today...
, for example, the eyes are caudal to the nose, and rostral to the back of the head.
Left and right (lateral), and medial
Left and right lateral are used in the same sense as they are in other vertebrates, as is medial. The left-right axis is rarely used in medicine; instead, the mediolateral axis is used almost exclusively.Appendages
As in other vertebrates, the terms "proximal" and "distal" are used to describe the point of attachment to, and part of an appendage furthest away from, the body, respectively. However, other terms are used for direction in the appendages, given the unique position of the limbs (in standard anatomical position) in humans.Arms
Hand
A hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...
of the hand, and posterior can be used to describe the back of the hand and arm.
However, presumably for improved clarity, the directional term palmar (Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
palma; palm of the hand) is usually used for the anterior of the hand, and dorsal is used to describe the back of the hand. Thus, by connecting the extremes, dorsopalmar axis is formed. Most commonly, "dorsopalmar" is used when describing the hand, although it is sometimes applied to the arm as a whole (see Fig. 12).
For the third axis, the mediolateral axis suffices, although if referring to the limb alone, "medial" may refer to the centre of the arm itself.
Relative directions
Also, in common usage, the segments of the digestive system closest to the mouthMouth
The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....
are termed proximal, as opposed to those closest to the anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
, which are termed distal. The terms oral "of the mouth" and aboral "away from the mouth" are also used.
Relative directions in the limbs
Specialized terms are used to describe location on appendages, parts that have a point of attachment to the main trunk of the body. Structures that are close to the point of attachment of the body are proximal or central, while ones more distant from the attachment point are distal or peripheral. For example, the hands are at the distal end of the arms, while the shoulders are at the proximal ends. These terms can also be used relatively to organs, for example the proximal end of the urethraUrethra
In anatomy, the urethra is a tube that connects the urinary bladder to the genitals for the removal of fluids out of the body. In males, the urethra travels through the penis, and carries semen as well as urine...
is attached to the bladder
Urinary bladder
The urinary bladder is the organ that collects urine excreted by the kidneys before disposal by urination. A hollow muscular, and distensible organ, the bladder sits on the pelvic floor...
.
In the limbs of most animals, the terms cranial and caudal are used in the regions proximal to the carpus (the wrist
Wrist
In human anatomy, the wrist is variously defined as 1) the carpus or carpal bones, the complex of eight bones forming the proximal skeletal segment of the hand;...
, in the forelimb) and the tarsus (the ankle
Ankle
The ankle joint is formed where the foot and the leg meet. The ankle, or talocrural joint, is a synovial hinge joint that connects the distal ends of the tibia and fibula in the lower limb with the proximal end of the talus bone in the foot...
in the hindlimb). Objects and surfaces closer to or facing toward the head are cranial; those facing away or farther from the head are caudal.
Distal to the carpal joint, the term dorsal replaces cranial and palmar replaces caudal. Similarly, distal to the tarsal joint the term dorsal replaces cranial and plantar replaces caudal. For example, the top of a dog
Dog
The domestic dog is a domesticated form of the gray wolf, a member of the Canidae family of the order Carnivora. The term is used for both feral and pet varieties. The dog may have been the first animal to be domesticated, and has been the most widely kept working, hunting, and companion animal in...
's paw
Paw
A paw is the soft foot of a mammal, generally a quadruped, that has claws or nails. A hard foot is called a hoof. Paws are used to pad feet for walking and increase friction.- Common characteristics :...
is its dorsal surface; the underside, either the palmar (on the forelimb) or the plantar (on the hindlimb) surface.
The sides of the forearm are named after its bones: Structures closer to the radius
Radius (bone)
The radius is one of the two large bones of the forearm, the other being the ulna. It extends from the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna, which exceeds it in length and size. It is a long bone, prism-shaped and slightly curved longitudinally...
are radial, structures closer to the ulna
Ulna
The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form...
are ulnar, and structures relating to both bones are referred to as radioulnar. Similarly, in the lower leg, structures near the tibia
Tibia
The tibia , shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates , and connects the knee with the ankle bones....
(shinbone) are tibial and structures near the fibula are fibular (or peroneal).
Volar (sometimes used as a synonym for "palmar") refers to the underside, for both the palm and the sole (plantar), as in volar pads on the underside of hands, fingers, feet and toes.
