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Anatomical terms of location
Anatomical terms of location
Anatomical terms of location
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Anatomical terminology

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.

Introduction

[edit]
Because of differences in the way humans and other animals are structured, different terms are used according to the neuraxis and whether an animal is a vertebrate or invertebrate.

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.[1][2] Much of this information has been standardised in internationally agreed vocabularies for humans (Terminologia Anatomica, Terminologia Neuroanatomica, and Terminologia Embryologica),[3][4] with Nomina Anatomica Veterinaria and Nomina Embryologica Veterinaria used for animal anatomy.[5]

Different terms are used for those vertebrates that are bipedal and those that are quadrupedal.[1] The reasoning is that the neuraxis, and therefore the standard anatomical position is different between the two groups.[2] Unique terms are also used to describe invertebrates, because of their wider variety of shapes and symmetries.[6]

Standard anatomical position

[edit]
Main article: Standard anatomical position
A male and female human in the 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, even when its appendages are in another position. This helps to avoid confusion in terminology when referring to the same animal in different postures.[7] In humans, this refers to the body in a standing position with arms at the side and palms facing forward.[8][7] In quadrupeds this is an animal standing upright with all four feet on the ground and the head facing forward.[9] For a fish this is belly down with neutral appendages.[10]

Planes

[edit]
Main article: Anatomical plane
The standard anatomical planes of both a human and a goat displaying three anatomical planes:
  the median plane
  the transverse plane
  the coronal plane (dorsal plane in quadrupeds)

Anatomical terms describe structures with relation to three main anatomical planes.[8] Anatomical planes are useful in a number of fields including medical imaging, embryology, and the study of movement.[11]

The three main plane orientations are:

  • The sagittal planes, also called the parasagittal planes or paramedian planes, are planes that divide the body into left and right.[1][12] The central one of these is the median plane, also called the midsagittal plane, which passes through the head, spinal cord, navel and, in many animals, the tail.[13]
  • The coronal plane or frontal plane divides the body into front and back parts.[8] In quadrupeds this plane is termed the dorsal plane and divides the body into dorsal (towards the backbone) and ventral (towards the belly) parts.[14][6]
  • The transverse plane, also called the axial plane or horizontal plane, is perpendicular to the other two planes.[8]

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.[15]

Axes

[edit]
Anatomical axes in a human, similar for other orthograde bipedal vertebrates

The three axes of a vertebrate, are formed in embryonic development before and during the gastrulation stage.[16] 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.[16][6]

  • The left-right axis, also known as the horizontal or frontal axis[16]
  • The craniocaudal axis, also known as the rostrocaudal, longitudinal or cephalocaudal[16]
  • The anteroposterior axis, also known as the dorsoventral, or sagittal axis[17][18]

An organism that is round, or asymmetrical may have different axes.[6]

Main terms

[edit]

See also: Anatomical terminology § Standard terms

Superior and inferior

[edit]

In the standard human anatomical position, superior (from Latin super 'above') or cranial, describes something that is nearer to the head, and inferior (from Latin inferus 'below') or caudal describes what is below, and nearer to the feet.[7] Examples are the superior mediastinum, and inferior mediastinum. Neuroanatomy examples are the superior colliculus, and the inferior colliculus.[12] 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.[1]

Anterior and posterior

[edit]
"Anterior" redirects here. For other uses, see Anterior (disambiguation).

Anterior (from Latin ante 'before') describes what is in front, and posterior (from Latin post 'after') describes what is to the back of something.[19] For example, for many fish the gill openings are posterior to the eyes and anterior to the tail. In veterinary anatomy, these terms are reserved for some structures of the head, instead using cranial and caudal throughout the rest of the body.[14]

Dorsal and ventral

[edit]

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.[19]

The dorsal (from Latin dorsum 'back') 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.[18][12]

The ventral (from Latin venter 'belly') surface refers to the front, or lower side, of an organism, or organ such as the undersurface of the tongue.[18]

In a fish, the dorsal fin is on the upper surface and its ventral fins (pelvic fins) are on the belly or undersurface.[20]

The terms are used in other contexts, for example in dorsal and ventral gun turrets on a bomber aircraft.

