Anatomical terms of location
Template:Short description Template:Good article Template:Anatomical terminology series
Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provides a definition of what is at the front ("anterior"), behind ("posterior") and so on. As part of defining and describing terms, the body is described through the use of anatomical planes and axes.
The meaning of terms that are used can change depending on whether a vertebrate is a biped or a quadruped, due to the difference in the neuraxis, or if an invertebrate is a non-bilaterian. A non-bilaterian has no anterior or posterior surface for example but can still have a descriptor used such as proximal or distal in relation to a body part that is nearest to, or furthest from its middle.
International organisations have determined vocabularies that are often used as standards for subdisciplines of anatomy. For example, Terminologia Anatomica, Terminologia Neuroanatomica, and Terminologia Embryologica for humans and Nomina Anatomica Veterinaria for animals. These allow parties that use anatomical terms, such as anatomists, veterinarians, and medical doctors, to have a standard set of terms to communicate clearly the position of a structure.
IntroductionEdit
Standard anatomical terms of location have been developed, usually based on Latin and Greek words, to enable all biological and medical scientists, veterinarians, medical 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.<ref name="Dyce2010">Template:Cite book</ref><ref name="Gray's">Template:Cite book</ref> Much of this information has been standardised in internationally agreed vocabularies for humans (Terminologia Anatomica, Terminologia Neuroanatomica, and Terminologia Embryologica),<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Fipat">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> with Nomina Anatomica Veterinaria and Nomina Embryologica Veterinaria used for animal anatomy.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Different terms are used for those vertebrates that are bipedal and those that are quadripedal.<ref name="Dyce2010"/> The reasoning is that the neuraxis, and therefore the standard anatomical position is different between the two groups.<ref name="Gray's"/> Unique terms are also used to describe invertebrates, because of their wider variety of shapes and symmetries.<ref name="Kardong2019">Template:Cite book</ref>
Standard anatomical positionEdit
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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, even when its appendages are in another position. This helps to avoid confusion in terminology when referring to the same animal in different postures.<ref name="libre3">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In humans, this refers to the body in a standing position with arms at the side and palms facing forward.<ref name="Gray2016">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="libre3"/> In quadrupeds this is an animal standing upright with all four feet on the ground and the head facing forward.<ref>Template:Cite book</ref> For a fish this is belly down with neutral appendages. <ref>Template:Cite book</ref>
PlanesEdit
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Anatomical terms describe structures with relation to four main anatomical planes.<ref name="Gray2016"/> Anatomical planes are useful in a number of fields including medical imaging, embryology, and the study of movement.<ref name="libre2025">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
The four main planes are:
- The median plane, also called the midsagittal plane, divides the body into left and right parts starting from the sagittal suture.<ref name="Gray2016"/> The median plane passes through the head, spinal cord, navel and, in many animals, the tail.<ref name="Hyman">Template:Cite book</ref>
- The sagittal planes, also called the parasagittal planes or paramedian planes, are planes that are parallel to the median plane.<ref name="Dyce2010"/><ref name="Moore1" />
- The dorsal plane divides the body into dorsal (towards the backbone) and ventral (towards the belly) parts.<ref>Template:Cite book</ref> In human anatomy coronal plane is preferred, or sometimes the frontal plane, and the description may reference splitting the body into front and back parts<ref name="Gray2016"/>, but this phrasing is not as clear for animals with a horizontal spine like quadrupeds or fish.
