Anatomical terms of location
This position provides a definition of what is at the front ("anterior"), behind ("posterior") and so on.
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 standard vocabularies for subdisciplines of anatomy, for example, Terminologia Anatomica for humans, and Nomina Anatomica Veterinaria for animals.
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.
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.
Unique terms are used to describe animals without a backbone (invertebrates), because of their wide variety of shapes and symmetry.
Standard anatomical position
Main article: 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 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.
Main article: Anatomical plane
Anatomical terms describe structures with relation to four main anatomical planes:
- 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 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 a quadruped, this divides the animal into anterior and posterior sections.
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.
Example axes are:
- The anteroposterior axis
- The cephalocaudal axis
- The dorsoventral axis
Examples of axes in specific animals are shown below.
Several terms are commonly seen and used as prefixes:
- Sub- (from Latin sub '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
- Hypo- (from Ancient Greek ὑπό 'under') is used to indicate something that is beneath. For example, the hypoglossal nerve supplies the muscles beneath the tongue.
- Infra- (from Latin infra 'under') is used to indicate something that is within or below. For example, the infraorbital nerve runs within the orbit.
- Inter- (from Latin inter '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 '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 '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
For example, in the anatomical position, the most superior part of the human body is the head and the most inferior is the feet.
Anterior and posterior
"Anterior" redirects here.
For other uses, see Anterior (disambiguation).
Medial and lateral
These terms describe how close something is to the midline, or the medial plane.
Lateral (from Latin lateralis 'to the side') describes something to the sides of an animal, as in "left lateral" and "right lateral".
Medial (from Latin medius '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.
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 '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 'same'): on the same side as another structure. For example, the left arm is ipsilateral to the left leg.
- Bilateral (from Latin bis '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 '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 'knock-kneed') and valgus (from Latin 'bow-legged') 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
"Proximal" and "distal" redirect here.
For the linguistic terms, see Demonstrative § Distal and proximal demonstratives.
For the dental terms, see Glossary of dentistry.
The terms proximal (from Latin proximus 'nearest') and distal (from Latin distare '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.
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.
Central (from Latin centralis) describes something close to the centre.
For example, the great vessels run centrally through the body; many smaller vessels branch from these.
For example, the arm is peripheral to the body.
Superficial and deep
These terms refer to the distance of a structure from the surface.
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.
Superficial (from Latin superficies 'surface') describes something near the outer surface of the organism.
Dorsal and ventral
These two terms, used in anatomy and embryology, describe something at the back (dorsal) or front/belly (ventral) of an organism.
The dorsal (from Latin dorsum 'back') 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 (from Latin venter 'belly') surface refers to the front, or lower side, of an organism.
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 '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 'skull') or cephalic (from Greek κεφαλή 'head') describes how close something is to the head of an organism.
- Caudal (from Latin cauda '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
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.
The position is often abbreviated.
Because the sacrum and coccyx are fused, they are not often used to provide the location.
References may also take origin from superficial anatomy, made to landmarks that are on the skin or visible underneath.
Anatomical lines are used to describe anatomical location.
Mouth and teeth
Main article: Dental terminology
Special terms are used 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.
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.
Hands and feet
Several anatomical terms are particular to the hands and feet.
For improved clarity, the directional term palmar (from Latin palma 'palm of the hand') is usually used to describe the front of the hand, and dorsal is the back of the hand.
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.
Similarly, in the forearm, for clarity, the sides are named after the bones.
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.
- Axial (from Latin axis '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
- Luminal (from Latin lumen 'light, opening'): on the—hollow—inside of an organ's lumen (body cavity or tubular structure); adluminal is towards, abluminal is away from the lumen. Opposite to outermost (the adventitia, serosa, or the cavity's wall).
- Parietal (from Latin paries '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 '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 '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
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 dorsal/ventral directions the same, and the inferior/superior directions 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.
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.
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).
The part of the organism attached to the substrate is usually referred to as the basal end (from Latin basis 'support/foundation'), whereas the end furthest from the attachment is referred to as the apical end (from Latin apex 'peak/tip').
Radially symmetrical organisms
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).
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 '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.
Special terms are used for spiders.
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.
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).
- Geometric terms of location
- Proper right and proper left
- Reflection symmetry
- Sinistral and dextral
Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Anatomical terms of location.