Anatomical terms of location

Overview
Anatomical terms of location are descriptive terms to help identify relative positions or directions within a species. While these terms are standardized within specific fields of biology, they can differ dramatically from one discipline to another.

The problem in consistency of naming occurs when ambiguous terms like "top" are used, which would be considered the head of a human, whereas for a flounder, the "top" would be the left or right side. In human anatomy, all naming is based on positions relative to the body in a standing (standard anatomical) position with arms at the side and thumbs out. In veterinary anatomy, many terms are given relative to the parts of the body, often in relation to the spine, which allows for consistency in terminology between vertebrate species that naturally assume a variety of positions. While the ears would be superior to (above) the shoulders in a human, this terminology fails when describing the armadillo, where the shoulders are above the ears. In veterinary terminology, the ears would be cranial to (towards the head) the shoulders in the armadillo, the dog, the kangaroo, or any other vertebrate. 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 to (towards the abdomen) in all vertebrates. While the universal vertebrate terminology used in veterinary medicine would work in human medicine, the human positional terms are too well established to change.

Purpose
In the sciences dealing with the anatomy of animals, precise anatomical terms of location are necessary for a variety of reasons.

Two major problems arise with common usage, however. First, they tend to be language-specific, requiring translation into equivalent, or almost-equivalent, terms in other languages. They are not universal terms that may be readily understood by zoologists speaking other languages. 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 second, and larger, problem is caused by the very nature of animals. Most animals are capable of moving relative to their environment. So while "up" might refer to the top of someone's head when they are standing upright, the same term ("up") would describe their belly while they are lying down.

Therefore, 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, tentacles, etc...) can change position with respect to the main body, it is important that any positional descriptive terms refer to the organism 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. 8, 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 (arms and legs) are placed in unnatural positions reminiscent of the supine position imposed on cadavers 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 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 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.

Medical (human) 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) &mdash; 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.

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 Bilateria. However, for historical and other reasons, standard human directional terminology has several differences from that used for other bilaterally-symmetrical organisms.

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.

Dorsal and ventral
The next most obvious end-points are the back and belly. These are termed the dorsal end (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.

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. (Note that these fins are not aligned anteroposteriorly, either &mdash; 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 geometric 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 vernacular "left-right" is preferentially used in English and some other languages. This is likely due to the adoption of the Latin "sinister" 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 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 "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 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".

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 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 within the abdominal cavity, and is covered with a lining called the visceral peritoneum.
 * 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 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

Why zootomy and androtomy terms differ ?
Although it can be argued that the standard directional nomenclature used for vertebrate zootomy can and should be used for medical anatomy, the differences persist. The differences in terminology arose (and are perpetuated) for three primary reasons:


 * Early human anatomical studies (being within the realm of medicine) were historically conducted separately from, and without reference to, those being done by zootomists.
 * Early zoological and human anatomical studies occurred before modern understanding of the process of biological evolution, and humans were widely viewed as "different" from (and "superior to") all other animals, and thus meriting their own terminology.
 * Unlike most tetrapod vertebrates, humans are not quadrupedal (walking on four legs), but rather are secondarily bipedal (walking on two legs). Human bipedalism causes shifts in the angle of the appendages (arms and legs) and head, with respect to the main body. Thus, it can be (and is) argued that separate terminology is necessary to adequately describe the unique bipedal stance of humans.

Unfortunately, the persistence of medical terminology as distinct from that used for other vertebrates tends to be confusing. For a quick comparison of equivalent terminology used in vertebrate and human anatomy.

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 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
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 quadruped 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 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.

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 palm 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 mouth 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.

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 urethra 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 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 (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.

General usage
Three basic reference planes are used in zoological anatomy.
 * A sagittal plane is 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 midline &mdash; i.e. it would pass through midline structures such as the navel or spine, 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, also known as an axial plane or cross-section, divides the body into cranial and caudal (head and tail) portions.

For post-embryonic 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 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 plane and the sagittal plane are examples of longitudinal planes.

Usage in human anatomy
Sometimes the orientation of certain planes needs to be distinguished, for instance in medical imaging 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 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 angles) and the right-hand rule 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 an Y-Z plane, perpendicular 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, 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 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 (ie. 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.
 * 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, a point midway between the anterior superior iliac spine and the pubic symphysis.
 * 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.
 * 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 spine (e.g. the 4th cervical vertebra, abbreviated "C4"), or the rib cage (e.g. the 5th intercostal space, abbreviated "5ICS").

Additional Resources

 * Atkins, B.T., Duval, A., Lewis, H.M.A., and Milne, R.C. (1993) Collins-Robert French-English, English-French Dictionary, 2nd ed. HarperCollins, Glasgow UK and Le Robert, Paris, France. ISBN 0-06-275513-7.
 * Barber, K., ed. (1998) The Canadian Oxford English Dictionary. Oxford University Press, Don Mills, ON, Canada. ISBN 0-19-541120-X.
 * Campbell, N.A. and Reece, J.B. (2005) Biology, 7th ed. Pearson, San Francisco, CA, USA. ISBN 0-8053-7166-4.
 * Hickman, C.P., Jr., Roberts, L.S. and Larson, A. (2003) Animal Diversity, 3rd ed. McGraw-Hill, New York, NY, USA. ISBN 0-07-234903-4.
 * Houseman, J. (2003) Digital Zoology, v. 2.0. McGraw-Hill, New York, NY, USA. ISBN 0-07-256481-4.
 * Kardong, K. (2005) Vertebrates: Comparative Anatomy, Function, Evolution, 4th ed. McGraw-Hill, New York, NY, USA. ISBN 0-07-290956-0.
 * Marieb, E.N. (1995) Human Anatomy and Physiology, 3rd ed. Benjamin/Cummings, Redwood City, CA, USA. ISBN 0-8053-4281-8.
 * Miller, S.A. (2002) General Zoology Laboratory Manual, 5th ed. McGraw-Hill, New York, NY, USA. ISBN 0-07-250837-0 (Mix from ISBN 0-07-243559-3).
 * Ruppert, E.E., Fox, R.S. and Barnes, R.D. (2004) Invertebrate Zoology: A Functional Evolutionary Approach, 7th ed. Thomson, Belmont, CA, USA. ISBN 0-03-025982-7.
 * Tortora, G.J. and Derrickson, B. (2006) Principles of Anatomy and Physiology, 11th ed. Wiley, Hoboken, NJ, USA. ISBN 0-471-68934-3.
 * Wischnitzer, S. (1993) Atlas and Dissection Guide for Comparative Anatomy, 5th ed. W.H. Freeman, New York. ISBN 0-71-672374-3.