The terms valgus and varus
Varus deformity
In orthopedics, a varus deformity is a term for the inward angulation of the distal segment of a bone or joint. The opposite of varus is called valgus.The terms varus and valgus always refer to the direction that the distal segment of the joint points....
are used to refer to angulation of the distal part of a limb at a joint. For example, at the elbow
Elbow
The human elbow is the region surrounding the elbow-joint—the ginglymus or hinge joint in the middle of the arm. Three bones form the elbow joint: the humerus of the upper arm, and the paired radius and ulna of the forearm....
joint, in the anatomical position, the forearm and the upper arm do not lie in a straight line, but the forearm is angulated laterally with respect to the upper arm by about 5–10°. The forearm is said to be "in valgus". Angulation at a joint may be normal (as in the elbow) or abnormal.
General usage
Three basic reference planes are used in zoological anatomy.- A sagittal planeSagittal planeSagittal plane is a vertical plane which passes from front to rear dividing the body into right and left sections.-Variations:Examples include:...
, being a plane parallel to the sagittal sutureSagittal sutureThe sagittal suture is a dense, fibrous connective tissue joint between the two parietal bones of the skull. The term is derived from the Latin word Sagitta, meaning "arrow". The derivation of this term may be demonstrated by observing how the sagittal suture is notched posteriorly, like an arrow,...
, divides the body into sinister and dexter (left and right) portions.- The midsagittal or median plane is in the mid line; i.e. it would pass through mid line structures such as the navelNavelThe navel is a scar on the abdomen caused when the umbilical cord is removed from a newborn baby...
or spineVertebral columnIn human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...
, and all other sagittal planes (also referred to as parasagittal planes) are parallel to it. Median can also refer to the midsagittal plane of other structures, such as a digit.
- The midsagittal or median plane is in the mid line; i.e. it would pass through mid line structures such as the navel
- A coronal or frontal plane divides the body into dorsal and ventral (back and front, or posterior and anterior) portions.
- A transverse planeTransverse planeThe transverse plane is an imaginary plane that divides the body into superior and inferior parts. It is perpendicular to the coronal and sagittal planes....
, also known as an axial plane or cross-section, divides the body into cranial and caudal (head and tail) portions.
For post-embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
nic humans a coronal plane is vertical and a transverse plane is horizontal, but for embryos and quadrupeds a coronal plane is horizontal and a transverse plane is vertical.
When describing anatomical motion, these planes describe the axis along which an action is performed. So by moving through the transverse plane, movement travels from head to toe. For example, if a person jumped directly up and then down, their body would be moving through the transverse plane in the coronal and sagittal planes.
Some of these terms come from Latin. Sagittal means "like an arrow", a reference to the position of the spine that naturally divides the body into right and left equal halves, the exact meaning of the term "midsagittal", or to the shape of the sagittal suture, which defines the sagittal plane and is shaped like an arrow.
A longitudinal plane is any plane perpendicular to the transverse plane. The coronal plane and the sagittal plane
Sagittal plane
Sagittal plane is a vertical plane which passes from front to rear dividing the body into right and left sections.-Variations:Examples include:...
are examples of longitudinal planes.
Usage in human anatomy
Sometimes the orientation of certain planes needs to be distinguished, for instance in medical imagingMedical imaging
Medical imaging is the technique and process used to create images of the human body for clinical purposes or medical science...
techniques such as sonography
Medical ultrasonography
Diagnostic sonography is an ultrasound-based diagnostic imaging technique used for visualizing subcutaneous body structures including tendons, muscles, joints, vessels and internal organs for possible pathology or lesions...
, CT scans, MRI scans
Magnetic resonance imaging
Magnetic resonance imaging , nuclear magnetic resonance imaging , or magnetic resonance tomography is a medical imaging technique used in radiology to visualize detailed internal structures...
, or PET scans
Positron emission tomography
Positron emission tomography is nuclear medicine imaging technique that produces a three-dimensional image or picture of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide , which is introduced into the body on a...