Medial and lateral

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These terms describe how close something is to the median plane.[2][19] 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.[19] 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 (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.[12]
  • Bilateral (from Latin bis 'twice'): on both sides of the body.[12] For example, bilateral orchiectomy means removal of testes on both sides of the body.
  • Unilateral (from Latin unus 'one') one-sided or single-sided: on one side of the body.[12] For example, unilateral deafness is hearing impairment in one ear.[21]

Varus (from Latin 'bow-legged') and valgus (from Latin 'knock-kneed' ) 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.[22]

Proximal and distal

[edit]
"Proximal" and "distal" redirect here. For the linguistic terms, see Demonstrative § Distal and proximal demonstratives. For the dental terms, see Glossary of dentistry.
Anatomical directional reference
Look up proximal  or distal in Wiktionary, the free dictionary.

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.[23] 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.[24]

"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.[24]

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.[25]

Rostral, cranial, and caudal

[edit]
In the human skull, the terms rostral and caudal are adapted to the curved neuraxis of Hominidae, rostrocaudal meaning the region on C shape connecting rostral and caudal regions.

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.[12][19]
  • Cranial (from Greek κρανίον 'skull') or cephalic (from Greek κεφαλή 'head') describes how close something is to the head of an organism.[12]
  • Caudal (from Latin cauda 'tail') describes how close something is to the trailing end of an organism.[19]

These terms are generally preferred in veterinary medicine and not used as often in human medicine.[26][27] For example, in horses, 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 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.[2] The "rostrocaudal axis" refers to the curved line of the neuraxis from the forehead (rostral) towards the tail end (caudal).

Central and peripheral

[edit]

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 (from Latin centralis) describes something at, or close to the centre.[28] For example, the great vessels run centrally through the body; many smaller vessels branch from these.

Peripheral (from Latin peripheria, originally from Ancient Greek) describes something that is situated nearer to the body's surface, such as a peripheral nerve.[29]

Superficial and deep

[edit]

These terms refer to the distance of a structure from the surface.[2][30]

Deep (from Old English) describes something further away from the surface of the organism.[30] 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 'due to depth').[1]

Superficial (from Latin superficies 'surface') describes something near the outer surface of the organism.[1] For example, in skin, the epidermis is superficial to the subcutis.[30]

Combined terms

[edit]
Dorsolateral prefrontal cortex highlighted in dark green

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.[31] 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.[32] Another term anteromedial is used for example in the anteromedial central arteries.[33]

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.[34]

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.[17]

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).[35] Combined terms were once generally hyphenated, but typically the hyphen is omitted.[36]

Modifiers

[edit]
Terms can be modified with prefixes and suffixes. In this image showing the jellyfish species Chrysaora, the prefix 'ab-', is used to indicate something that is 'away from' the mouth, for example the aboral. Other terms are combined to indicate axes, such as proximodistal axis.

Several terms are commonly seen and used as prefixes:[37]

  • 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.[37] For example, subcutaneous means beneath the skin.
  • Hypo- (from Ancient Greek ὑπό 'under') is used to indicate something that is beneath.[37] 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.[37] 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.[37] For example, the supraorbital ridges are above the eyes.
  • Ab- (from Latin ab 'away'), and ad- (from Latin ad 'towards') are used to indicate that something is towards (ad-) or away from (ab-) something else.[37] 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 (from Latin al 'pertaining to, of the') For example femoral neck.
  • -ad (from Latin ad 'towards'), equivalent to '-ally', is a suffix createing the adverb form to indicate that something moves towards (-ad) something else.[38] For example, "distad" means "in the distal direction,"[39] 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.[2][1][38]

Other terms and special cases

[edit]

Anatomical landmarks

[edit]
See also: List of anatomical lines

The location of anatomical structures can also be described in relation to different anatomical landmarks used in anatomy, surface anatomy, surgery, and radiology.[40]

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.[40] 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.[40] 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 teeth

[edit]
Main article: Dental terminology

Special terms are used to describe the mouth and teeth.[2] 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.[41][42] 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.[43] Terms used to describe structures include "buccal" (from Latin bucca 'cheek') and "palatal" (from Latin palatum 'palate') referring to structures close to the cheek and hard palate respectively.[43]

Hands and feet

[edit]
"Plantar" redirects here. For the fictional frog family, see List of Amphibia characters.
Anatomical terms used to describe a human hand

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 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 (from Latin palma 'palm of the hand') 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.[44] But palmar is used exclusively for the palm of the hand, and plantar is used exclusively for the sole of the foot.[44][45]

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.[46] Similarly, in the lower leg, structures near the tibia (shinbone) are tibial and structures near the fibula are fibular (or peroneal).

