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Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans. Furthermore, the terms are not language-specific, so that with little, or ideally no, translation, they can be understood by all zoologists.
While these terms are standardized within specific fields of biology, there are unavoidable, sometimes dramatic, differences between some disciplines. For example, differences in terminology remain a problem that, to some extent, still separates the terminology of human anatomy from that used in the study of various other zoological categories.
The vertebrates and Craniata share a substantial heritage of common structure, allowing much of the same terminology to be used for all the species. To avoid ambiguities this terminology is based on the anatomy of each animal in a standard way. For example, stated positions are relative; the top of a human being will be seen as the head, whereas the top of a fish such as a flounder could refer to either its left or its right side. Most animals, furthermore, are capable of moving relative to their environment. So while "up" might refer to the direction of a standing human's head, the same term ("up") might be used to refer to the direction of the belly of a supine human. It is also necessary to employ some specific anatomical knowledge in order to apply the terminology unambiguously: For example, while the ears would be superior to (above) the shoulders in a human, this fails when describing the armadillo, where the shoulders are above the ears. Thus, in veterinary terminology, the ears would be cranial to (i.e., "toward the head from") the shoulders in the armadillo, the dog, the kangaroo, or any other terrestrial vertebrate, including the human. Likewise, while the belly is considered anterior to (in front of) the back in humans, this terminology fails for the flounder, the armadillo, and the dog. In veterinary terms, the belly would be ventral ("toward the abdomen") in all vertebrates.
In human anatomy, as explained below, all naming is based on positions relative to the body in a standing, standard anatomical position with arms at the side and palms facing forward (thumbs out). While the universal vertebrate terminology used in veterinary medicine would work in human medicine, the human terms are thought to be too well established to be worth changing.
For invertebrates, standard application of locational terminology often becomes difficult or debatable at best when the differences in morphology are so radical that common concepts are not homologous and do not refer to common concepts. For example, many species are not even bilaterally symmetrical. In these species, terminology depends on their type of symmetry (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, even though the meaning of some of the terms often is context-sensitive.
Because animals can change orientation with respect to their environment, and because appendages like limbs and tentacles, can change position with respect to the main body, positional descriptive terms need to refer to the animal as in its standard anatomical position.
Thus, all descriptions are with respect to the organism in its standard anatomical position, even when the organism in question has appendages in another position. For example, see Fig. 9, where the tentacles are curved, and therefore not in anatomical position. However, a straight position is assumed when describing the proximo-distal axis. This helps avoid confusion in terminology when referring to the same organism in different postures.
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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 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. The penis in the anatomical position is as if erect or lying against the stomach, hence the dorsal surface of the penis is actually anterior when the penis is pointing down between the legs.
In humans, the anatomical position of the skull has been agreed by international convention to be the Frankfurt plane, a position in which the lower margins of the orbits, the orbitales, and the upper margins of the ear canals, 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.
Three basic reference planes are used in zoological anatomy.
When describing anatomical motion, these planes describe the axis along which an action is performed. So by moving through the transverse plane, movement travels from head to toe. For example, if a person jumped directly up and then down, their body would be moving through the transverse plane in the coronal and sagittal planes.
Some of these terms come from Latin. Sagittal means "like an arrow", a reference to the position of the spine that naturally divides the body into right and left equal halves, the exact meaning of the term "midsagittal", or to the shape of the sagittal suture, which defines the sagittal plane and is shaped like an arrow.
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 Z-axis going from front to back, the X-axis going from left to right, and the Y-axis going from up to down. The Z-axis is always forward (Tait-Bryan angles) and the right-hand rule applies.
The axes and the sagittal plane are the same for bipeds and quadrupeds, but the orientation of the coronal and transverse planes switch. The axes on particular pieces of equipment may or may not correspond to axes of the body, especially since the body and the equipment may be in different relative orientations.
