In fields of anatomy, 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 palms facing forwards (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.
The second, and larger, problem is caused by the very nature of animals. Most animals are capable of moving relative to their environment (see Fig. 1). 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.
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.
The polar opposite to the anterior end is the posterior end (Latin post; after). Another term for posterior is caudal (Latin caudum; tail) — 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). In veterinary medicine, caudo-cranial is preferred between head and tail, and rostro-caudal between nose and neck. Less-used synonyms would be rostrocaudal or cephalocaudal axes (see Table 1). For brevity, the term anteroposterior is often abbreviated to read AP (or A-P) axis. As well as defining the anteroposterior axis, the terms "anterior" and "posterior" also define relative positions along the axis. Thus, in the fish in Fig. 2, the gill openings are posterior relative to the eyes, but anterior to the tail.
|Directional term||Defined Axis||Synonyms||Axis runs...|
|Anterior||Anteroposterior||Rostrocaudal1, Craniocaudal1, Cephalocaudal2||...from head end to opposite end of body or tail.|
|Dorsal||Dorsoventral||—||...from spinal column (back) to belly (front).|
|Left (lateral)||Left-right||Dextro-sinister2, Sinistro-dexter2||...from left to right sides of body.|
|Medial||Mediolateral3||—||...from centre of organism to one or other side.|
|Left or right (lateral)|
|Proximal||Proximodistal||—||...from tip of an appendage (distal) to where it joins the body (proximal).|
(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.)
As with anteroposterior, the terms "dorsal" and "ventral" are also used to describe relative positions along the dorsoventral axis. Thus, the pectoral fins are dorsal to the anal fin, but ventral to the dorsal fin in Fig. 2. (Note that these fins are not aligned anteroposteriorly, either — the dorsal fin being posterior to the pectoral, and anterior to the anal fins, respectively.)
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" (see below).
Together, the AP, DV and LR (or ML) axes allow for precise three-dimensional descriptions of location within any bilaterally-symmetrical organism, whether vertebrate or invertebrate. In practice, the terms can cause some confusion when, unlike the fish shown in Fig. 2, the organism in question is not strictly linear in form, which includes most tetrapods (see Figs. 3 and 4). For example, the AP axis in Fig. 3 does not appear to be at right angles to the DV axis. Rather, it is a depiction of the approximate average AP axis, when all body segments are included.
When considering any one segment, the dorsoventral axis is perpendicular to the AP axis. Thus, in Fig. 4, the DV axis of the tail would run from the "back" of the tail (posterior end of the trunk), to the "underside" of the tail (near the legs) — nearly parallel to the AP axis of the main body.
As a general rule of thumb, if the body is included in consideration, the AP axis of the main body would be used, as would the DV and ML axes perpendicular to it. However, if considering only one segment, the AP axis would shift to reflect the axes shown in Fig. 4, with the DV and ML axes shifting correspondingly. 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 (as well as bipeds) can be described uniformly.
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 vertebrate fins (see Fig. 2) and limbs (see Figs. 3 and 4), but properly apply to any structure that extends (and can at least potentially move separately) from the main body. Thus, "appendage" would also include such structures as external ears (pinnae) and hair (in mammals), feathers (in birds) and scales (fish, reptiles and birds). As well, varieties of tentacles or other projections from the body in 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 tetrapod 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).
Some elongated protists have distinctive ends of the body. In such organisms, the end with a mouth (or equivalent structure, such as the cytostome in Paramecium or Stentor), or the end that usually points in the direction of the organism's locomotion (such as the end opposite the flagellum in Euglena), is normally designated as the anterior end. The opposite end then becomes the posterior end, and by connecting them, an anteroposterior axis is formed. Properly, this terminology would 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 sessile (normally attached to a surface, as in Fig. 7, bottom, and Fig. 8).
Organisms that are attached to a substrate, such as sponges (Fig. 8), or some animal-like protists also have distinctive ends. The part of the organism attached to the substrate is usually referred to as the basal end (Latin basis; support or foundation), whereas the end furthest from the attachment is referred to as the apical end (Latin apex; peak, tip). Thus, by joining the two ends, an apical-basal (or basal-apical) axis is formed (see Fig. 8). Transverse axes may be defined indifferently in any direction perpendicular to this axis, as there is no symmetry present.
Unlike spherical and asymmetrical organisms, radially-symmetrical animals always have one distinctive axis.
Cnidarians 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 cnidarians and comb jellies), have a definite proximodistal axis (Fig. 8). Unlike vertebrates, cnidarians (jellyfish, sea anemones, corals) have no other distinctive axes, and multiple radial axes are possible (Fig. 10).
It is noteworthy that some "biradially-symmetrical" comb jellies have distinct "tentacular" and "pharyngeal" axes, and are thus anatomically equivalent to bilaterally-symmetrical animals. As well, adult echinoderms (starfish, sea urchins, sea cucumbers) are pentaradial, and have only five symmetrical radial axes (unlike the multiple axes in cnidarians).
Lateral, dorsal, and ventral have no meaning in such organisms, and all can be replaced by the generic term peripheral (Latin peri-; around; see Table 2). Medial can be used, but in the case of radiates indicates the central point of these organisms, rather than a central axis (as in vertebrates). Thus, as there are many possible radial axes, there are multiple medio-peripheral (half-) axes (Fig. 10).
|Bilateral Bauplans||Radial Bauplans|
|Anterior||Rostral, Cranial, Cephalic2||Oral||Apical3|
(1) Includes both Radiates and adult Echinoderms.
(2) Rarely used.
(3) Only in organisms attached to a substrate.
(4) Vertebrate equivalents are meaningless in radial animals.
(5) Roughly equivalent to "superficial".
(6) Roughly equivalent to "deep".
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, see Table 3 (below).
|Vertebrate zootomy||Human anatomy|
|Anterior||Rostral, Cranial, Cephalic1||Superior||Same1|
(1) Rarely used.
(2) Used only to describe one side of an appendage.
(3) Strictly relative term, used with other locational descriptors.
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.
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.
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.
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.
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.
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.
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