Standard
anatomical terms of location are employed in science which deal with the
anatomyAnatomy 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
animalAnimals are a major group of mostly 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...
s to avoid ambiguities which 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.
Unfortunately, while these terms are standardized within specific fields of
biologyBiology is the natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy...
, 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)
anatomyHuman anatomy, which, with physiology and biochemistry, is a complementary basic medical science is primarily the scientific study of the morphology of the adult human body. Anatomy is subdivided into gross anatomy and microscopic anatomy...
(sometimes called
androtomyAndrotomy 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
CraniataCraniata is a proposed clade of chordate animals that contains the Myxini , Petromyzontida , and Gnathostomata as living representatives. Craniata includes all animals with a skull, or cranium, as the name suggests.Craniata as an unranked taxon replaces the former use of Vertebrata...
(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
headIn anatomy, the head of an animal is the rostral part that usually comprises the brain, eyes, ears, nose, and mouth...
of a
humanHumans are bipedal primates belonging to the species Homo sapiens in Hominidae, the great ape family. They are the only surviving member of the genus Homo. Humans have a highly developed brain, capable of abstract reasoning, language, introspection, and problem solving...
but the left or right side of a
flounderFlounder is an ocean-dwelling flatfish species that is located off the Canadian and U.S. east coast of the Northern Atlantic, and the Pacific Ocean, in coastal lagoons and estuaries.-Taxonomy:...
. Most animals, furthermore, are capable of moving relative to their environment (see Fig. 1). So while "up" might refer to the direction of a standing human's head, the same term ("up") might be thought to point the direction to the belly for a
supineThe 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...
human (at least, a sufficiently
stoutObesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health, leading to reduced life expectancy and/or increased health problems...
one). It is also necessary to employ some specific anatomical knowledge in order to apply the terminology unambiguously:
E.g. while the ears would be
superior to (above) the shoulders in a human, this fails when describing the
armadilloArmadillos are small placental mammals, known for having a leathery armor shell. The Dasypodidae are the only surviving family in the order Cingulata, part of the superorder Xenarthra along with the anteaters and sloths...
, where the shoulders are above the ears. Thus in veterinary terminology, the ears would be
cranial to (
i.e. "towards the head from") the shoulders in the armadillo, the
dogThe 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 domestic dog has been one of the most widely kept working and companion animals in human history...
, the
kangarooA 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 genus Macropus, Red Kangaroo, Antilopine Kangaroo, Eastern and Western Grey Kangaroo...
, or any other vertebrate, including the human. Similarly, 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 (although it could work for the kangaroo). In veterinary terms, the belly would be
ventral ("towards 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 forwards (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 changeThe 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.The finding has been observed in many fields, including political science and economics....
.
For
invertebrateAn invertebrate is an animal without a vertebral column. The group includes 95% of all animal species — all animals except those in the Chordate subphylum Vertebrata ....
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.
Standard anatomical position
Because animals can change orientation with respect to their environment, and because any appendages (arms, legs,
tentacleTentacles can refer to the elongated flexible organs that are present in some animals, especially invertebrates, and sometimes to the hairs of the leaves of some insectivorous plants. Usually, they are used for feeding, feeling and grasping...
s,
etc.) can change position with respect to the main body, it is important that any positional descriptive terms refer to the
organismIn biology, an organism is any living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole...
when it is in its
standard anatomical position.
Thus, and very importantly,
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 zootomy, 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 positionThe 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...
imposed on cadavers during autopsy. Therefore, the body has its feet together (or slightly separated), and its arms are rotated outward so that the
palmsThe hands are the two intricate, prehensile, multi-fingered body parts normally located at the end of each arm of a primate. They are the chief organs for physically manipulating the environment, used for both gross motor skills and fine motor skills...
are forward, and the thumbs 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.
Skull
In humans, the anatomical position of the
skullThe skull is a bony structure found in the head of many animals. The skull supports the structures of the face and protects the head against injury....
has been agreed by international convention to be the
Frankfurt planeThe 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
orbitsIn anatomy, the orbital bone is the cavity or socket of the skull in which the eye and its appendages are situated.It can also mean the skin which surrounds the eye of a bird....