- The transverse plane, also called the axial plane or horizontal plane, is perpendicular to the other two planes.<ref name="Gray2016"/> In an animal with a vertical craniocaudal axis, like a human, this plane is parallel to the ground; in an animal with a horizontal craniocaudal axis, like a quadruped, the coronal plane divides the animal into anterior and posterior sections.<ref name="Kardong2019"/>
Sagittal planes and transverse planes are used as anatomical lines to delineate bodily regions. There are several transverse planes with clinical relevance in the division of the torso into sections. They include the transpyloric plane, the subcostal plane, and the transumbilical plane.<ref name="medscape2025">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
AxesEdit
The three axes of a vertebrate, are formed in embryonic development before and during the gastrulation stage.<ref name="Abas2022">Template:Cite journal</ref> Distinct ends of the embryo are chosen, and the axis is named according to those directions. The three main axes of a bilaterally symmetrical animal that intersect at right angles, are the left-right, the craniocaudal, and the anteroposterior axes.<ref name="Abas2022"/><ref name="Kardong2019"/>
- The left-right axis, also known as the horizontal or frontal axis<ref name="Abas2022"/>
- The craniocaudal axis, also known as the rostrocaudal, longitudinal or cephalocaudal<ref name="Abas2022"/>
- The anteroposterior axis, also known as the dorsoventral, or sagittal axis<ref name="UNSW2025">{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref><ref name="amigo2">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
An organism that is round, or asymmetrical may have different axes.<ref name="Kardong2019"/>
Main termsEdit
Template:Anchor Template:See also
Superior and inferiorEdit
In the standard human anatomical position, superior (Template:Ety) or cranial, describes something that is nearer to the head, and inferior (Template:Ety) or caudal describes what is below, and nearer to the feet.<ref name="libre3"/> Examples are the superior mediastinum, and inferior mediastinum. Neuroanatomy examples are the superior colliculus, and the inferior colliculus.<ref name="Moore1">Template:Cite book</ref> In veterinary anatomy, the terms superior and inferior are not used except to describe the eye, eyelids, lips and inner ear, using instead dorsal and ventral.<ref name="Dyce2010"/>
Anterior and posteriorEdit
Anterior (Template:Ety) describes what is in front, and posterior (Template:Ety) describes what is to the back of something.<ref name="Purves1">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> For example, for many fish the gill openings are posterior to the eyes and anterior to the tail.
Dorsal and ventralEdit
These two terms, used in veterinary anatomy, are also used in human anatomy mostly in neuroanatomy, and embryology, to describe something at the back (dorsal, posterior) or front (ventral, anterior) of an organ, or organism.<ref name="Gray2016"/><ref name="Purves1"/>
The dorsal (Template:Ety) surface, (also dorsum) of an organism or organ, refers to the back, or upper side, such as in the human, the dorsum of the tongue, the dorsum of the hand, and the dorsum of the foot. If talking about the skull, the dorsal side is the top.<ref name="amigo2"/><ref name="Moore1"/>
Template:Anchor The ventral (Template:Ety) surface refers to the front, or lower side, of an organism, or organ such as the undersurface of the tongue.<ref name="amigo2"/>
In a fish, the dorsal fin is on the upper surface and its ventral fins (pelvic fins) are on the belly or undersurface.<ref name="SCDNR">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
The terms are used in other contexts, for example in dorsal and ventral gun turrets on a bomber aircraft.
Medial and lateralEdit
These terms describe how close something is to the median plane.<ref name="Gray's"/><ref name="Purves1"/> Lateral (Template:Ety) describes something to the sides of an animal, as in "left lateral" and "right lateral". Medial (Template:Ety) describes structures close to the median plane, or closer to the median plane than another structure.<ref name="Purves1"/> For example, in a human, the arms are lateral to the torso. The genitals are medial to the legs. Temporal has a similar meaning to lateral but is restricted to the head.
The terms "left" and "right", or sinistral and dextral, refer to the halves of a bilaterally symmetrical body divided by the median plane.
Terms derived from lateral include:
- Contralateral (Template:Ety): 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 (Template:Ety): on the same side as another structure. For example, the left arm is ipsilateral to the left leg.<ref name="Moore1"/>
- Bilateral (Template:Ety): on both sides of the body.<ref name="Moore1"/> For example, bilateral orchiectomy means removal of testes on both sides of the body.