. One imagines a human in the anatomical position, and an X-Y-Z coordinate system
Cartesian coordinate system
A Cartesian coordinate system specifies each point uniquely in a plane by a pair of numerical coordinates, which are the signed distances from the point to two fixed perpendicular directed lines, measured in the same unit of length...
with the Z-axis going from front to back, the X-axis going from left to right, and the Y-axis going from up to down. The Z-axis axis is always forward (Tait-Bryan angles) and the right-hand rule
Right-hand rule
In mathematics and physics, the right-hand rule is a common mnemonic for understanding notation conventions for vectors in 3 dimensions. It was invented for use in electromagnetism by British physicist John Ambrose Fleming in the late 19th century....
applies.
- A transverse (also known as axial or horizontal) plane is an X-Z plane, parallel to the ground, which (in humans) separates the superior from the inferior, or put another way, the head from the feet.
- A coronal (also known as frontal) plane is a Y-X plane, perpendicularPerpendicularIn geometry, two lines or planes are considered perpendicular to each other if they form congruent adjacent angles . The term may be used as a noun or adjective...
to the ground, which (in humans) separates the anterior from the posterior, the front from the back, the ventral from the dorsal. - A sagittal (also known as lateral) plane is an Y-Z plane, perpendicular to the ground, which separates left from right. The midsagittal plane is the specific sagittal plane that is exactly in the middle of the body.
The axes and the sagittal plane are the same for bipeds and quadrupeds, but the orientation of the coronal and transverse planes switch. The axes on particular pieces of equipment may or may not correspond to axes of the body, especially since the body and the equipment may be in different relative orientations.
Occasionally, in medicine, abdominal
Abdomen
In vertebrates such as mammals the abdomen constitutes the part of the body between the thorax and pelvis. The region enclosed by the abdomen is termed the abdominal cavity...
organs may be described with reference to the trans-pyloric plane, which is a transverse plane passing through the pylorus
Pylorus
The pylorus is the region of the stomach that connects to the duodenum . It is divided into two parts:* the pyloric antrum, which connects to the body of the stomach.* the pyloric canal, which connects to the duodenum....
.
Anatomical planes in animal brains
In discussing the neuroanatomyNeuroanatomy
Neuroanatomy is the study of the anatomy and organization of the nervous system. In contrast to animals with radial symmetry, whose nervous system consists of a distributed network of cells, animals with bilateral symmetry have segregated, defined nervous systems, and thus we can begin to speak of...
of animals, particularly rodent
Rodent
Rodentia is an order of mammals also known as rodents, characterised by two continuously growing incisors in the upper and lower jaws which must be kept short by gnawing....
s used in neuroscience
Neuroscience
Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. However, it is currently an interdisciplinary science that collaborates with other fields such as chemistry, computer science, engineering, linguistics, mathematics,...
research, a simplistic convention has been to name the sections of the brain according to the homologous human sections. Hence, what is technically a transverse (orthogonal) section with respect to the body length axis of a rat (dividing anterior from posterior) may often be referred to in rat neuroanatomical coordinates as a coronal section, and likewise a coronal section with respect to the body (i.e. dividing ventral from dorsal) in a rat brain is referred to as transverse. This preserves the comparison with the human brain, whose length axis in rough approximation is rotated with respect to the body axis by 90 degrees in the ventral direction. It implies that the planes of the brain are not necessarily the same as those of the body.
However, the situation is more complex, since comparative embryology shows that the length axis of the neural tube (the primordium of the brain) has three internal bending points, namely two ventral bendings at the cervical and cephalic flexures (cervical flexure roughly between the medulla oblongata and the spinal cord, and cephalic flexure between the diencephalon and the midbrain), and a dorsal (pontine or rhombic) flexure at the midst of the hindbrain, behind the cerebellum. The latter flexure mainly appears in mammals and sauropsids (reptiles and birds), whereas the other two, and principally the cephalic flexure, appear in all vertebrates (the sum of the cervical and cephalic ventral flexures is the cause of the 90 degree angle mentioned above in humans between body axis and brain axis). This more realistic concept of the longitudinal structure of vertebrate brains implies that any section plane, except the sagittal plane, will intersect variably different parts of the same brain as the section series proceeds across it (relativity of actual sections with regard to topological morphological status in the ideal unbent neural tube). Any precise description of a brain section plane therefore has to make reference to the anteroposterior part of the brain to which the description refers (e.g., transverse to the midbrain, or horizontal to the diencephalon). A necessary note of caution is that modern embryologic orthodoxy indicates that the brain's true length axis finishes rostrally somewhere in the hypothalamus where basal and alar zones interconnect from left to right across the median line; therefore, the axis does not enter the telencephalic area, although various authors, both recent and classic, have assumed a telencephalic end of the axis. The causal argument for this lies in the end of the axial mesoderm -mainly the notochord, but also the prechordal plate- under the hypothalamus. Early inductive effects of the axial mesoderm upon the overlying neural ectoderm is the mechanism that establishes the length dimension upon the brain primordium, jointly with establishing what is ventral in the brain (close to the axial mesoderm) in contrast with what is dorsal (distant from the axial mesoderm). Apart of the lack of a causal argument for introducing the axis in the telencephalon, there is the obvious difficulty that there is a pair of telencephalic vesicles, so that a bifid axis is actually implied in these outdated versions.
