Encyclopedia
In human and zoological
anatomy , several terms are used to describe the location of organs and other structures in the body of
bilateral animals. These terms, generally derived from
Latin, are listed and explained here.
In some cases, the terminology in
human anatomy may differ from that in general anatomy, which also applies to other species .
Standard anatomical position
Where anatomical terms are to be used, it is appropriate that the subject be studied in the
standard anatomical position. This is typically a posture similar to that seen during life. For humans the body is considered as standing erect, feet together and toes pointed forward, arms at the sides and palms facing forward . In other species, e.g. quadrupeds, the standard anatomical position is described as standing erect with the head facing forwards in a neutral position.
In humans, the anatomical position of the
skull has been agreed by international convention to be the Frankfurt plane, a position where the lower margins of the orbits and the upper margins of the
ear canals all lie in the same horizontal plane. This is a good approximation to the position where the subject is standing upright and facing forwards.
Note that the standard anatomical position does not correspond to the position in which the subject is examined or studied. Humans are typically examined lying on the back; some quadrupeds are examined lying on the side.
Directions
General usage
Animals typically have one end with a head and mouth, with the opposite end often having the anus and tail. The head end is the
cranial end; the tail end is the
caudal end. Within the head itself,
rostral refers to the direction toward the end of the nose, and caudal is still used to refer to the tail direction.
The surface or side of the body normally oriented upwards, away from the pull of gravity,
is the
dorsal side; the opposite side, typically the one closest to the ground when walking on all legs, swimming or flying, is the
ventral side. For example: in
vertebrates, the spine or nerve cord is located on the
dorsal side of the organism. A dolphin's dorsal fin is, unsurprisingly, on the
dorsal side. A cow's udder is on the
ventral side.
Scales on the belly of a snake are called
ventral scales.
On the limbs or other appendages, a point closer to the main body is "
proximal"; a point farther away is "
distal".
The right and left side are always given as viewed from the animal that is described.
Usage in human anatomy
In humans, portions of the body which are closer to the head end are "
superior" ; those which are farther away are "
inferior" -- "superior" corresponds to
cranial , or cephalic , and "inferior" corresponds to
caudal . Objects near the front are "
anterior"; those near the rear are "
posterior" -- these correspond to the terms "
ventral", forward surface, and "dorsal" rear surface.
The terms "anterior" and "posterior" are confusing when referring to most animals, however, and are particularly unsuitable for
quadrupeds. In this case, "rostral/cranial" and "caudal" are more appropriate. However the word posterior is commonly used as a substantive meaning the buttocks or an adjective referring to them.
In humans,
anatomical terms of motion refer to changes away from the standard anatomical position, such as the position of the forearm with the palm directed anteriorly is known as the
supine position. Where the palm faces posteriorly , this is the
prone position. Turning the hand from prone to supine is called
supination; turning the hand from supine to prone is
pronation. Pronation results in crossing of the radius with respect to the
ulna. To avoid confusion with "medial" and "lateral", different terms are used when describing the sides of the forearm .
Relative directions
Structures near the midline are called
medial and those near the sides of animals are called
lateral. Therefore, medial structures are closer to the midsagittal plane, lateral structures are further from the midsagittal plane. Structures in the midline of the body are
median, or medial. For example, your cheeks are lateral to your nose and the tip of the nose is in the median line.
Ipsilateral means on the same side,
contralateral means on the other side and
bilateral means on both sides.
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
urethra is attached to the bladder.
Structures on or closer to the body's surface are
superficial and those further inside are
profound or
deep .
When speaking of inner organs,
visceral means attached to or associated with an organ, while
parietal refers to a structure associated with or attached to the body wall . For example, whilst the pleura is a single structure, for convenience the term "visceral pleura" is used to refer to that part attached to the outer surface of the lung, and "parietal pleura" to refer to that part attached to the inside of the chest wall.
Relative directions in the limbs
In the limbs of most animals, the terms
cranial and
caudal are used in the regions proximal to the carpus and the tarsus . 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
dog's paw is its
dorsal surface; the underside, either the
palmar or the
plantar 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 are
tibial and structures near the fibula are
fibular .
Volar, a term which refers to both the palm and the sole, is sometimes used as a synonym for "palmar".
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 or abnormal.
Planes
General usage
Three basic reference planes are used in zoological anatomy.
- A sagittal plane divides the body into left and right portions.
- The midsagittal plane is in the midline, i.e. it would pass through midline structures such as the navel or spine, and all other sagittal planes are parallel to it.
- A coronal plane divides the body into dorsal and ventral portions.
- A transverse plane divides the body into cranial and caudal portions.
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".
Usage in human anatomy
Sometimes the orientation of certain planes needs to be distinguished, for instance in medical imaging techniques such as
CT scans,
MRI scans or
PET scans. One imagines a human in the
anatomical position, and an X-Y-Z
coordinate system with the X-axis going left to right, the Y-axis passing up and down, and the Z-axis going front and back.
By convention, when displaying axial images, the image is positioned as if the viewer were standing at the feet of a reclining subject, looking towards the head.
- A transverse plane is an X-Z plane, parallel to the ground, which separates the superior from the inferior, or put another way, the head from the feet.
- A coronal plane is an X-Y plane, perpendicular to the ground, which separates the anterior from the posterior, the front from the back, the ventral from the dorsal.
- A sagittal plane is an Y-Z plane, perpendicular to the ground and to the coronal plane, which separates left from right. The midsagittal plane is the specific sagittal plane that is exactly in the middle of the body.
Occasionally, in medicine, abdominal organs may be described with reference to the
trans-pyloric plane which is a transverse plane passing through the pylorus.
Anatomical planes in animal brains
In discussing the neuroanatomy of animals, particularly rodents used in neuroscience research, the covention has been to name the sections of the brain according to the homologous human sections. Hence, what is technically a transverse section with respect the body of a rat may often be referred to in rat neuroanatomical coordinates as a coronal section and likewise a coronal section with respect to the body in a rat
brain is referred to as transverse. This preserves the comparison with the human brain which is rotatated 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 . Parallel are the anterior axillary line, which passes through the anterior axillary skinfold, and the posterior axillary line, which passes through the posterior axillary line.
- The mid-clavicular line, a line running vertically down the surface of the body passing through the midpoint of the clavicle
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- The mid-pupillary line, a line running vertically down the face through the midpoint of the pupil when looking directly forwards.
- The mid-inguinal point, which is the point midway between the anterior superior iliac spine and the pubic tubercle.
- Tuffier's line, which is a transverse line passing across the lumbar spine between the posterior iliac crests.
Additionally, reference may be made to structures at specific levels of the spine , or the rib cage .
Relative motions
Example - scale nomenclature in Snakes
The nomenclature of scales in snakes is very important as the scales are an important diagnostic at family and species level. The names of scales are intimately linked to the position on the body. An example is shown in the form of an annotated photograph of the head of the
Buff-striped Keelback , a common grass snake of South Asia. For the main article see
Snake scales.
See also
External links