- Unilateral (Template:Ety) one-sided or single-sided: on one side of the body.<ref name="Moore1"/> For example, unilateral deafness is hearing impairment in one ear.<ref name="Dodson">Template:Cite journal</ref>
Varus (Template:Ety) and valgus (Template:Ety ) are terms used to describe angulation or bowing of a bone or joint within the coronal plane, where the distal portion deviates towards (varus) or away from (valgus) the midline.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Proximal and distalEdit
The terms proximal (Template:Ety) and distal (Template:Ety) are used to describe parts of a feature that are close to or distant from the main mass of the body, respectively.<ref name="Seer2025">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Thus the upper arm in humans is proximal and the hand is distal. The main mass is taken as the center, the chest, or the heart.<ref name="meaning"/>
"Proximal and distal" are frequently used when describing appendages, such as fins, tentacles, and limbs. 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.<ref name="meaning">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
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.<ref>Template:Cite journal </ref>
Rostral, cranial, and caudalEdit
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 (Template:Ety) describes something situated toward the oral or nasal region, or in the case of the brain, toward the tip of the frontal lobe.<ref name="Moore1"/><ref name="Purves1"/>
- Cranial (Template:Ety) or cephalic (Template:Ety) describes how close something is to the head of an organism.<ref name="Moore1"/>
- Caudal (Template:Ety) describes how close something is to the trailing end of an organism.<ref name="Purves1"/>
These terms are generally preferred in veterinary medicine and not used as often in human medicine.<ref name="Hickman et al. 2003">Hickman, C. P. Jr., Roberts, L. S. and Larson, A. Animal Diversity. McGraw-Hill 2003 Template:ISBN</ref><ref name="Miller 2002">Miller, S. A. General Zoology Laboratory Manual McGraw-Hill, Template:ISBN and Template:ISBN</ref> For example, in horses, the eyes are caudal to the nose and rostral to the back of the head.<ref name="Dyce2010"/>
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 gross anatomy and refers more to the front of the face than the superior aspect of the organism. But it is used in embryology, and neuroanatomy. Similarly, the term "caudal" is used more in embryology and neuroanatomy, and only occasionally in human gross anatomy.<ref name="Gray's"/> The "rostrocaudal axis" refers to the curved line of the neuraxis from the forehead (rostral) towards the tail end (caudal).
Central and peripheralEdit
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.
Central (Template:Ety) describes something at, or close to the centre.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> For example, the great vessels run centrally through the body; many smaller vessels branch from these.
Peripheral (Template:Ety, originally from Ancient Greek) describes something that is situated nearer to the body's surface, such as a peripheral nerve.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Superficial and deepEdit
These terms refer to the distance of a structure from the surface.<ref name="Gray's"/><ref name="libre4">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Deep (Template:Ety) describes something further away from the surface of the organism.<ref name="libre4"/> 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" (Template:Ety).<ref name="Dyce2010"/>
Superficial (Template:Ety) describes something near the outer surface of the organism.<ref name="Dyce2010"/> For example, in skin, the epidermis is superficial to the subcutis.<ref name="libre4"/>
Combined termsEdit
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), or to a named organ such as the anterolateral tibial tubercle.<ref name="Moore2018">Template:Cite book</ref> The term can also describe the direction and location of something that enters or courses through the body such as the anterolateral system in the spinal cord, and the anterolateral central arteries.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Another term anteromedial is used for example in the anteromedial central arteries.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
In the more internal brain and spinal cord of the central nervous system the terms dorsal and ventral and their combinations are often used in place of anterior and posterior. In these organs numerous references need to be used, and in the brain for example the prefrontal cortex has the divisions of the dorsomedial prefrontal cortex, and the dorsolateral prefrontal cortex. And the dorsomedial region has subcompartments that make use of other terms such as the anterior cingulate cortex, and infralimbic cortex. Structures such as the anterior cingulate cortex may be divided anatomically based on cognitive (dorsal), and emotional (ventral) components.<ref name="Bush00">Template:Cite journal</ref>
Proximodistal is the axis of an appendage such as an arm or a leg, taken from its tip at the distal part to where it joins the body at the proximal part.<ref name="UNSW2025"/>
In radiology, various X-ray views uses terminology based on where the X-ray beam enters and leaves the body, including the front to back view (anteroposterior), the back to front view (posteroanterior), and the side view (lateral).<ref>Template:Cite book</ref> Combined terms were once generally hyphenated, but typically the hyphen is omitted.<ref>Template:Cite dictionary</ref>
ModifiersEdit
Several terms are commonly seen and used as prefixes:<ref name="Terms2025">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
- Sub- (Template:Ety) is used to indicate something that is beneath, or something that is subordinate to or lesser than.<ref name="Terms2025"/> For example, subcutaneous means beneath the skin.