In discussing the neuroanatomy of animals, particularly rodents used in neuroscience research, a simplistic convention has been to name the sections of the brain according to the homologous human sections. Hence, what is technically a transverse (orthogonal) section with respect to the body length axis of a rat (dividing anterior from posterior) may often be referred to in rat neuroanatomical coordinates as a coronal section, and likewise a coronal section with respect to the body (i.e. dividing ventral from dorsal) in a rat brain is referred to as transverse. This preserves the comparison with the human brain, whose length axis in rough approximation is rotated with respect to the body axis by 90 degrees in the ventral direction. It implies that the planes of the brain are not necessarily the same as those of the body.
However, the situation is more complex, since comparative embryology shows that the length axis of the neural tube (the primordium of the brain) has three internal bending points, namely two ventral bendings at the cervical and cephalic flexures (cervical flexure roughly between the medulla oblongata and the spinal cord, and cephalic flexure between the diencephalon and the midbrain), and a dorsal (pontine or rhombic) flexure at the midst of the hindbrain, behind the cerebellum. The latter flexure mainly appears in mammals and sauropsids (reptiles and birds), whereas the other two, and principally the cephalic flexure, appear in all vertebrates (the sum of the cervical and cephalic ventral flexures is the cause of the 90 degree angle mentioned above in humans between body axis and brain axis). This more realistic concept of the longitudinal structure of vertebrate brains implies that any section plane, except the sagittal plane, will intersect variably different parts of the same brain as the section series proceeds across it (relativity of actual sections with regard to topological morphological status in the ideal unbent neural tube). Any precise description of a brain section plane therefore has to make reference to the anteroposterior part of the brain to which the description refers (e.g., transverse to the midbrain, or horizontal to the diencephalon). A necessary note of caution is that modern embryologic orthodoxy indicates that the brain's true length axis finishes rostrally somewhere in the hypothalamus where basal and alar zones interconnect from left to right across the median line; therefore, the axis does not enter the telencephalic area, although various authors, both recent and classic, have assumed a telencephalic end of the axis. The causal argument for this lies in the end of the axial mesoderm -mainly the notochord, but also the prechordal plate- under the hypothalamus. Early inductive effects of the axial mesoderm upon the overlying neural ectoderm is the mechanism that establishes the length dimension upon the brain primordium, jointly with establishing what is ventral in the brain (close to the axial mesoderm) in contrast with what is dorsal (distant from the axial mesoderm). Apart from the lack of a causal argument for introducing the axis in the telencephalon, there is the obvious difficulty that there is a pair of telencephalic vesicles, so that a bifid axis is actually implied in these outdated versions.
As humans are bilaterally symmetrical organisms, anatomical directions in humans can usually be correctly described using the same terms as those for vertebrates and other members of the taxonomic group Bilateria. However, for historical and other reasons, standard human directional terminology has several differences from that used for other bilaterally symmetrical organisms.
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 be used with respect to only the head and the main body (trunk), and not when considering the limbs.
As with vertebrate directional terms, superior and inferior can be used in a relative sense in humans, but can not be uniformly applied to other organisms with varying normal anatomical positions. For example, the shoulders are superior to the navel, but inferior to the eyes in humans. In any tetrapod, the shoulders are cranial to the belly, but caudal to the eyes.
In human anatomical usage, anterior refers to the "front" of the individual, and is synonymous with ventral, other than in the head. Similarly, posterior, refers to the "back" of the subject, and is synonymous with dorsal, other than in the head (see Table 3). When referring to the body as a whole the terms "dorsal" and "ventral" are used infrequently in human anatomy. However, they are applied commonly in referring to limb position. 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.
Lateral refers to the sides, as in 'left lateral' and 'right lateral'. Medial refers to the middle.
Left and right lateral are used in the same sense as they are in other vertebrates, referring to the individual's left and right. The left-right axis is rarely used in medicine; instead, the mediolateral axis is used almost exclusively.
