All vertebrates, including humans, have the same basic body plan: they are strictly bilaterally symmetrical in early embryonic stages and largely bilaterally symmetrical in adulthood. If they are divided down the middle, in other words, they have mirror-image left and right halves. For those 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.
While the 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 the one that is used in the study of various other zoological categories.
Standard anatomical and zoological terms of location have been developed, usually based on Latin and Greek words, to enable all biological and medical scientists, veterinarians, doctors and anatomists to precisely delineate and communicate information about animal bodies and their organs, even though the meaning of some of the terms often is context-sensitive. Much of this information has been standardised in internationally agreed vocabuliaries for humans (Terminologia Anatomica) and animals (Nomina Anatomica Veterinaria). 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 humans, one type of vertebrate, and other animals that stand on two feet (bipeds), terms that are used are different from those that stand on four (quadrupeds). One reason is that humans have a different neuraxis and another is that unlike animals that rest on four limbs, humans are considered when describing anatomy as being in the standard anatomical position, which is standing up with arms outstretched. Thus, what is on "top" of a human is the head, whereas the "top" of a dog may be its back, and the "top" of a flounder could refer to either its left or its right side.
For invertebrates, it is difficult to use anatomical terms of location in a standard way, as animal shape can differ so radically that common concepts are not homologous and do not refer to common concepts. For example, many species are not even bilaterally symmetrical. In those species, terminology depends on their type of symmetry, if they have any.
Standard anatomical position
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, terms to describe position need to refer to an animal when it is in its standard anatomical position. This means descriptions are with respect to the organism in its standard anatomical position, even when the organism in question has appendages in another position. This helps avoid confusion in terminology when referring to the same organism in different postures. In humans, this refers to the body in a standing position with arms at the side and palms facing forward, with thumbs out and to the sides.
Many anatomical terms can 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" or an "i" is added in prefixing it to the qualified term. For example, a view of an animal from an aspect at once dorsal and lateral might be called a "dorsolateral" view; and the effect of dorsolateral flattening in an organism such as a krait gives its body a triangular cross section. Again, in describing the morphology of an organ or habitus of an animal such as many of the Platyhelminthes, one might speak of it as "dorsiventrally" flattened as opposed to bilaterally flattened animals such as ocean sunfish.
Where desirable three or more terms may be agglutinated or concatenated, as in "anteriodorsolateral". 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 median plane, which divides the body into left and right. This passes through the head, spinal cord, navel, and, in many animals, the tail.
- The sagittal planes, which are parallel to the median plane.
- The frontal plane, also called the coronal plane, which divides the body into front and back.
- The horizontal plane, also known as the transverse plane, which is perpendicular to the other two planes. In a human, this plane is parallel to the ground; in an animal[clarification needed]
The axes of the body are lines drawn about which an organism is roughly symmetrical. To do this, distinct ends of an organism are chosen, and the axis is named according to those directions. An organism that is symmetrical on both sides has three main axes that intersect at right angles. An organism that is round or not symmetrical may have different axes.
|Axis||Directional term||Directed towards|
|Rostrocaudal,[a] craniocaudal,[a] cephalocaudal,[b] longitudinal||Rostral, cranial, cephalad||
|Dorsoventral||Dorsal||Back, spinal column|
|Left-right, dextro-sinister,[b] sinistro-dexter,[b] frontal, horizontal||Left (sinister)||Left-hand side|
|Right (dexter)||Right-hand side|
|Mediolateral[c]||Medial||Towards the Center line|
|Lateral||Away from the Center Line|
|Proximal/distal||Proximal||Point at which appendage joins the body|
|Distal||Extremity of appendage|
Anatomical axes in a human, similar for other orthograde bipedal vertebrates
Anatomical axes and directions in a fish
Several terms are commonly seen and used as prefixes:
- Sub- (from Latin sub, meaning 'preposition beneath, close to, nearly etc') is used to indicate something that is beneath, or something that is subordinate to or lesser than. For example, subcutaneous means beneath the skin, and "subglobular" may mean smaller than a globule
- Hypo- (from Ancient Greek ὑπό, meaning 'under') is used to indicate something that is beneath. For example, the hypoglossal nerve supplies the muscles beneath the tongue.