, the orbitales, and the upper margins of the
ear canalThe 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 26 mm in length and 7 mm in diameter.- Structure:...
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 vertebrates (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 (
invertebrateAn invertebrate is an animal without a vertebral column. The group includes 95% of all animal species — all animals except those in the Chordate subphylum Vertebrata ....
) organisms as well.
Vertebrate directional terms
To begin, distinct, polar-opposite ends of the
organismIn biology, an organism is any living system . In at least some form, all organisms are capable of response to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole...
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.
Anterior and posterior
The most obvious end-points are the "nose" and "tail" (see Fig. 2). Anatomically, 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 (
GreekGreek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical...
kephalē; head). For reasons of broader applicability, especially in organisms without distinct heads (many invertebrates), "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 which strictly applies only to vertebrates, 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.
Table 1: Defined Axes in VertebrateVertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony...
ZoologyZoology, also spelled zoölogy, is the branch of biology that focuses on the structure, function, behavior, and evolution of animals. The zoologist's pronunciation of "zoology" is , though a common spelling pronunciation is .-Systems of classification:...
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 An appendage in the broadest sense is an additional or subsidiary part existing on, or added to, something which can generally still function if the appendage has never existed or is later provided or grown, or will still perform a primary function if the appendage is removed.- Biological context...
(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.) |
Dorsal and ventral
The next most obvious end-points are the back and belly. These are termed the
dorsal endIn 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;
abdomenIn 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 finA dorsal fin is a fin located on the backs of some fish, whales, dolphins, and porpoises, as well as the ichthyosaurs. Depending on the species, an animal can have up to three of them. Its main purpose is to stabilize the animal against rolling and assist in sudden turns...
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 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 DV axes. Obviously, 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. Properly, this is called the
dextro-sinistral (or, more uncommonly, the
sinistro-dextral)
axis, from the Latin
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.
In practice, and contradictory to the practice with other anatomical terms of location, the
vernacularVernacular is the native language of a country or a locality. In general linguistics, it is used to describe local languages as opposed to lingua francas, official standards or global languages. It is sometimes applied to nonstandard dialects of a global language...
"left-right" is preferentially used in English and some other languages. This is likely due to the adoption of the Latin "
sinisterEvil, in many cultures, is a broad term used to describe what are seen as subjectively harmful deeds that are labeled as such to steer moral support. Evil is usually contrasted with good, which describes acts that are subjectively beneficial to the observer. In some religions, evil is an active...
" to mean "evil" in English and other languages (
e.g. sinistre in French has the same connotation).
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 finA dorsal fin is a fin located on the backs of some fish, whales, dolphins, and porpoises, as well as the ichthyosaurs. Depending on the species, an animal can have up to three of them. Its main purpose is to stabilize the animal against rolling and assist in sudden turns...
is
right of the left pectoral fin, but is
left of the right
eyeEyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system...
. 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 "towards 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. Properly, the ML axis is a half axis; practically, 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
operculumThe 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. The operculum is composed of four fused bones; the opercle, preopercle, interopercle, and subopercle...
, but
lateral to the
heartThe heart is a muscular organ found in all vertebrates 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
Together, the AP, DV and LR (or ML) axes allow for precise three-dimensional descriptions of location within any bilaterally-symmetrical organism, whether
vertebrateVertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony...
or
invertebrateAn invertebrate is an animal without a vertebral column. The group includes 95% of all animal species — all animals except those in the Chordate subphylum Vertebrata ....
. 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. Alternatively, 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 generally 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.
Proximal and distal
The term
proximal (Latin
proximus; nearest) is used to describe 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 appendages 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
appendages. "Appendages" would include
vertebrateVertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony...
finsFINS is a network protocol used by Omron PLCs, over different physical networks like Ethernet, Controller Link, DeviceNet and RS-232C....
(see Fig. 2) and
limbsA 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 (pinnae) and
hairHair is a protein filament that grows through the epidermis from follicles deep within the dermis. The fine, soft hair found on many nonhuman mammals is typically called fur; wool is the characteristically curly hair found on sheep and goats. Found exclusively in mammals, hair is one of the...