Rotational direction

[edit]
Image showing an anteverted uterus lying above the bladder (above), compared with a retroverted uterus undergoing bimanual examination facing towards the rectum (below)

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.[47][48]

  • Anteversion (from Latin anteversus) describes an anatomical structure being tilted further forward than normal, whether pathologically or incidentally.[47] For example, a person's uterus typically is anteverted, tilted slightly forward. A misaligned pelvis may be anteverted, that is to say tilted forward to some relevant degree.[49]
  • Retroversion (from Latin retroversus) describes an anatomical structure tilted back away from something.[48] An example is a retroverted uterus.[48]

Other directional terms

[edit]

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 'axle'): around the central axis of the organism or the extremity.[50] Two related terms, "abaxial" and "adaxial", refer to locations away from and toward the central axis of an organism, respectively[51]
  • Luminal (from Latin lumen 'light, opening'): on the—hollow—inside of an organ's lumen (body cavity or tubular structure);[52][53] adluminal is towards, abluminal is away from the lumen.[54] Opposite to outermost (the adventitia, serosa, or the cavity's wall).[55]
  • Terminal (from Latin terminus 'boundary or end') at the extremity of a usually projecting structure; forming the end of a structure such as an axon terminal.[56]
  • Visceral (from Latin viscera 'internal organs'): 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.[57]
  • Parietal (from Latin paries 'wall'): pertaining to the wall of a body cavity as the parietal pleura lining the thoracic cavity, contrasted with visceral pleura.[57]
  • Aboral (away from oral) is used to denote a location in an organism that is further from the mouth.

Other animals

[edit]

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.[13] 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.[58] 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;[19] 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.[19] 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.[59]

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.

  • Anatomical terms of location in a dog
    Anatomical terms of location in a dog
  • Anatomical terms of location in a kangaroo
    Anatomical terms of location in a kangaroo
  • Anatomical terms of location in most fish
    Anatomical terms of location in most fish
  • Anatomical terms of location in a horse
    Anatomical terms of location in a horse
  • Flatfish are asymmetric, with both eyes lying on the same side of the head.
    Flatfish are asymmetric, with both eyes lying on the same side of the head.

Non-bilaterian organisms

[edit]
Asymmetrical and spherical body shapes. (a) An organism with an asymmetrical, amoeboid body plan (Amoeba proteus – an amoeba). (b) An organism with a spherical amoeboid body plan (Actinophrys sol – a heliozoan).

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 organisms

[edit]

Four individuals of Phaeodactylum tricornutum, a diatom with a fixed elongated shape

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.[60]

Organisms where the ends of the long axis are distinct (Paramecium caudatum, above, and Stentor roeselii, below).

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.[60] 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).[61]

A cluster of Euplectella aspergillum sponges (Venus flower baskets), showing the apical–basal axes

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 (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

[edit]

Radially symmetrical organisms include those in the group Radiata – primarily Cnidarians (jellyfish, sea anemones and corals, and the comb jellies).[62] 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.[63] Echinoderm larvae are not included, since they are bilaterally symmetrical.[63]

Cnidarians have an incomplete digestive system, meaning that one end of the organism has a mouth, the oral end (from Latin ōrālis 'of the mouth'), and the opposite aboral end (from Latin ab- 'away from') has no opening from the gut (coelenteron).[62] They are radially symmetric around the oral-aboral axis.[62] Having only the single distinctive axis, "lateral", "dorsal", and "ventral" have no meaning, 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.[64]

Comb jellies have a biradial symmetry about only two planes, a tentacular plane, and a pharyngeal plane.[65]

Spiders

[edit]
See also: Glossary of spider terms and Spider anatomy

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.[66] 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.[67] Usually, the eyes are arranged in two roughly parallel, horizontal and symmetrical rows of eyes.[67] 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).[67]

  • Aspects of spider anatomy. This aspect shows the mainly prolateral surface of the anterior femora, plus the typical horizontal eye pattern of the Sparassidae.
    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
    Typical arrangement of eyes in the Lycosidae, with PME being the largest
  • In the Salticidae the AME are the largest.
    In the Salticidae the AME are the largest.