As in other vertebrates, the terms "proximal" and "distal" are used to describe parts of a feature that are respectively close to or distant from some anatomical point of reference. The point of reference varies, but is normally the point of attachment to the main body for limbs and appendages, and a point considered to represent the origin for other features. For example the side of a tooth furthest from the centre of the mouth is its distal side. However, other terms are used for direction in the appendages, given the unique position of the limbs (in standard anatomical position) in humans.
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.
In standard anatomical position, the palms of the hands point anteriorly. Thus, anterior can be used to describe the palm of the hand, and posterior can be used to describe the back of the hand and arm.
However, presumably for improved clarity, the directional term palmar (Latin palma; palm of the hand) is usually used for the anterior of the hand, and dorsal is used to describe the back of the hand. Thus, by connecting the extremes, dorsopalmar axis is formed. Most commonly, "dorsopalmar" is used when describing the hand, although it is sometimes applied to the arm as a whole (see Fig. 12).
For the third axis, the mediolateral axis suffices, although if referring to the limb alone, "medial" may refer to the centre of the arm itself.
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 toward the head are cranial; those facing away or farther from the head are caudal.
Distal to the carpal joint, the term dorsal replaces cranial and palmar replaces caudal. Similarly, distal to the tarsal joint the term dorsal replaces cranial and plantar replaces caudal. For example, the top of a dog's paw is its dorsal surface; the underside, either the palmar (on the forelimb) or the plantar (on the hindlimb) surface.
The sides of the forearm are named after its bones: Structures closer to the radius are radial, structures closer to the ulna are ulnar, and structures relating to both bones are referred to as radioulnar. Similarly, in the lower leg, structures near the tibia (shinbone) are tibial and structures near the fibula are fibular (or peroneal).
Volar (sometimes used as a synonym for "palmar") refers to the underside, for both the palm and the sole (plantar), as in volar pads on the underside of hands, fingers, feet and toes.
The terms valgus and varus are used to refer to angulation of the distal part of a limb at a joint. For example, at the elbow joint, in the anatomical position, the forearm and the upper arm do not lie in a straight line, but the forearm is angulated laterally with respect to the upper arm by about 5–10°. The forearm is said to be "in valgus". Angulation at a joint may be normal (as in the elbow) or abnormal.
Commonly when, for example, one anatomical feature is nearer the caudal end than another, one may use an expression such as "nearer the caudal end" or "caudal to". However, an unambiguous and concise convention is to use the Latin suffix -ad, meaning "towards", or sometimes "to". So for example, "distad" means "in the distal direction", and "distad of the femur" means "beyond the femur in the distal direction". The suffix may be used very widely, as in the following examples: anteriad (towards the anterior), apicad (towards the apex), basad (towards the basal end), caudad, centrad, cephalad (towards the cephalic end), craniad, dextrad, dextrocaudad, dextrocephalad, distad, dorsad, ectad (towards the ectal, or exterior, direction), entad (towards the interior), laterad, mediad, mesad, neurad, orad, posteriad, proximad, rostrad, sinistrad, sinistrocaudad, sinistrocephalad, ventrad.
In humans, reference may take origin from superficial anatomy, made to landmarks that are on the skin or visible underneath. As with planes, lines and points are imaginary. Examples include:
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 (invertebrate) organisms as well.
To begin with, distinct, polar-opposite ends of the organism are chosen. By definition, each pair of opposite points defines an axis. In a bilaterally symmetrical organism, there are 6 polar opposite points, giving three axes that intersect at right angles—the x, y, and z axes familiar from three-dimensional geometry.
The most obvious end-points are the "nose" and "tail" (see Fig. 2). 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 κρανίον (kranion); skull), or the cephalic end (Greek κεφάλι (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 cauda; tail, though in humans this refers to the feet i.e. inferior rather than posterior)—a term that strictly applies only to vertebrates, and therefore less preferred, except in veterinary medicine where these terms are standard, as accepted for example in the Nomina Anatomica Veterinaria.