- Infra- (from Latin infra, meaning 'under') is used to indicate something that is within or below. For example, the infraorbital nerve runs within the orbit.
- Inter- (from Latin inter, meaning 'between') is used to indicate something that is between. For example, the intercostal muscles run between the ribs.
- Super- or Supra- (from Latin super, supra, meaning 'above, on top of') is used to indicate something that is above something else. For example, the supraorbital ridges are above the eyes.
Other terms are used as suffixes, added to the end of words:
- -ad (from Latin ad, meaning 'towards') and ab- (from Latin ab) are used to indicate that something is towards (-ad) or away from (-ab) something else. For example, "distad" means "in the distal direction", and "distad of the femur" means "beyond the femur in the distal direction". Further examples may include cephalad (towards the cephalic end), craniad, and distad.
Superior and inferior
Superior (from Latin super, meaning 'above') describes what is above something and inferior (from Latin inferus, meaning 'below') describes what is below it. For example, in the anatomical position, the most superior part of the human body is the head and the most inferior is the feet. As a second example, in humans, the neck is superior to the chest but inferior to the head.
Anterior and posterior
Anterior (from Latin ante, meaning 'before') describes what is in front, and posterior (from Latin post, meaning 'after') describes what is to the back of something. For example, in a dog the nose is anterior to the eyes and the tail is considered the most posterior part; in many fish the gill openings are posterior to the eyes, but anterior to the tail. In projectional radiography terminology, an anteroposterior (AP) projection is taken with the X-ray generator anteriorly (such as in the front of a human), and the X-ray detector posteriorly. In contrast, a posteroanterior (PA) projection is taken with the X-ray generator posteriorly.
Medial and lateral
These terms describe how close something is to the midline, or the medial plane. Lateral (from Latin lateralis, meaning 'to the side') describes something to the sides of an animal, as in "left lateral" and "right lateral". Medial (from Latin medius, meaning 'middle') describes structures close to the midline, or closer to the midline than another structure. For example, in a human, the arms are lateral to the torso. The genitals are medial to the legs.
The terms "left" and "right" are sometimes used, or their Latin alternatives (Latin: dexter, lit. 'right'; Latin: sinister, lit. 'left'). However, as left and right sides are mirror images, using these words is somewhat confusing, as structures are duplicated on both sides. For example, it is very confusing to say the dorsal fin of a dolphin is "right of" the left pectoral fin, but is "left of" the right eye, but much easier and clearer to say "the dorsal fin is medial to the pectoral fins".
Terms derived from lateral include:
- Contralateral (from Latin contra, meaning 'against'): on the side opposite to another structure. For example, the right arm and leg are controlled by the left, contralateral, side of the brain.
- Ipsilateral (from Latin ipse, meaning 'same'): on the same side as another structure. For example, the left arm is ipsilateral to the left leg.
- Bilateral (from Latin bis, meaning 'twice'): on both sides of the body. For example, bilateral orchiectomy means removal of testes on both sides of the body.
- Unilateral (from Latin unus, meaning 'one'): on one side of the body. For example, a stroke can result in unilateral weakness, meaning weakness on one side of the body.
Varus (from Latin, meaning 'bow-legged') and valgus (from Latin, meaning 'knock-kneed') are terms used to describe a state in which a part further away is abnormally placed towards (varus) or away from (valgus) the midline.
Proximal and distal
The terms proximal (from Latin proximus, meaning 'nearest') and distal (from Latin distare, meaning 'to stand away from') are used to describe parts of a feature that are close to or distant from the main mass of the body, respectively. Thus the upper arm in humans is proximal and the hand is distal.
These terms are frequently used when describing appendages such as fins, tentacles, limbs or any structure that extends that can potentially move separately from the main body. Although the direction indicated by "proximal" and "distal" is always respectively towards or away from the point of attachment, a given structure can be either proximal or distal in relation to another point of reference. Thus the elbow is distal to a wound on the upper arm, but proximal to a wound on the lower arm.