- Hypo- (Template:Ety) is used to indicate something that is beneath.<ref name="Terms2025"/> For example, the hypoglossal nerve supplies the muscles beneath the tongue.
- Infra- (Template:Ety) is used to indicate something that is within or below. For example, the infraorbital nerve runs within the orbit.
- Inter- (Template:Ety) is used to indicate something that is between.<ref name="Terms2025"/> For example, the intercostal muscles run between the ribs.
- Super- or Supra- (Template:Ety) is used to indicate something that is above something else.<ref name="Terms2025"/> For example, the supraorbital ridges are above the eyes.
- Ab- (Template:Ety), and ad- (Template:Ety) are used to indicate that something is towards (ad-) or away from (ab-) something else.<ref name="Terms2025"/> For example abduction and adduction refer to muscular movement away from, and towards the midline of the body, respectively.
Other terms are used as suffixes, added to the end of words:
- -al (Template:Ety) For example femoral neck.
- -ad (Template:Ety), equivalent to '-ally', is a suffix createing the adverb form to indicate that something moves towards (-ad) something else.<ref name= "GGHDHD">Template:Cite book</ref> For example, "distad" means "in the distal direction,"<ref name="MW2025c">{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref> as in "arterial blood flows distad/distally." Further examples may include cephalad (towards the cephalic end), orad, craniad, and proximad. The terms "proximally" and "distally" are in more common use in human and veterinary anatomic textbooks, while "proximad" and "distad," are used commonly in insect anatomy.<ref name="Gray's"/><ref name="Dyce2010"/><ref name= "GGHDHD"/>
Other terms and special casesEdit
Anatomical landmarksEdit
Template:See also The location of anatomical structures can also be described in relation to different anatomical landmarks used in anatomy, surface anatomy, surgery, and radiology.<ref name="Butler">Template:Cite book</ref>
Structures may be described as being at the level of a specific vertebra, depending on the section of the vertebral column the structure is at.<ref name="Butler" /> 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 the location.
References may also take origin from surface anatomy, made to landmarks that are on the skin or visible underneath.<ref name="Butler" /> For example, structures may be described relative to the anterior superior iliac spine, the medial malleolus or the medial epicondyle.
Anatomical lines are theoretical lines, using either horizontal transverse planes, or vertical sagittal planes, used to describe anatomical location. For examples, the mid-clavicular line is used as part of the cardiac examination to feel the apex beat of the heart, and the axillary lines are reference lines for the underarm region. Other types of lines in anatomy include the curved nuchal lines on the occipital bone, and the gluteal lines on the ilium.