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.
|Axis||Directional term||Directed towards|
|Anteroposterior (rostrocaudal1,craniocaudal1, cephalocaudal2)||Anterior||Head end|
|Dorsoventral||Dorsal||Back, spinal column|
|Left-right (dextro-sinister2, sinistro-dexter2)||Left (sinister)||Left-hand side|
|Right (dexter)||Right-hand side|
|Lateral||Left and right|
|Proximal/distal||Proximal||Point at which appendage joins the body|
|Distal||Extremity of appendage|
(1) Fairly common usage.
(2) Uncommon usage.
(3) Equivalent to one-half of the left-right axis.
(The terms "intermediate", "ipsilateral", "contralateral", "superficial", and "deep", while indicating directions, are relative terms and thus do not properly define fixed anatomical axes. Also, while the "rostrocaudal" and anteroposterior directionality are equivalent in a significant portion of the human body, they are different directions in other parts of the body.)
The next end-points are the back and belly. These are termed the dorsal end (Latin dorsum; back) and the ventral end (Latin venter; belly), respectively. By connecting the outermost points the dorsoventral axis is formed (sometimes hyphenated: dorso-ventral). This is commonly abbreviated to DV (or D-V) axis. The DV axis, by definition, is perpendicular (at right angles to) the AP axis at all times (see below).
As with anteroposterior, the terms "dorsal" and "ventral" are also used to describe relative positions along the dorsoventral axis. Thus, the pectoral fins are dorsal to the anal fin, but ventral to the dorsal fin in Fig. 2. (Note that these fins are not aligned anteroposteriorly, either: the dorsal fin is posterior to the pectoral fins, and is anterior to the anal fins.)
The last axis, by geometric definition, must be at right angles to both the AP and the DV axes. The left side and right side of the organism are the outermost points between the two "sides" of the organism. When connected, these points form the left-right axis (commonly abbreviated to LR (or L-R) axis. In Latin, this is called the dextro-sinistral (or, less commonly, the sinistro-dextral) axis, from dexter (right) and sinister (left). The "left" and "right" refer to the sides of the organism, and not those of the observer.
"Left-right" is typically used in English and some other languages.[clarification needed]
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., in the previous example, there is both a right eye and a left eye, forcing one to specify which is used as a reference).
To counter this clumsiness of usage, the directional term lateral (Latin lateralis; "to the side") is used as a modifier for both sides, yielding the left lateral and right lateral sides. As an opposite to lateral, the term median (Latin medius; "middle") is used to define a point in the centre of the organism (where the left-right axis intersects the midsagittal plane —see below), and the term medial means "toward the median plane". Thus, rather than "left-right" axis and its inherent clumsiness of usage, the term mediolateral (also sometimes hyphenated medio-lateral) axis is frequently used. Sometimes this is abbreviated to ML (or M-L) axis. In proper usage, the ML axis is a half-axis; in practice, its usage is less clumsy and less linguistically biased than "left-right". The terms may still be used relatively to describe locations along the LR axis. Thus, in Fig. 2 the gills are medial to the operculum, but lateral to the heart.
The usage "mediolateral" is strictly used to describe relative position along the left-right axis, to avoid confusion with the terms "superficial" and "deep" (see below).
The term proximal (Latin proximus; nearest) describes where the appendage joins the body, and the term distal (Latin distare; to stand away from) is used for the point furthest from the point of attachment to the body. Since 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 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 penis in many vertebrates and some invertebrates, would be included.
By connecting the two points, the proximodistal (sometimes hyphenated to proximo-distal) axis is created. (The abbreviation AB axis is occasionally, but not commonly, used.) As before, the terms "proximal" and "distal" can be used as relative terms to indicate where structures lie along the proximodistal axis. Thus, the "elbow" is proximal to the hoof, but distal to the "shoulder" in Figs. 3 and 4.