(in mammals), feathers (in birds) and
scalesIn 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...
(
fishA fish is any aquatic vertebrate animal that is typically ectothermic , covered with scales, and equipped with two sets of paired fins and several unpaired fins...
, reptiles and birds). As well, varieties of tentacles or other projections from the body in invertebrates 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. (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
tetrapodTetrapods are vertebrate animals having four feet, legs or leglike appendages. Amphibians, reptiles, dinosaurs/birds, and mammals are all tetrapods, and even the limbless snakes are tetrapods by descent...
vertebrates, 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).
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 (Latin ipse; self/same): on the same side as another structure. Thus, the left arm is ipsilateral to the left leg.
- Contralateral (Latin contra; against): on the opposite from 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 navel
The navel is a scar on the abdomen, caused when the umbilical cord is removed from a newborn baby. All placental mammals have a navel...
is intermediate to (or intermediate between) the left arm and the contralateral (right) leg.
- Visceral (Latin viscus; internal organs, flesh): associated with organs within the body's cavities. The stomach is a viscus
In anatomy, a viscus is an internal organ of an animal , in particular an internal organ of the thorax or abdomen. 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 peritoneumThe 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 higher vertebrates and some invertebrates...
.
- Parietal (Latin paries "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 bone
The parietal bones are bones in the human skull and form, by their union, the sides and roof of the cranium. Each bone is irregularly quadrilateral in form, and has two surfaces, four borders, and four angles.-External:The external surface [Fig...
of the skull or associated structures.)
- Axial (Latin axis from Greek axōn "axle"): Towards the central axis of the organism or an extremity.
- Abaxial : away from the central axis of the organism or extremity
Invertebrate directional terms
The large variety of body shapes 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.
Asymmetrical and spherical organisms
In organisms with a changeable shape, such as
amoeboidAmoeboids are single-celled life-forms characterized by an irregular shape."Amoeboid" and "amoeba" are often used interchangeably even by biologists, and especially refers to a creature moving by using pseudopods. 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 slideA microscope slide was originally a 'slider' made of ivory or bone, containing specimens held between disks of translucent mica. These were popular in Victorian England until the Royal Microscopical Society introduced the standardized microscope slide in the form of a thin sheet of glass used to...
s, where they appear essentially two-dimensional. In some cases a third axis can be defined, particularly where a non-terminal
cytostomeA 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
Some elongated protists have distinctive ends of the body. In such organisms, the end with a mouth (or equivalent structure, such as the
cytostomeA 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
ParameciumParamecia, also known as Lady Slippers, due to their appearance, are a group of unicellular ciliate protozoa, which are commonly studied as a representative of the ciliate group, and range from about 50 to 350 μm in length. Simple cilia cover the body, which allow the cell to move with a...
or
StentorIn Greek mythology, Stentor was a herald of the Greek forces during the Trojan War. His name has given rise to the adjective "stentorian", meaning loud-voiced, for which he was famous. Homer said his "voice was as powerful as hundred voices of other men". He died after his defeat by Hermes in a...
), or the end that usually points in the direction of the organism's
locomotionAnimal locomotion, which is the act of self-propulsion by an animal, has many manifestations, including running, 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
flagellumA flagellum is a tail-like structure that projects from the cell body of certain prokaryotic and eukaryotic cells, and functions in locomotion. There are some notable differences between prokaryotic and eukaryotic flagella, such as protein composition, structure, and mechanism of propulsion...
in
EuglenaEuglena is a genus of unicellular protists, of the class Euglenoidea of the phylum Euglenozoa . They are single-celled organisms. Currently, over 1000 species of Euglena have been described. Marin et al. revised the genus to include several species without chloroplasts, formerly classified as...
), 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 only apply 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
sessileIn 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 rock, or the hull of a ship in the case of barnacles. Corals lay down their own substrate....
(normally attached to a surface, as in Fig. 7, bottom, and Fig. 8).
