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Standardanatomical terms of location are used to describe unambiguously theanatomy ofanimals, includinghumans. The terms, typically derived fromLatin orGreek roots, describe something in itsstandard 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 ofanatomical planes andanatomical axes.
The meaning of terms that are used can change depending on whether an animal isbipedal orquadrupedal. Additionally, forinvertebrates, some terms may not have any meaning at all; for example, an animal that isradially 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, andTerminologia Embryologica for humans andNomina Anatomica Veterinaria for animals. These allow parties that use anatomical terms, such asanatomists,veterinarians, andmedical doctors, to have a standard set of terms to communicate clearly the position of a structure.
Standardanatomical andzoological terms of location have been developed, usually based on Latin andGreek words, to enable all biological and medical scientists,veterinarians,doctors andanatomists 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.[1][2] Much of this information has been standardised in internationally agreed vocabularies for humans (Terminologia Anatomica,[2] andTerminologia Embryologica),[3] and for animals (Nomina Anatomica Veterinaria).[1]
Different terms are used for thosevertebrates that arebipedal and those that arequadripedal.[1] The reasoning is that theneuraxis is different between the two groups, and so is what is considered thestandard anatomical position, such as how humans tend to be standing upright and with their arms at their sides, palms forward.[2] Unique terms are also used to describeinvertebrates, because of their wider variety of shapes andsymmetries.[4]
Becauseanimals can change orientation with respect to their environment, and becauseappendages likelimbs andtentacles can change position with respect to the main body, terms to describe position need to refer to an animal when it is in itsstandard 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.[5] In humans, this refers to the body in a standing position with arms at the side and palms facing forward.[6][5]
Anatomical terms describe structures with relation to four mainanatomical planes.[6] Anatomical planes are useful in a number of fields includingmedical imaging,embryology, and the study of movement.[7]
The four main planes are:
Sagittal planes and transverse planes are used asanatomical lines to delineatebodily regions. There are several transverse planes with clinical relevance in the division of the torso into sections. They include thetranspyloric plane, thesubcostal plane, and thetransumbilical plane.[10]
The threeaxes of a vertebrate, are formed inembryonic development before and during thegastrulation stage.[11] Distinct ends of the embryo are chosen, and the axis is named according to those directions. The three main axes of abilaterally symmetrical animal that intersect at right angles, are the left-right, the craniocaudal, and the anteroposterior axes.[11][4]
An organism that is round, orasymmetrical may have different axes.[4]
In thestandard human anatomical position,superior (from Latin super 'above') orcranial, describes something that is nearer to the head, andinferior (from Latin inferus 'below') orcaudal describes what is below, and nearer to the feet.[5] Examples are thesuperior mediastinum, andinferior mediastinum. Neuroanatomy examples are thesuperior colliculus, and theinferior colliculus.[9]
Anterior (from Latin ante 'before') describes what is in front, andposterior (from Latin post 'after') describes what is to the back of something.[14] For example, for manyfish thegill openings are posterior to the eyes and anterior to the tail.
These two terms, are also used inhuman anatomy (mostly inneuroanatomy),embryology, andzoology, to describe something at the back (dorsal, posterior) or front/belly (ventral, anterior) of an organ, or organism.[6][14]
Thedorsal (from Latin dorsum 'back') surface, (alsodorsum) of an organism ororgan, refers to the back, or upper side, such as in the human, the dorsum of thetongue, thedorsum of the hand, and thedorsum of the foot. If talking about the skull, the dorsal side is the top.[13][9]
Theventral (from Latin venter 'belly') surface refers to the front, or lower side, of an organism, or organ such as the undersurface of the tongue.[13]
In afish, thedorsal fin is on the upper surface and itsventral fins (pelvic fins) are on the belly or undersurface.[15]
The terms are used in other contexts, for example indorsal and ventral gun turrets on a bomber aircraft.
These terms describe how close something is to the medial plane.[2][14]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 median plane, or closer to the median plane than another structure.[14] For example, in a human, the arms are lateral to thetorso. Thegenitals are medial to the legs.Temporal has a similar meaning to lateral but is restricted to the head.
The terms "left" and "right", orsinistral and dextral, refer to the halves of a bilaterally symmetrical body divided by the median plane.