Choosing terms for the other two axes perpendicular to the proximodistal axis could be variable, as they would also depend on the position of the limb. For that reason, when considering any organism, the other two axes are considered to be relative to the appendage when in standard anatomical position. This is roughly defined for all organisms, as in the normal position when at rest and not moving. For tetrapod 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).
Similar to appendages that branch out from the body, the directions of blood vessels may be labeled with the terms Ostial (referring to the Ostium or opening where the vessel branches off) and distal (the extreme end away from the branch point).
Teeth may be aligned with their main axes within the jaw, but some different relationships require special terminology as well; for example teeth also can be rotated, and in such contexts terms like "anterior" or "lateral" become ambiguous. Therefore, fields such as osteology, palaeontology and dentistry apply special terms of location in oral and dental description.
In addition to the three primary axes (AP, DV and the ML half-axis) and the proximodistal axis of appendages of bilaterally symmetrical animals, there are several classes of directional terms. These are mainly of use in referring to strictly relative situations and directions, and as such do not and cannot define fixed axes. These terms include:
Most terms of anatomical location are relative to linear motion (translation) along the X- Y- and Z-axes, but there are other degrees of freedom as well, in particular, rotation around any of those three axes.
Anteversion and retroversion are complementary anatomical terms of location, describing the degree to which an anatomical structure is rotated forwards (towards the front of the body) or backwards (towards the back of the body) respectively, relative to some datum position. The terms also describe the positioning of surgical implants, such as in arthroplasty.
The above terms may be combined, either to indicate a position in two axes simultaneously or to indicate the direction of a movement relative to the body. For example, "anterolateral" indicates a position that is both anterior and lateral to the body axis (such as the bulk of the pectoralis major muscle). In radiology, an X-ray image may be said to be "anteroposterior", indicating that the beam of X-rays pass from their source to patient's anterior body wall through the body to exit through posterior body wall.
There is no definite limit to the contexts in which terms may be modified to qualify each other in such combinations. generally the modifier term is truncated and an "o" is added in prefixing it to the qualified term. For example a view of an animal from an aspect dorsal and lateral might be called "dorsolateral". Where desirable three or more terms may be agglutinated or concatenated, as in "anteriodorsolateral" (or "anterodorsolateral"). Such terms sometimes used to be hyphenated, but the modern tendency is to omit the hyphen. There is however little basis for any strict rule to interfere with choice of convenience in such usage.
The terms of zootomy and androtomy came into usage at a time when all scientific communication took place in Latin. In their original Latin forms the respective meanings of "anterior" and "posterior" are in front of (or before) and behind (or after), those of "dorsal" and "ventral" are toward the spine and toward the belly, and those of "superior" and "inferior" are above and below. From these meanings it can be seen that in the most general terms the anterior/posterior axis is oriented to the direction of forward motion, the dorsal/ventral axis is oriented to the anatomy of the vertebrate torso, and the superior/inferior axis is oriented to gravity.
For almost all vertebrates, including almost all bipeds, these axes all provide a consistent reference for anatomical positions across species—with the inferior/superior axis being roughly the same as the dorsal/ventral axis, and therefore redundant. Humans, however, have the rare property of having a torso oriented perpendicular to their direction of forward motion—while their head orientation remains consistent with other vertebrates on this axis. This makes the dorsal/ventral axis on humans redundant with the anterior/posterior axis, and the inferior/superior axis necessary. Because of this difference with humans, the anterior/posterior and inferior/superior axes are inconsistent between humans and other vertebrates in torso anatomy but consistent in head anatomy. As all three of these axes are used in the naming of anatomical structures, and most human anatomical structures are shared by other animals, these differences can lead to considerable confusion. For example, in the naming of brain structures, the non-human context of the dorsal/ventral axis was used. Therefore, in human anatomy, "dorsal" can refer to two different (perpendicular) directions—the posterior direction in the context of the torso, and the superior direction in the context of the brain. Confusingly then, the "dorsal" direction in the human brain, being perpendicular to the "dorsal" direction in the human torso, conflicts with the direction that might be inferred from the literal Latin meaning of "dorsum": a back or a mountain ridge.