Organisms that are attached to a
substrateIn biology a substrate is the surface a plant or animal lives upon. The substrate can include biotic or abiotic materials. For example, encrusting algae that lives on a rock can be substrate for another animal that lives above the algae on the rock. See also substrate .-External links:*...
, 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.
Radially-symmetrical organisms
Radially symmetrical organisms include those in the group
RadiataThe Radiata are the radially symmetric animals of the Eumetazoa subregnum. The term Radiata has had various meanings in the history of classification...
— primarily
jellyfish, sea anemones and coralsCnidaria is a phylum containing over 10,000 species of animals found exclusively in aquatic, mostly marine, environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey...
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
symmetrySymmetry 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
larvaA larva is a young form of animal with indirect development, going through or undergoing metamorphosis ....
e are
not included, since they are bilaterally symmetrical.
Unlike spherical and asymmetrical organisms, radially-symmetrical animals always have one distinctive axis.
CnidariaCnidaria is a phylum containing over 10,000 species of animals found exclusively in aquatic, mostly marine, environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey...
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, appendages that move independently of the body (tentacles in
cnidariaCnidaria is a phylum containing over 10,000 species of animals found exclusively in aquatic, mostly marine, environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey...
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-symmetricalThe 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. The bilateralia are a subregnum of animals, including the majority...
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
BilateralThe 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. The bilateralia are a subregnum of animals, including the majority...
Bauplans |
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 substrateIn biology a substrate is the surface a plant or animal lives upon. The substrate can include biotic or abiotic materials. For example, encrusting algae that lives on a rock can be substrate for another animal that lives above the algae on the rock. See also substrate .-External links:*...
.
(4) Vertebrate equivalents are meaningless in radial animals.
(5) Roughly equivalent to "superficial".
(6) Roughly equivalent to "deep". |
Medical (human) directional terms
As we are bilaterally-symmetrical organisms, anatomical directions in humans can correctly be described using the same terms as those for vertebrates and other members of the taxonomic group
BilateriaThe 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. The bilateralia are a subregnum of animals, including the majority...
. 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 adrotomy came into usage at a time where all scientific communication was done 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
towards the spine and
towards 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; conversely, the dorsal/ventral axis is inconsistent between humans and other vertebrates in head anatomy but consistent in torso 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 very many anatomical structures.
For a quick comparison of equivalent terminology used in vertebrate and human anatomy, see Table 3 (below).
Table 3: Equivalent Directional Terms used in
VertebrateVertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony...
ZoologyZoology, also spelled zoölogy, is the branch of biology that focuses on the structure, function, behavior, and evolution of animals. The zoologist's pronunciation of "zoology" is , though a common spelling pronunciation is .-Systems of classification:...
and Human AnatomyAnatomy 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...
VertebrateVertebrates are members of the subphylum Vertebrata, chordates with backbones or spinal columns. About 58,000 species of vertebrates have been described. Vertebrata is the largest subphylum of chordates, and contains many familiar groups of large land animals. Vertebrates comprise cyclostomes, bony...
zootomy |
Human torso |
Human head |
| Direction |
Synonyms |
Direction |
Synonyms |
Direction |
Synonyms |
| Anterior |
Rostral, Cranial, Cephalic1 |
Superior |
Same1 |
Anterior |
Rostral |
| Posterior |
Caudal |
Inferior |
Caudal1 |
Posterior |
Caudal |
| Dorsal |
— |
Posterior |
Dorsal |
Superior |
Dorsal |
| Ventral |
— |
Anterior |
Ventral |
Inferior |
Ventral |
| Left (lateral) |
Sinister1 |
Same |
— |
Same |
— |
| Right (lateral) |
Dexter1 |
Same |
— |
Same |
— |
| Medial |
— |
Same |
— |
Same |
— |
| Proximal |
— |
Same |
— |
Same |
— |
| Distal |
— |
Same |
— |
Same |
— |
| Intermediate2 |
— |
Same |
— |
Same |
— |
| Ipsilateral2 |
— |
Same |
— |
Same |
— |
| Contralateral2 |
— |
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 (Latin
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 only be used with respect to the head and 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
shouldersShoulders 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
navelThe navel is a scar on the abdomen, caused when the umbilical cord is removed from a newborn baby. All placental mammals have a 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
Anterior and
posterior, as used in medical/human anatomical descriptions are major sources of confusion to those accustomed to standard vertebrate directional terminology, and
vice versa. The confusion arises from the differences in
standard anatomical positions of
quadrupedQuadrupedalism is a form of land animal locomotion using four limbs or legs. An animal or machine that usually moves in a quadrupedal manner is known as a quadruped, meaning "four feet"...
vertebrates and bipedal humans.