Terms derived from lateral include:
Varus (from Latin 'bow-legged') andvalgus (from Latin 'knock-kneed' ) are terms used to describe a state in which a body part is abnormally placed towards (varus) or away from (valgus) the midline.[17]
The termsproximal (from Latin proximus 'nearest') anddistal (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.[18] Thus the upper arm in humans is proximal and the hand is distal. The main mass is taken as the center, thechest, or theheart.[19]
"Proximal and distal" are frequently used when describingappendages, such asfins,tentacles, andlimbs. 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.[19]
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 overallmoiety of a given compound.[20]
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:
These terms are generally preferred in veterinary medicine and not used as often in human medicine.[21][22] For example, inhorses, the eyes are caudal to the nose and rostral to the back of the head.[1]
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 humangross anatomy and refers more to the front of the face than the superior aspect of the organism. But it is used inembryology, andneuroanatomy. Similarly, the term "caudal" is used more in embryology and neuroanatomy, and only occasionally in human gross anatomy.[2] The "rostrocaudal axis" refers to the curved line of theneuraxis from the forehead (rostral) towards the tail end (caudal).
Central and peripheral refer to the distance towards and away from the centre of something.[23] That might be an organ, a region in the body, or an anatomical structure. For example, thecentral nervous system and theperipheral nervous systems.
Central (from Latin centralis) describes something close to the centre.[23] For example, thegreat vessels run centrally through the body; many smaller vessels branch from these.
Peripheral (from Latin peripheria, originally fromAncient Greek) describes something further away from the centre of something.[24] For example, the arm is peripheral to the body.
These terms refer to the distance of a structure from the surface.[2][25]
Deep (from Old English) describes something further away from the surface of the organism.[25] For example, theexternal oblique muscle of the abdomen is deep to the skin. "Deep" is one of the few anatomical terms of location derived fromOld English rather than Latin – the anglicised Latin term would have been "profound" (from Latin profundus 'due to depth').[1]
Superficial (from Latin superficies 'surface') describes something near the outer surface of the organism.[1] For example, inskin, theepidermis is superficial to thesubcutis.[25]
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 thepectoralis major muscle), or to a named organ such as theanterolateral tibial tubercle.[26] The term can also describe the direction and location of something that enters or courses through the body such as theanterolateral system in thespinal cord, and theanterolateral central arteries.[27] Another termanteromedial is used for example in theanteromedial central arteries.[28]
In the more internalbrain andspinal cord of thecentral nervous system the termsdorsal 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 theprefrontal cortex has the divisions of thedorsomedial prefrontal cortex, and thedorsolateral prefrontal cortex. And the dorsomedial region has subcompartments that make use of other terms such as theanterior cingulate cortex, andinfralimbic cortex. Structures such as the anterior cingulate cortex may be divided anatomically based on cognitive (dorsal), and emotional (ventral) components.[29]
Proximodistal is the axis of anappendage 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.[12]
Inradiology, variousX-ray views usesterminology 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).[30] Combined terms were once generally hyphenated, but typically the hyphen is omitted.[31]
Several terms are commonly seen and used asprefixes:[32]
Other terms are used assuffixes, added to the end of words:
The location of anatomical structures can also be described in relation to differentanatomical landmarks used in anatomy,surface anatomy, surgery, and radiology.[35]
Structures may be described as being at the level of a specificspinal vertebra, depending on the section of thevertebral column the structure is at.[35] The position is often abbreviated. For example, structures at the level of the fourthcervical vertebra may be abbreviated as "C4", at the level of the fourththoracic vertebra "T4", and at the level of the thirdlumbar vertebra "L3". Because thesacrum 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.[35] For example, structures may be described relative to theanterior superior iliac spine, themedial malleolus or themedial 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 thecardiac examination to feel theapex beat of theheart, and theaxillary lines are reference lines for theunderarm region. Other types of lines in anatomy include the curvednuchal lines on theoccipital bone, and thegluteal lines on theilium.
Special terms are used to describe the mouth and teeth.[2] Fields such asosteology,paleontology anddentistry 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.[36][37] For example, the terms "distal" and "proximal" (or "mesial") are used for surfaces of individual teeth relative to the midpoint of thedental arch, and "medial" and "lateral" are used in the standard sense relative to the median plane.[38] Terms used to describe structures include "buccal" (from Latin bucca 'cheek') and "palatal" (from Latin palatum 'palate') referring to structures close to thecheek andhard palate respectively.[38]
Several anatomical terms are particular to the hands and feet.[2] 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 thepalm of the hand; Similarly is "posterior", used to describe the back of the hand and arm. This confusion can arise because the forearm canpronate andsupinate and flip the location of the hand. For improved clarity, the directional termpalmar (from Latin palma 'palm of the hand') is commonly used to describe the front of the hand, anddorsal is the back of the hand. Thepalmar fascia ispalmar to thetendons of muscles which flex the fingers, and thedorsal venous arch is so named because it is on the dorsal side of the foot.