See also

[edit]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Anatomical terms of location

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Anatomical terms of location are standardized directional descriptors used in human to unambiguously specify the relative positions of body structures, regardless of the body's actual orientation, thereby facilitating clear communication among medical professionals and researchers. These terms are derived primarily from Latin and Greek roots and are applied relative to a reference framework known as the . The serves as the foundational reference for all such terms, defined as the body standing upright with feet parallel and slightly apart, arms hanging at the sides, palms facing forward (anteriorly), head and eyes directed forward, and mouth closed. This position eliminates ambiguity in descriptions, such as distinguishing anterior from posterior, by assuming a neutral, erect posture. Key anatomical terms of location include pairs that describe spatial relationships along major axes: superior (or cranial), meaning toward the head or upper part of the body (e.g., the nose is superior to the mouth); and inferior (or caudal), meaning toward the feet or lower part (e.g., the feet are inferior to the knees). Anterior (or ventral) refers to the front of the body (e.g., the kneecap is anterior to the thigh), while posterior (or dorsal) indicates the back (e.g., the spine is posterior to the heart). Medial denotes toward the midline of the body (e.g., the big toe is medial to the little toe), and lateral means away from the midline (e.g., the ears are lateral to the nose). For limbs, proximal describes closer to the trunk or point of attachment (e.g., the elbow is proximal to the wrist), whereas distal indicates farther from it (e.g., the fingers are distal to the shoulder). Additionally, superficial refers to structures nearer the body surface (e.g., skin), and deep to those farther inward (e.g., muscles beneath the skin). These terms are essential in clinical settings, , and , as they enable precise anatomical descriptions that transcend barriers and support applications in , , and . They also integrate with body planes—such as the sagittal, coronal, and transverse—for further spatial orientation.

Fundamentals

Standard anatomical position

The serves as the foundational reference posture in human anatomy, defined as the body standing upright with the feet flat on the ground and slightly apart, arms hanging at the sides, palms facing forward (supinated), head and eyes directed straight ahead, and toes pointing forward. This posture ensures that the head and are erect, the upper limbs are positioned close to the body with forearms supinated, and the lower limbs are fully extended with feet parallel. It provides a neutral, reproducible orientation for describing the location of structures throughout the body. The primary purpose of the is to establish a universal starting point for anatomical descriptions, thereby eliminating ambiguity in communication among healthcare professionals, researchers, and educators regardless of a patient's actual posture. By standardizing this reference, it facilitates precise localization of organs, tissues, and landmarks, which is essential for procedures, imaging, and education. This position also orients the body to consistently define anatomical planes and axes in subsequent analyses. In clinical contexts, adjustments to the standard position are common, such as the (lying flat on the back with arms at the sides and palms up) or (lying face down), where directional terms are still referenced back to the standard to maintain consistency during surgeries, examinations, or imaging.

Anatomical planes

Anatomical planes are imaginary flat surfaces that divide the into sections for descriptive and analytical purposes in , facilitating the visualization of internal structures relative to the standard anatomical position where the body stands upright, facing forward, with arms at the sides and palms facing forward. These planes provide a standardized framework for , surgical planning, and anatomical study, with three principal planes commonly used: the sagittal, coronal, and transverse planes. Oblique planes serve as additional, non-perpendicular variations for more complex sectional views. The is a vertical plane that passes longitudinally through the body, dividing it into left and right portions, and runs parallel to the body's long axis in the . When this plane passes directly through the midline, it is termed the midsagittal or median plane, bisecting the body into two equal halves. For visualization, a sagittal section of the reveals the cerebral hemispheres separated by the , while in the , it displays the anterior and posterior horns of gray matter. The , also known as the frontal plane, is another vertical plane that divides the body into anterior (front) and posterior (back) portions, oriented perpendicular to the . This plane is particularly useful in imaging techniques like (MRI) to assess frontal views of structures. An example is a coronal section of the , which separates the lungs and heart from the posterior ribs and spine, highlighting the mediastinum's contents. The , alternatively called the horizontal or axial plane, is a horizontal division that separates the body into superior (upper) and inferior (lower) parts, to both the sagittal and coronal planes, and parallel to the ground in the . It is widely employed in computed tomography (CT) scans for cross-sectional analysis. For instance, a transverse section through the at the level of the liver divides it into upper and lower lobes, illustrating vascular and biliary structures in a circular view. Oblique planes cut through the body at angles not aligned with the principal planes, offering versatile perspectives in advanced imaging modalities such as CT or MRI for irregular structures. These planes are non-perpendicular to the body's axes and are particularly valuable when standard sections do not adequately capture anatomical relationships, such as in diagonal views of the to assess articulations.