This terminology is also employed in molecular biology and therefore by extension is also used in chemistry, specifically referring to the atomic loci of molecules from the overall moiety of a given compound.
Central and peripheral
Central and peripheral refer to the distance towards and away from the centre of something. That might be an organ, a region in the body, or an anatomical structure. For example, the Central nervous system and the peripheral nervous systems.
Superficial and deep
These terms refer to the distance of a structure from the surface or structure.
Deep (from Old English) describes something further away from the surface of the organism. For example, the external oblique muscle of the abdomen is deep to the skin. "Deep" is one of the few anatomical terms of location derived from Old English rather than Latin – the anglicised Latin term would have been "profound" (from Latin profundus, meaning 'due to depth').
Dorsal and ventral
Cranial and caudal
Specific terms exist to describe how close or far something is to the head or tail of an animal. To describe how close to the head of an animal something is, three distinct terms are used:
- Rostral (from Latin rostrum, meaning 'beak, nose') describes something situated toward the oral or nasal region, or in the case of the brain, toward the tip of the frontal lobe.
- Cranial (from Greek κρανίον, meaning 'skull') or cephalic (from Greek κεφαλή, meaning 'head') describe how close something is to the head of an organism.
- Caudal (from Latin cauda, meaning 'tail') describes how close something is to the trailing end of an organism.
For example, in horses, the eyes are caudal to the nose and rostral to the back of the head.
These terms are generally preferred in veterinary medicine and not used as often in human medicine. In humans, "cranial" and "cephalic" are used to refer to the skull, with "cranial" being used more commonly. The term "rostral" is rarely used in human anatomy, apart from embryology, and refers more to the front of the face than the superior aspect of the organism. Similarly, the term "caudal" is used more in embryology and only occasionally used in human anatomy. This is because the brain is situated at the superior part of the head whereas the nose is situated in the anterior part. Thus, the "rostrocaudal axis" refers to a C shape (see image).
Other terms and special cases
Structures may be described as being at the level of a specific spinal vertebra, depending on the section of the vertebral column the structure is at. The position is often abbreviated. For example, structures at the level of the fourth cervical vertebra may be abbreviated as "C4", at the level of the fourth thoracic vertebra "T4", and at the level of the third lumbar vertebra "L3". Because the sacrum and coccyx are fused, they are not often used to provide location.
References may also take origin from superficial anatomy, made to landmarks that are on the skin or visible underneath. For example, structures may be described relative to the anterior superior iliac spine, the medial malleolus or the medial epicondyle.
Mouth and teeth
Special terms are used to describe the mouth and teeth. Fields such as osteology, palaeontology and dentistry apply special terms of location to describe the mouth and teeth. This is because although teeth may be aligned with their main axes within the jaw, 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. Terms such as "distal" and "proximal" are also redefined to mean the distance away or close to the mandibular symphysis. Terms used to describe structures include "buccal" (from Latin bucca, meaning 'cheek') and "palatal" (from Latin) referring to structures close to the cheek and hard palate respectively.
Hands and feet
Several anatomical terms are particular to the hands and feet.
For improved clarity, the directional term palmar (from Latin palma, meaning 'palm of the hand') is usually used to describe the front of the hand, and dorsal is the back of the hand. 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 palmar fascia is palmar to the tendons of muscles which flex the fingers, and the dorsal venous arch is so named because it is on the dorsal side of the foot.
Volar can also be used to refer to the underside of the palm or sole, which are themselves also sometimes used to describe location as palmar and plantar. For example, volar pads are those on the underside of hands, fingers, feet, and toes.
These terms are used to avoid confusion when describing the median surface of the hand and what is the "anterior" or "posterior" surface – "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. This confusion can arise because the forearm can pronate and supinate.
Similarly, in the forearm, for clarity, the sides are named after the 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).
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 terms describing an anatomical structure that is rotated forwards (towards the front of the body) or backwards (towards the back of the body), relative to some other position. They are particularly used to describe the curvature of the uterus.