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, or "toward the tail"/rostral, or "toward the beak"), doing so would require the renaming of very many anatomical structures.
For a quick comparison of equivalent terminology used in vertebrate and human anatomy, see Table 3 (below).
|Table 3: Equivalent directional terms used in|
vertebrate zoology and human anatomy
|Vertebrate zootomy||Human torso||Human head|
|Anterior||Rostral, Cranial, Cephalic1||Anterior||Ventral, Front||Anterior||Rostral, Front|
|Posterior||Caudal||Posterior||Dorsal, Back||Posterior||Caudal, Back|
|Dorsal||—||Superior||Same1, Up||Superior||Dorsal, Up|
|Ventral||—||Inferior||Caudal1, Down||Inferior||Ventral, Down|
|lateral||Away from the middle||Same||—||Same||—|
|Proximal||Away from extremity||Same||—||Same||—|
(1) Rarely used.
(2) Strictly relative term, used with other locational descriptors.
Together, the AP, DV and LR (or ML) axes allow for precise three-dimensional descriptions of location within any bilaterally symmetrical organism, whether vertebrate or invertebrate. In practice, the terms can cause some confusion when, unlike the fish shown in Fig. 2, the organism in question is not strictly linear in form, which includes most tetrapods (see Figs. 3 and 4). For example, the AP axis in Fig. 3 does not appear to be at right angles to the DV axis. Rather, it is a depiction of the approximate average AP axis, when all body segments are included.
When considering any one segment, the dorsoventral axis is perpendicular to the AP axis. Thus, in Fig. 4, the DV axis of the tail would run from the "back" of the tail (posterior end of the trunk), to the "underside" of the tail (near the legs) — nearly parallel to the AP axis of the main body.
As a general rule of thumb, if the body is included in consideration, the AP axis of the main body would be used, as would the DV and ML axes perpendicular to it. However, if considering only one segment, the AP axis would shift to reflect the axes shown in Fig. 4, with the DV and ML axes shifting correspondingly. In alternative manner, to avoid confusion, AP, DV, and ML terms are used strictly in relation to the main body, and the terms proximal and distal are used for body segments such as the head, neck, and tail (see below).
To avoid this confusion, in veterinary medicine, the terms anterior, posterior, superior, and inferior are in general avoided except for certain structures within the head. By using the terms cranial, caudal, dorsal and ventral, all tetrapod organisms (including bipeds) can be described uniformly.
In humans, the directions "rostral" and "caudal" often become confused with anterior and posterior, or superior and inferior. The difference between the two is most easily visualized when looking at the head, as can be seen in the image to the right. From the most caudal of positions in the nervous system (of a person) to a nearby, rostral area, it is equally accurate to say the area in question is rostral as to say it is superior. However, in the frontal lobes of the telencephalon, to say an area is rostral to a nearby area is equivalent to saying it is anterior. Those two lines lie on planes perpendicular to one another. This occurs, as becomes clear in the diagram, due to the intuitive yet curious curving "C" shape of rostrocaudal directionality when discussing the human brain.
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 vertebrate anatomy, and appropriate novel terms are applied as needed. Some such borrowed terms are widely applicable in most invertebrates; for example proximal, literally meaning "near" refers to the part of an appendage nearest to where it joins the body, and distal, literally meaning "standing away from" is used for the part furthest from the point of attachment. In all cases, the usage of terms is dependent on the bauplan of the organism.
In organisms with a changeable shape, such as amoeboid organisms, most directional terms are meaningless, since the shape of the organism is not constant and no distinct axes are fixed. Similarly, in spherically symmetrical organisms, there is nothing to distinguish one line through the centre of the organism from any other. An indefinite number of triads of mutually perpendicular axes could be defined, but any such choice of axes would be useless, as nothing would distinguish a chosen triad from any others. In such organisms, only terms such as superficial and deep, or sometimes proximal and distal, are usefully descriptive.