Mouth and teethEdit
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Special terms are used to describe the mouth and teeth.<ref name="Gray's"/> Fields such as osteology, paleontology 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.<ref name="Folkens2000">Template:Cite book</ref><ref name=SmithDodson2003>Template:Cite journal</ref> For example, the terms "distal" and "proximal" (or "mesial") are used for surfaces of individual teeth relative to the midpoint of the dental arch, and "medial" and "lateral" are used in the standard sense relative to the median plane.<ref name=Rajkumar2017>Template:Cite book</ref> Terms used to describe structures include "buccal" (Template:Ety) and "palatal" (Template:Ety) referring to structures close to the cheek and hard palate respectively.<ref name=Rajkumar2017 />
Hands and feetEdit
Several anatomical terms are particular to the hands and feet.<ref name="Gray's"/> Additional terms may be used to avoid confusion when describing the surfaces of the hand and what is the "anterior" or "posterior" surface. The term "anterior", while anatomically correct, can be confusing when describing the palm of the hand; Similarly is "posterior", used to describe the back of the hand and arm. This confusion can arise because the forearm can pronate and supinate and flip the location of the hand. For improved clarity, the directional term palmar (Template:Ety) is commonly used to describe the front of the hand, and dorsal is the back of the hand. 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.
In humans, volar can also be used synonymously with palmar to refer to the palm of the hand, and can also be used to refer to the sole of the foot.<ref name="Collins3">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> But palmar is used exclusively for the palm of the hand, and plantar is used exclusively for the sole of the foot.<ref name="Collins3"/><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Similarly, in the limbs for clarity, the sides are named after the bones. In the forearm, 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, such as the distal radioulnar joint.<ref name="Dyan">Template:Cite journal</ref> Similarly, in the lower leg, structures near the tibia (shinbone) are tibial and structures near the fibula are fibular (or peroneal).
Rotational directionEdit
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.<ref name="Collins2025a">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref name="Collins2025b">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
- Anteversion (Template:Ety) describes an anatomical structure being tilted further forward than normal, whether pathologically or incidentally.<ref name="Collins2025a"/> 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.<ref name="Zhou2024">Template:Cite journal</ref>
- Retroversion (Template:Ety) describes an anatomical structure tilted back away from something.<ref name="Collins2025b"/> An example is a retroverted uterus.<ref name="Collins2025b"/>
Other directional termsEdit
Several other terms are also used to describe location. These terms are not used to form the fixed axes. Terms include:
- Axial (Template:Ety): around the central axis of the organism or the extremity.<ref>{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref> Two related terms, "abaxial" and "adaxial", refer to locations away from and toward the central axis of an organism, respectively<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
- Luminal (Template:Ety): on the—hollow—inside of an organ's lumen (body cavity or tubular structure);<ref>{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> adluminal is towards, abluminal is away from the lumen.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Opposite to outermost (the adventitia, serosa, or the cavity's wall).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
- Terminal (Template:Ety) at the extremity of a usually projecting structure; forming the end of a structure such as an axon terminal.<ref>{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref>
- Visceral (Template:Ety): associated with the innermost layer of an organ within the body. For example, the visceral pleura covering the lungs, contrasted with the parietal pleura lining the thoracic cavity.<ref name="radiopedia">{{#invoke:citation/CS1|citation
|CitationClass=web }}</ref>
- Parietal (Template:Ety): pertaining to the wall of a body cavity as the parietal pleura lining the thoracic cavity, contrasted with visceral pleura.<ref name="radiopedia"/>
- Aboral (away from oral) is used to denote a location in an organism that is further from the mouth.
Other animals and organismsEdit
Different terms are used because of different body plans in animals, whether animals stand on two or four legs, and whether an animal is symmetrical or asymmetrical. For example, as humans are bilaterally symmetrical, anatomical descriptions usually use the same terms as those for other vertebrates.<ref name="Hyman"/> However, the standard human anatomical position means that their anterior/posterior and ventral/dorsal directions are the same, so the inferior/superior directions are used due to longstanding tradition instead of cranial/caudal, which apply regardless of position, as in other species.<ref name= "Latin Tucker">Template:Cite book</ref> The term "rostral" used to refer to the beak or nose in some animals is used less frequently in humans, with the exception of parts of the brain;<ref name="Purves1"/> while humans do not have a visible tail (the coccygeal vertebrae are present and commonly called the "tailbone") the term "caudal" that refers to the tail-end is also sometimes used in humans and animals without tails to refer to the hind part of the body.<ref name="Purves1"/> Flounder and other flatfish which lie on the seabed on their left or right side are asymmetric, with both eyes on the 'up' side, making anatomical nomenclature a challenge.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Invertebrates have a large variety of body shapes that can present a problem when trying 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, meaning "near" refers to the part of an appendage nearest to where it joins the body, and distal, 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.