Anatomical axes

Anatomical axes refer to the imaginary lines around which the body or its segments rotate during movement, providing a framework for describing joint actions in three-dimensional space. These axes are perpendicular to the cardinal anatomical planes and are fundamental for analyzing motion in fields such as and . The three principal axes—sagittal, transverse, and longitudinal—are defined relative to the , where the body stands upright with arms at the sides and palms facing forward. The sagittal axis, also known as the anteroposterior axis, runs horizontally from anterior to posterior through the body or limb. Rotations around this axis occur in the frontal (coronal) plane, such as abduction and adduction of the limbs. For example, raising the arm laterally away from the midline involves rotation around the sagittal axis at the . The longitudinal axis, or vertical axis, extends superior to inferior along the of the body or limb. Movements around this axis take place in the , including rotations like internal and external turning of the or . This axis is crucial for torsional motions that twist segments relative to the body's central line. The transverse axis, also called the mediolateral axis, runs horizontally from medial to lateral, perpendicular to the sagittal axis. Rotations about this axis occur in the , facilitating actions such as flexion and extension at joints like the or . For instance, bending the toward the upper demonstrates rotation around the transverse axis. In and , these axes enable precise quantification of joint movements, aiding in the assessment of athletic performance, rehabilitation, and ergonomic by breaking down complex motions into components along each axis. Mathematically, the anatomical axes correspond to the lines of a adapted to the body: the transverse axis aligns with the x-axis (right-left), the sagittal axis with the y-axis (anterior-posterior), and the longitudinal axis with the z-axis (superior-inferior), allowing rotations to be modeled using or rotation matrices in .

Primary directional terms

Superior and inferior

In anatomical terminology, the terms superior and inferior describe positions along the vertical axis of the body relative to its long axis, assuming the where the body stands upright with feet together, arms at the sides, and palms facing forward. Superior, also known as cranial or cephalic, refers to a location toward the head or upper part of the body, indicating a higher position. Conversely, inferior, or caudal, denotes a position toward the feet or lower part of the body, signifying a lower location. These terms are essential for precise descriptions in human , as the body's upright orientation in the standard position aligns the vertical axis from head to feet. The , a horizontal plane to the long axis, divides the body into superior and inferior portions, facilitating the application of these terms in sectional . For example, the heart is superior to the diaphragm, as it resides in the above the muscular dome separating the from the . Similarly, the lungs are inferior to the clavicles, with their primary bulk extending downward from the collarbones into the . In clinical contexts, such as , superior and inferior terms guide patient positioning and image interpretation; for instance, a superior-inferior axial view orients the projection from head to foot to assess structures like the orthogonally. This standardized usage ensures unambiguous communication among healthcare professionals, aiding in diagnostics for conditions involving vertical displacements, such as hernias or tumors.