In human anatomical usage,
anterior refers to the "front" of the individual, and is
synonymous with ventral. Similarly, posterior, in medical anatomy refers to the "back" of the subject, and is synonymous with dorsal (see Table 3). The terms "dorsal" and "ventral" are used in human anatomy, but infrequently when referring to the body as a whole. Thus, 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 eyes are
posterior to the
noseAnatomically, a nose is a protuberance in vertebrates that houses the nostrils, or nares, which admit and expel air for respiration in conjunction with the mouth....
, but
anterior to the back of the head in humans.
However, in the
horseThe horse is a hoofed mammal, a subspecies of one of seven extant species of the 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
In standard anatomical position, the palms of the hands point anteriorly. Thus,
anterior can be (and sometimes is) used to describe the
palmThe hands are the two intricate, prehensile, multi-fingered body parts normally located at the end of each arm of a primate. They are the chief organs for physically manipulating the environment, used for both gross motor skills and fine motor skills...
of the hand, and
posterior can be (and sometimes is) 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.
Relative directions
Also, in common usage, the segments of the digestive system closest to the
mouthThe mouth, buccal cavity, or oral cavity is the first portion of the alimentary canal that receives food and begins digestion by mechanically breaking up the solid food particles into smaller pieces and mixing them with saliva...
are termed
proximal, as opposed to those closest to the
anusThe anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to expel 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, such as bones; food...
, 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
urethraIn anatomy, the urethra is a tube which connects the urinary bladder to the outside of the body. The urethra has an excretory function in both sexes to pass urine to the outside, and also a reproductive function in the male, as a passage for semen.The external urethral sphincter is a striated...
is attached to the
bladderIn anatomy, the urinary bladder is the organ that collects urine excreted by the kidneys prior to 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
wristIn human anatomy, the wrist is variously defined as 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
ankleIn human anatomy, 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 towards the head are
cranial; those facing away or further 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
dogThe 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 domestic dog has been one of the most widely kept working and companion animals in human history...
's
pawA 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
radiusThe radius is the bone of the forearm that extends from the lateral side of the elbow to the thumb side of the wrist. The radius is situated on the lateral side of 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
ulnaThe ulna is a long bone, prismatic in form. In anatomical position the ulna is placed at the medial side of the forearm closest to the body, parallel with the radius on both arms.-Articulations:The ulna articulates with:...
are
ulnar, and structures relating to both bones are referred to as
radioulnar. Similarly, in the lower leg, structures near the
tibiaThe 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.The tibia is named for the greek aulos flute, also known as a tibia.-In humans:...
(shinbone) are
tibial and structures near the
fibulaThe fibula or calf bone is a bone located on the lateral side of the tibia, with which it is connected above and below. It is the smaller of the two bones, and, in proportion to its length, the most slender of all the long bones...
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
varusIn 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
elbowThe 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 plane
A sagittal plane of the human body is an imaginary plane that travels vertically from the top to the bottom of the body along the Y axis, dividing it into left and right portions.-Variations:Examples include:...
, being a plane parallel to the sagittal suture, 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 navel
The navel is a scar on the abdomen, caused when the umbilical cord is removed from a newborn baby. All placental mammals have a navel...
or spineIn human anatomy, the vertebral column is a column usually consisting of 33 vertebrae, the sacrum, intervertebral discs, and the coccyx situated in the dorsal aspect of the torso, separated by spinal 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.
- A coronal or frontal plane divides the body into dorsal and ventral (back and front, or posterior and anterior) portions.
