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Standard terms for unambiguous description of relative placement of body parts

Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. 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 anatomical axes. The meaning of terms that are used can change depending on whether an organism is bipedal or quadrupedal. Additionally, for some animals such as invertebrates, some terms may not have any meaning at all; for example, an animal that is radially symmetrical will have no anterior surface, but can still have a description that a part is close to the middle ("proximal") or further from the middle ("distal"). International organisations have determined vocabularies that are often used as standards for subdisciplines of anatomy. For example, Terminologia Anatomica 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.

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 vocabularies for humans (Terminologia Anatomica) and animals (Nomina Anatomica Veterinaria). Different terms are used for groups of creatures with different body layouts, such as bipeds (creatures that stand on two feet, such as humans) and quadrupeds. The reasoning is that the neuraxis is different between the two groups, and so is what is considered the standard anatomical position, such as how humans tend to be standing upright and with their arms reaching forward. Thus, the "top" of a human is the head, whereas the "top" of a dog would be the back, and the "top" of a flounder may be on either the left or right side. Unique terms are also used to describe invertebrates as well, because of their wider variety of shapes and symmetry.

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 as if the organism is 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 passes from their source to patient's anterior body wall through the body to exit through posterior body wall. Combined terms were once generally hyphenated, but the modern tendency is to omit the hyphen.

Superior describes what is above something and inferior 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 describes what is in front, and posterior describes what is to the back of something. For example, for a dog the nose is anterior to the eyes and the tail is considered the most posterior part; for many fish the gill openings are posterior to the eyes but anterior to the tail.

The terms proximal and distal 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. "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. The terms are also applied to internal anatomy, such as to the reproductive tract of snails. Unfortunately, different authors use the terms in opposite senses. Some consider "distal" as further from a point of origin near the centre of the body and others as further from where the organ reaches the body's surface; or other points of origin may be envisaged. 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 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 describes something close to the centre. For example, the great vessels run centrally through the body; many smaller vessels branch from these. Peripheral (, originally from Ancient Greek) describes something further away from the centre of something. For example, the arm is peripheral to the body.

These terms refer to the distance of a structure from the surface. Deep 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"
Superficial describes something near the outer surface of the organism. For example, in skin, the epidermis is superficial to the subcutis.

These two terms, used in anatomy and embryology, describe something at the back (dorsal) or front/belly (ventral) of an organism. The dorsal surface of an organism refers to the back, or upper side, of an organism. If talking about the skull, the dorsal side is the top. The ventral surface refers to the front, or lower side, of an organism. For example, in a fish, the pectoral fins are dorsal to the anal fin, but ventral to the dorsal fin.

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 describes something situated toward the oral or nasal region, or in the case of the brain, toward the tip of the frontal lobe. Cranial or cephalic describes how close something is to the head of an organism. Caudal 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).

The location of anatomical structures can also be described in relation to different anatomical landmarks. They are used in anatomy, surface anatomy, surgery, and radiology. 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. For example, the terms "distal" and "proximal" are also redefined to mean the distance away or close to the dental arch, and "medial" and "lateral" are used to refer to the closeness to the midline of the dental arch. Terms used to describe structures include "buccal" and "palatal" referring to structures close to the cheek and hard palate respectively. adluminal is towards, abluminal is away from the lumen. Opposite to outermost (the adventitia, serosa, or the cavity's wall). Parietal : 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 at the extremity of a usually projecting structure. For example, " an antenna with a terminal sensory hair". Visceral and viscus : 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.

Different terms are used because of different body plans in animals, whether animals stand on one or two legs, and whether an animal is symmetrical or not, as discussed above. For example, as humans are approximately bilaterally symmetrical organisms, anatomical descriptions usually use the same terms as those for other vertebrates. However, humans stand upright on two legs, meaning their anterior/posterior and ventral/dorsal directions are the same, and the inferior/superior directions are necessary. Humans do not have a beak, so a term such as "rostral" used to refer to the beak in some animals is instead used to refer to part of the brain; humans do also not have a tail so a term such as "caudal" that refers to the tail end may also be used in humans and animals without tails to refer to the hind part of the body. In invertebrates, the large variety of body shapes 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, 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.

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. Organisms that are attached to a substrate, such as sponges, animal like protists also have distinctive ends. The part of the organism attached to the substrate is usually referred to as the basal end , whereas the end furthest from the attachment is referred to as the apical end .

Radially symmetrical organisms include those in the group Radiata primarily jellyfish, sea anemones and corals and the comb jellies. and are thus anatomically equivalent to bilaterally symmetrical animals.

Special terms are used for spiders. 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. 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. Usually, the eyes are arranged in two roughly parallel, horizontal and symmetrical rows of eyes. 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).