Anterior and posterior

In the , where the body is upright with feet parallel and palms facing forward, the term anterior (also known as ventral) describes the direction toward the front of the body, specifically facing the face or the belly side. Conversely, posterior (also known as dorsal) refers to the direction toward the back of the body. These terms provide a consistent framework for describing the relative positions of structures along the front-to-back axis, independent of the body's actual orientation. For instance, the lies anterior to the vertebral column, positioning it closer to the front of the . The preference for anterior and posterior over ventral and dorsal in descriptions of adult human anatomy stems from the bipedal posture, which aligns the front-back axis with the ventral (belly-facing) and dorsal (back-facing) orientations established in . In embryonic development, ventral denotes the anterior aspect facing the or belly region, while dorsal indicates the posterior aspect toward the or back; this nomenclature persists in adult human contexts like but shifts to anterior/posterior for gross anatomical clarity to avoid confusion with superior/inferior directions in upright humans. An example is the kidneys, which are positioned posterior to the intestines within the , behind the peritoneal lining that encases much of the digestive tract. In non-human vertebrates, particularly quadrupeds, anterior and posterior primarily indicate head-to-tail progression, with dorsal and ventral distinctly separating the upper back surface from the lower belly surface due to the horizontal body orientation. For human anatomy, however, the bipedal adaptation integrates these concepts such that anterior/posterior effectively overlays ventral/dorsal, emphasizing front-back relations in the that bisects the body into anterior and posterior halves. This standardized usage ensures precise communication in medical and scientific contexts, facilitating descriptions of organ relationships and surgical approaches.

Medial and lateral

In human anatomy, the terms medial and lateral describe relative positions of body structures along the left-right axis, with reference to the midline of the body as viewed in the standard anatomical . These directional terms are essential for precisely locating anatomical features and are defined relative to the median plane, which is a specific passing through the body's longitudinal midline, dividing it into symmetrical left and right halves. Medial indicates a position toward or closer to this midline or median plane; for instance, the (breastbone) is medial to the . In contrast, lateral denotes a position away from or farther from the midline; the ears, for example, are lateral to the . These terms apply throughout the body but are particularly useful in describing features in the , where medial structures approach the central axis while lateral ones extend toward the periphery. Common examples illustrate their application: the is medial to the eyes, positioning it nearer the facial midline, whereas the are lateral to the pinky fingers when the hands are supinated in anatomical position. In contexts involving bilateral , such as the , medial and lateral inform related terms like contralateral (on the opposite side of the midline from a given structure) and ipsilateral (on the same side); for example, the left is ipsilateral to the left but contralateral to the right .

Proximal and distal

In anatomy, the terms proximal and distal describe the relative positions of body structures along the length of limbs or appendages, with reference to their point of attachment to the trunk or point of origin. These directional terms are essential for precise communication in medical and anatomical contexts, particularly when discussing the extremities such as the arms and legs. They help differentiate locations in a linear fashion from the body's core outward, complementing other terms like medial and lateral that address orientation relative to the midline. "Proximal" denotes a position nearer to the trunk or the origin of a limb, such as the for the or the for the lower limb. For instance, the is proximal to the because it is closer to the , the point of attachment to the . In contrast, "distal" refers to a position farther away from the trunk or origin, so the is distal to the , and the fingers are distal to the . These terms emphasize a along the proximodistal axis, facilitating descriptions in clinical examinations, surgical planning, and imaging interpretations. Proximal and distal are specifically applicable to appendicular structures—the limbs—and are not used for the or trunk, where terms like superior/inferior or anterior/posterior are more appropriate. This restriction ensures clarity, as the terms rely on a defined proximal reference point, which is absent in central body regions. In specialized fields like , "proximal" extends to describe the surfaces of teeth oriented toward adjacent teeth, namely the mesial (toward the midline) and distal (away from the midline) surfaces, which form contact areas between neighboring teeth.

Superficial and deep

In anatomy, the term superficial refers to a position closer to or at the surface of the body, while deep describes a position farther from the surface and toward the interior. These terms are used to indicate relative depth regardless of the body's orientation, providing a consistent framework for describing the layering of tissues and organs. For instance, the skin is considered superficial to underlying structures such as muscles and bones, whereas the heart lies deep to the chest wall. These directional terms are particularly relevant when discussing body layers, where superficial structures include the and of the skin, and the serves as an intermediate layer just beneath the skin but still relatively superficial compared to deeper muscles or organs. Bones, for example, are deep to the and skeletal muscles, emphasizing the progression from outer to inner anatomical regions. This layering concept aids in understanding tissue organization, such as how superficial veins are more accessible than deep arteries. In surgical contexts, superficial and deep terms guide precise interventions; for example, a superficial incision targets the skin and subcutaneous layers for procedures like biopsies, while deep dissection accesses internal organs to minimize damage to overlying tissues. In , these terms are essential for classifying lesions or conditions, such as superficial burns affecting only the versus deeper injuries involving subcutaneous fat, which informs treatment strategies like topical applications versus surgical . Accurate use of these terms enhances communication in clinical settings, including interpretation and wound documentation.