- Anteversion (from Latin anteversus) describes an anatomical structure being tilted further forward than normal, whether pathologically or incidentally. For example, a woman's uterus typically is anteverted, tilted slightly forward. A misaligned pelvis may be anteverted, that is to say tilted forward to some relevant degree.
- Retroversion (from Latin retroversus) describes an anatomical structure tilted back away from something. An example is a retroverted uterus.
Other directional terms
Several other terms are also used to describe location. These terms are not used to form the fixed axes. Terms include:
- Axial (from Latin axis, meaning 'axle'): around the central axis of the organism or the extremity. Two related terms, "abaxial" and "adaxial", refer to locations away from and toward the central axis of an organism, respectively
- Parietal (from Latin paries, meaning 'wall'): pertaining to the wall of a body cavity. For example, 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.
- Terminal (from Latin terminus, meaning 'boundary or end') at the extremity of a usually projecting structure. For example, "...an antenna with a terminal sensory hair".
- Visceral and viscus (from Latin viscera, meaning 'internal organs'): associated with organs within the body's cavities. For example, the stomach is covered with a lining called the visceral peritoneum as opposed to the parietal peritoneum. Viscus can also be used to mean "organ". For example, the stomach is a viscus within the abdominal cavity, and visceral pain refers to pain originating from internal organs.
- Aboral (opposite to oral) is used to denote a location along the gastrointestinal canal that is relatively closer to the anus.
Specific animals and other organisms
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 body plan of the organism.
Anatomical terms of location in a dog
Anatomical terms of location in a kangaroo
Anatomical terms of location in a fish.
Anatomical terms of location in a horse.
Anatomical terms of location in a human.
As humans are approximately bilaterally symmetrical organisms, anatomical descriptions usually use 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.
The terms of zootomy and anatomy came into use 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.
Humans, however, have the rare property of having an upright torso. This makes their anterior/posterior and dorsal/ventral directions the same, and the inferior/superior directions necessary.
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.
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/rostral, or "toward the tail"/"toward the beak"), doing so would require the renaming of many anatomical structures.
Asymmetrical and spherical organisms
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 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. Properly, this terminology would apply only to an organism that is always planktonic (not normally attached to a surface), although the term can also be applied to one that is sessile (normally attached to a surface).
Organisms that are attached to a substrate, such as sponges, 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 (from Latin basis, meaning 'support/foundation'), whereas the end furthest from the attachment is referred to as the apical end (from Latin apex, meaning 'peak/tip').
Radially symmetrical organisms
Radially symmetrical organisms include those in the group Radiata – primarily jellyfish, sea anemones and corals and the comb jellies. Adult echinoderms, such as starfish, sea urchins, sea cucumbers and others are also included, since they are pentaradial, meaning they have five 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 (from Latin ōrālis, meaning 'of the mouth'), and the opposite surface is the aboral end (from Latin ab-, meaning 'away from').
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 (from Ancient Greek περιφέρεια, meaning 'circumference'). 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. However, some biradially symmetrical comb jellies do have distinct "tentacular" and "pharyngeal" axes and are thus anatomically equivalent to bilaterally symmetrical animals.
Aurelia aurita, another species of jellyfish, showing multiple radial and medio-peripheral axes
The sea star Porania pulvillus, aboral and oral surfaces
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
- Standard anatomical position
- Anatomical terms of motion – Terms describing animal motion
- Anatomical terms of muscle
- Anatomical terms of bone
- Anatomical terms of neuroanatomy
Terms of orientation
- Handedness – Better performance or individual preference for use of a hand
- Laterality – Preference most humans show for one side of their body over the other
- Proper right and proper left – terms for unambiguous directions, like port and starboard
- Reflection symmetry
- Sinistral and dextral
- Direction (disambiguation)
- Port and starboard – Nautical terms for direction, another example of terms of directionality that do not depend on the location of the observer for things that are bilaterally symmetrical
- Cardinal Directions
- Blocking (stage)
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