In humans,volar can also be used synonymously withpalmar to refer to thepalm of the hand, and can also be used to refer to thesole of the foot.[39] Butpalmar is used exclusively for the palm of the hand, andplantar is used exclusively for the sole of the foot.[39][40]
Similarly, in thelimbs for clarity, the sides are named after the bones. In theforearm structures closer to theradius areradial, structures closer to theulna areulnar, and structures relating to both bones are referred to asradioulnar, such as thedistal radioulnar joint.[41] Similarly, in thelower leg, structures near thetibia (shinbone) aretibial and structures near thefibula arefibular (orperoneal).
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Anteversion andretroversion 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 theuterus.[42][43]
Several other terms are also used to describe location. These terms are not used to form the fixed axes. Terms include:
Different terms are used because of differentbody plans in animals, whether animals stand on two or four legs, and whether an animal is symmetrical or asymmetrical. For example, as humans arebilaterally symmetrical, anatomical descriptions usually use the same terms as those for other vertebrates.[8] 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.[53] 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;[14] while humans do not have a visible tail (thecoccygeal 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.[14]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.[54]
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.
In organisms with a changeable shape, such asamoeboid organisms, most directional terms are meaningless, since the shape of the organism is not constant and no distinct axes are fixed. Similarly, inspherically 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 assuperficial anddeep, or sometimesproximal anddistal, are usefully descriptive.
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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. Thelong orlongitudinal axis is defined by points at the opposite ends of the organism. Similarly, a perpendiculartransverse 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 areplanktonic (free-swimming)protists, and are nearly always viewed onmicroscope slides, where they appear essentially two-dimensional. In some cases a third axis can be defined, particularly where a non-terminalcytostome or other unique structure is present.[55]
Some elongatedprotists have distinctive ends of the body. In such organisms, the end with a mouth (or equivalent structure, such as thecytostome inParamecium orStentor), or the end that usually points in the direction of the organism'slocomotion (such as the end with theflagellum inEuglena), is normally designated as theanterior end. The opposite end then becomes theposterior end.[55] Properly, this terminology would apply only to an organism that is alwaysplanktonic (not normally attached to a surface), although the term can also be applied to one that issessile (normally attached to a surface).[56]
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Organisms that are attached to asubstrate, such assponges andanimal-like protists also have distinctive ends. The part of the organism attached to the substrate is usually referred to as thebasal end (from Latin basis 'support/foundation'), whereas the end furthest from the attachment is referred to as theapical end (from Latin apex 'peak/tip').
Radially symmetrical organisms include those in the groupRadiata – primarilyCnidarians (jellyfish,sea anemones andcorals, and thecomb jellies).[57] Adultechinoderms, such asstarfish,sea urchins,sea cucumbers and others are also included, since they have apentamerous symmetry having fivediscrete symmetric parts arranged around a central axis.[58] Echinodermlarvae are not included, since they arebilaterally symmetrical.[58]
Cnidarians have an incomplete digestive system, meaning that one end of the organism has a mouth, theoral end (from Latin ōrālis 'of the mouth'), and the oppositeaboral end (from Latin ab- 'away from') has no opening from the gut (coelenteron).[57] They are radially symmetric around theoral-aboral axis.[57] Having only the single distinctive axis, "lateral", "dorsal", and "ventral" have no meaning, and all can be replaced by the generic termperipheral (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 possibleradial axes andmedio-peripheral (half-)axes.[59]
Comb jellies have a biradial symmetry about only two planes, a tentacular plane, and a pharyngeal plane.[60]
Special terms are used forspiders. Two such terms are useful in describing views of the legs andpedipalps of spiders, and otherarachnids.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.[61] Most spiders have eight eyes in four pairs. All the eyes are on thecarapace of theprosoma, and their sizes, shapes and locations are characteristic of various spider families and othertaxa.[62] Usually, the eyes are arranged in two roughly parallel, horizontal and symmetrical rows of eyes.[62] 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).[62]
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| Bacteria and fungi | ||
| Protists | ||
| Plants | ||
| Invertebrates | ||
| Mammals | ||
| Other vertebrates | ||
| Glossaries | ||
| Related topics | ||