- Direcoes anatomicas.svg
Anatomical terms of location in a dog
- Anatomical-directions-kangaroo.svg
Anatomical terms of location in a kangaroo
- Anatomical Directions and Axes.JPG
Anatomical terms of location in most fish
- Horse Axes.JPG
Anatomical terms of location in a horse
- Pseudopleuronectes americanus.jpg
Flatfish are asymmetric, with both eyes lying on the same side of the head.
Non-bilaterian organismsEdit
In non-bilaterian 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 radially 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.
Elongated organismsEdit
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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.<ref name="Ruppert2004">Template:Cite book</ref>
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.<ref name="Ruppert2004" /> 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).<ref>Template:Cite book</ref>
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Organisms that are attached to a substrate, such as sponges and animal-like protists also have distinctive ends. The part of the organism attached to the substrate is usually referred to as the basal end (Template:Ety), whereas the end furthest from the attachment is referred to as the apical end (Template:Ety).
Radially symmetrical organismsEdit
Radially symmetrical organisms include those in the group RadiataTemplate:Snd primarily Cnidarians (jellyfish, sea anemones and corals, and the comb jellies).<ref name="Ruppert2004B">Template:Cite book</ref> Adult echinoderms, such as starfish, sea urchins, sea cucumbers and others are also included, since they have a pentamerous symmetry having five discrete symmetric parts arranged around a central axis.<ref name="Ruppert2004A">Template:Cite book</ref> Echinoderm larvae are not included, since they are bilaterally symmetrical.<ref name="Ruppert2004A"/>
Cnidarians have an incomplete digestive system, meaning that one end of the organism has a mouth, the oral end (Template:Ety), and the opposite aboral end (Template:Ety) has no opening from the gut (coelenteron).<ref name="Ruppert2004B"/> They are radially symmetric around the oral-aboral axis.<ref name="Ruppert2004B"/> Having only the single distinctive axis, "lateral", "dorsal", and "ventral" have no meaning, and all can be replaced by the generic term peripheral (Template:Ety). 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.<ref name="Oliveira2007">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
Comb jellies have a biradial symmetry about only two planes, a tentacular plane, and a pharyngeal plane.<ref name="Ruppert2004C">Template:Cite book</ref>
- Radiate Radial Axes.JPG
Aurelia aurita, another species of jellyfish, showing multiple radial and medio-peripheral axes
- Porania pulvillus.jpg
The sea star Porania pulvillus, aboral and oral surfaces
SpidersEdit
Special terms are used for spiders. Two such terms are useful in describing views of the legs and pedipalps of spiders, and other arachnids. 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.<ref name="Kaston1972">Template:Cite book</ref> Most spiders 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.<ref name=FOELIX2011>Template:Cite book</ref> Usually, the eyes are arranged in two roughly parallel, horizontal and symmetrical rows of eyes.Template:R Eyes are labelled according to their position as anterior and posterior lateral eyes (ALE) and (PLE); and anterior and posterior median eyes (AME) and (PME).Template:R
- Palystes superciliosus male anterior 2012 03 04 3452.JPG
Aspects of spider anatomy. This aspect shows the mainly prolateral surface of the anterior femora, plus the typical horizontal eye pattern of the Sparassidae.
- Hogna species female Lycosidae showing eye pattern EOS 027s.jpg
Typical arrangement of eyes in the Lycosidae, with PME being the largest
- Salticidae Male Anterior annotated.jpg
In the Salticidae the AME are the largest.