Surface and other landmarks in humans
In humans, reference may take origin from superficial anatomySuperficial anatomy
Superficial anatomy is a descriptive science dealing with anatomical features that can be studied by sight, without dissecting an organism...
, made to landmarks that are on the skin or visible underneath. As with planes, lines and points are imaginary. Examples include:
- The midaxillary line, a line running vertically down the surface of the body passing through the apex of the axilla (armpit). Parallel are the anterior axillary line, which passes through the anterior axillary skinfold, and the posterior axillary line, which passes through the posterior axillary skinfold.
- The mid-clavicular line, a line running vertically down the surface of the body passing through the midpoint of the clavicleClavicleIn human anatomy, the clavicle or collar bone is a long bone of short length that serves as a strut between the scapula and the sternum. It is the only long bone in body that lies horizontally...
. - The mid-pupillary line, a line running vertically down the face through the midpoint of the pupilPupilThe pupil is a hole located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils. In...
when looking directly forward. - The mid-inguinal point, a point midway between the anterior superior iliac spineIlium (bone)The ilium is the uppermost and largest bone of the pelvis, and appears in most vertebrates including mammals and birds, but not bony fish. All reptiles have an ilium except snakes, although some snake species have a tiny bone which is considered to be an ilium.The name comes from the Latin ,...
and the pubic symphysisPubic symphysisThe pubic symphysis or symphysis pubis is the midline cartilaginous joint uniting the superior rami of the left and right pubic bones. It is located anterior to the urinary bladder and superior to the external genitalia; for females it is above the vulva and for males it is above the penis...
.- mid-point of inguinal ligament = mid-point between anterior superior iliac spine and pubic tubercle
- Tuffier's line, which is a transverse line passing across the lumbar spine between the posterior iliac crestsIlium (bone)The ilium is the uppermost and largest bone of the pelvis, and appears in most vertebrates including mammals and birds, but not bony fish. All reptiles have an ilium except snakes, although some snake species have a tiny bone which is considered to be an ilium.The name comes from the Latin ,...
. - Mid-ventral line, the intersection between the ventral skin and the median plane.
In addition, reference may be made to structures at specific levels of the spine
Vertebral column
In human anatomy, the vertebral column is a column usually consisting of 24 articulating vertebrae, and 9 fused vertebrae in the sacrum and the coccyx. It is situated in the dorsal aspect of the torso, separated by intervertebral discs...
(e.g. the 4th cervical vertebra, abbreviated "C4"), or the rib cage (e.g., the 5th intercostal space
Intercostal space
The intercostal space is the space between two ribs . Since there are 12 ribs on each side, there are 11 intercostal spaces, each numbered for the rib superior to it.-Structures in intercostal space:* several kinds of intercostal muscle...
).
See also
- Anatomical terms of motionAnatomical terms of motionThe movement of body structures is accomplished by the contraction of muscles. Muscles may move parts of the skeleton relatively to each other, or may move parts of internal organs relatively to each other. All such movements are classified by the directions in which the affected structures are moved...
- Commonly used terms of relationship and comparison in dentistryCommonly used terms of relationship and comparison in dentistryThere are numerous commonly used terms of relationship and comparison that refer to different aspects of teeth and are frequently utilized in articles about dentistry...
- Human positions