- A transverse plane
The 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-
embryoAn 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 planeA coronal plane is any vertical plane that divides the body into anterior and posterior sections....
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 which 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 planeA coronal plane is any vertical plane that divides the body into anterior and posterior sections....
and the
sagittal planeA sagittal plane of the human body is an imaginary plane that travels vertically from the top to the bottom of the body along the Y axis, dividing it into left and right portions.-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 refers to the techniques and processes used to create images of the human body for clinical purposes or medical science .As a discipline and in its widest sense, it is part of biological imaging and incorporates...
techniques such as
sonographyDiagnostic sonography is an ultrasound-based diagnostic imaging technique used to visualize subcutaneous body structures including tendons, muscles, joints, vessels and internal organs for possible pathology or lesions. Obstetric sonography is commonly used during pregnancy and is widely...
, CT scans,
MRI scansMagnetic Resonance Imaging , or nuclear magnetic resonance imaging , is primarily a medical imaging technique most commonly used in radiology to visualize the internal structure and function of the body...
, or
PET scansPositron emission tomography is a nuclear medicine imaging technique which 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 systemA 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 X-axis going from front to back, the Y-axis going from left to right, and the Z-axis going from up to down. The X-axis axis is always forward (
Tait-Bryan anglesYaw, pitch, and roll, also known as Tait–Bryan angles, named after Peter Guthrie Tait and George Bryan, are a specific kind of Euler angles very often used in aerospace applications to define the relative orientation of a vehicle...
) and the
right-hand ruleIn 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 Zachariah William Cole in the late 1800s....
applies.
- A transverse (also known as axial or horizontal) plane is an X-Y 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-Z plane, perpendicular
In 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. Thus, referring to Figure 1, the line AB is the perpendicular to CD through the point B...
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 median) plane is an X-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,
abdominalIn 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
pylorusThe pylorus is the region of the stomach that connects to the duodenum. It is divided in 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 is the study of the anatomy of nervous tissue and neural structures of the nervous system. In vertebrate animals, the routes that the myriad nerves take from the brain to the rest of the body , and the internal structure of the brain in particular, are both extremely elaborate...
of animals, particularly
rodentRodentia 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
neuroscienceNeuroscience is the scientific study of the nervous system. Such studies span the structure, function, evolutionary history, development, genetics, biochemistry, physiology, pharmacology, informatics, computational neuroscience and pathology of the nervous system.The International Brain Research...
research, the convention has been to name the sections of the brain according to the homologous human sections. Hence, what is technically a
transverse section with respect to the body 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 which is rotated with respect to the body axis by
90 degrees in the ventral direction. It does mean that the planes of the rat brain are not necessarily the same as those of the body.
Surface and other landmarks in humans
In humans, reference may be made to landmarks which are on the skin or visible underneath. As with planes, lines and points are imaginary. Examples include:
- The mid-axillary 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 clavicle
In human anatomy, the clavicle or collar bone is classified as a long bone that makes up part of the shoulder girdle . It receives its name from the Latin clavicula because the bone rotates along its axis like a key when the shoulder is abducted. This movement is palpable...
.
- The mid-pupillary line, a line running vertically down the face through the midpoint of the pupil
The pupil is an opening 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...
when looking directly forwards.
- The mid-inguinal point, a point midway between the anterior superior iliac spine
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 symphysisThe pubic symphysis 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. In males, the suspensory...
.
- 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 crests
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.
Additionally, reference may be made to structures at specific levels of the
spineIn human anatomy, the vertebral column is a column usually consisting of 33 vertebrae, the sacrum, intervertebral discs, and the coccyx situated in the dorsal aspect of the torso, separated by spinal discs...
(e.g. the 4th cervical vertebra, abbreviated "C4"), or the rib cage (e.g. the 5th
intercostal spaceThe intercostal space is the space between two ribs . Since there are 12 ribs on each side, there are 11 intercostal spaces.-Importance of intercostal space:...
).
See also
- Anatomical terms of motion
The 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 dentistry
There are numerous commonly used terms of relationship and comparison that refer to different aspects of teeth and are frequently utilized in articles about dentistry...
External links