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 slides, where they appear essentially two-dimensional. In some cases a third axis can be defined, particularly where a non-terminal cytostome or other unique structure is present.
Some elongated protists have distinctive ends of the body. In such organisms, the end with a mouth (or equivalent structure, such as the cytostome in Paramecium or Stentor), or the end that usually points in the direction of the organism's locomotion (such as the end with the flagellum in Euglena), is normally designated as the anterior end. The opposite end then becomes the posterior end, and by connecting them, an anteroposterior axis is formed. Properly, this terminology would apply only to an organism that is always planktonic (not normally attached to a surface, as in Fig. 7 top), although the term can also be applied to one that is sessile (normally attached to a surface, as in Fig. 7, bottom, and Fig. 8).
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.
Radially symmetrical organisms include those in the group Radiata—primarily jellyfish, sea anemones and corals and the comb jellies. Adult echinoderms (sea stars (starfish), sea urchins, sea cucumbers and others) are also included, since they are pentaradial (i.e. they have fivefold discrete rotational symmetry). Echinoderm larvae are not included, since they are bilaterally symmetrical. Radially symmetrical organisms always have one distinctive axis.
Cnidarians (jellyfish, sea anemones and corals) 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).
Unlike vertebrates, cnidarians have no other distinctive axes. 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, rather than a central axis (as in vertebrates). Thus, there are multiple possible radial axes and medio-peripheral (half-) axes (Fig. 10). However, it is noteworthy that some “biradially symmetrical" comb jellies do have distinct "tentacular" and "pharyngeal" axes and are thus anatomically equivalent to bilaterally symmetrical animals.
|Table 2: Comparison of Directional Terms used in|
Radially Symmetrical1 and Bilaterally Symmetrical Animals
|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".
Figure 10: Aurelia aurita, another species of jellyfish, showing multiple radial and medio-peripheral axes.
Two specialized terms are useful in describing views of arachnid legs and pedipalps. Prolateral refers to the surface of a leg that is closest to the anterior end of an arachnid's body. Retrolateral refers to the surface of a leg that is closest to the posterior end of an arachnid's body.
Because of the unusual nature and positions of the eyes of the Araneae (spiders), and their importance in taxonomy, evolution and anatomy, special terminology with associated abbreviations has become established in arachnology. Araneae normally have eight eyes in four pairs. All the eyes are on the carapace of the prosoma, and their sizes, shapes and locations are characteristic of various spider families and other taxa. In some taxa not all four pairs of eyes are present, the relevant species having only three, two, or one pair of eyes. Some species (mainly troglobites) have no functional eyes at all.
In what is seen as the likeliest ancestral arrangement of the eyes of the Araneae, there are two roughly parallel, horizontal, symmetrical, transverse rows of eyes, each containing two symmetrically placed pairs, respectively called: anterior and posterior lateral eyes (ALE) and (PLE); and anterior and posterior median eyes (AME) and (PME).
As a rule it is not difficult to guess which eyes are which in a living or preserved specimen, but sometimes it can be. Apart from the fact that in some species one or more pairs may be missing, sometimes eyes from the posterior and anterior rows may be very close to each other, or even fused. Also, either one row or both might be so grossly curved that some of the notionally anterior eyes actually may lie posterior to some of the eyes in the posterior row. In some species the curve is so gross that the eyes apparently are arranged into two anteroposterior parallel rows of eyes.
|Aspects of spider anatomy; This aspect shows the mainly prolateral surface of the anterior femora, plus the typical horizontal eye pattern of the Sparassidae
Typical arrangement of eyes in the Lycosidae, with PME being the largest
In the Salticidae the AME are the largest
|This article cites its sources but does not provide page references. (February 2014)|