Secondary directional terms

Dorsal and ventral

In anatomical terminology, the term dorsal refers to the direction toward the back or upper surface of an , particularly in embryos and non-human animals, while ventral denotes the direction toward the belly or lower surface. These terms establish the dorsal-ventral axis, a fundamental in bilaterian animals that orients structures relative to the back and front. In for humans, dorsal aligns with posterior (back), and ventral with anterior (front), though the terms persist in specific contexts to maintain consistency with embryological and . The dorsal-ventral axis originates during early embryogenesis, specifically in the formation of the , where the epiblast layer positions dorsally relative to the , setting the initial polarity. The , precursor to the , develops dorsally from the ectodermal through , positioning it along the embryo's back. In contrast, the primitive gut tube arises ventrally from the , forming the epithelial lining of the as the embryo folds and incorporates into a ventral hollow cylinder. This separation underscores the axis's role in , with dorsal structures like the deriving from and ventral ones like the gut from , influenced by signaling molecules such as bone morphogenetic proteins for patterning. In human anatomy, dorsal and ventral terms overlap with posterior and anterior but are retained in neuroanatomy to describe spinal cord features, such as the dorsal root of a spinal nerve, which carries sensory afferents from the periphery to the central nervous system via the dorsal root ganglion. For instance, the palms of the hands represent the ventral surface in the anatomical position, facing forward when arms are at the sides. In comparative contexts, the dorsal fin of a fish exemplifies a dorsal structure, located along the back to aid in balance and propulsion during swimming. These applications highlight the terms' utility in preserving evolutionary and developmental continuity across species.

Rostral, cranial, and caudal

In anatomical terminology, the terms cranial (or cephalic) and caudal describe positions along the head-to-tail axis of the body, with cranial indicating a direction toward the head and caudal toward the tail or the inferior end of the body. These terms derive from Latin roots: "cranial" from "cranium," referring to the skull, and "caudal" from "cauda," meaning tail. In humans, who maintain an upright posture, cranial often aligns with superior, while caudal corresponds to inferior along the vertical axis. Rostral is a related term primarily used in to denote a direction toward the or oral () end, particularly within the (CNS). Etymologically, "rostral" comes from the Latin "rostrum," meaning , reflecting its historical association with the forward-projecting structure in animals. In the brain, rostral typically equates to anterior, such as describing the progression from the caudally to the rostrally. The exemplifies caudal elongation, extending from the inferiorly toward the sacral region, with segments arranged from rostral cervical levels to caudal lumbar and sacral levels. Rostral is preferred over anterior in CNS contexts to maintain consistency along the , accounting for the brain's developmental flexure where the orients forward while the aligns more vertically. This distinction avoids ambiguity in describing structures like the rostral migration of neural progenitors during development or the caudal extension of motor pathways.

Central and peripheral

In , the terms central and peripheral denote relative positions within a structure or the body, with central indicating proximity to or midpoint and peripheral indicating locations toward the outer boundaries or edges. These directional terms are particularly prominent in describing the organization of the nervous and vascular systems, where they highlight the distinction between core elements and their extensions outward. Unlike medial and lateral terms, which reference deviation from the body's sagittal midline, central and peripheral convey a radial orientation from a central axis or hub, emphasizing systemic distribution rather than left-right asymmetry. Within the nervous system, central refers to the (CNS), which encompasses the and as the body's primary processing core, protected within the and vertebral column. In contrast, peripheral describes the peripheral nervous system (PNS), consisting of cranial and spinal nerves that extend from the CNS to innervate muscles, organs, and sensory receptors throughout the body's periphery. This division is crucial in for diagnosing conditions such as , where damage affects the outer nerve branches while sparing the central core. In the vascular system, central applies to major vessels near the heart, such as central veins like the superior and , which collect from the body's core regions for return to the right atrium. Peripheral denotes vessels farther from the heart, including peripheral arteries (e.g., radial and femoral arteries) that distribute oxygenated to the limbs and superficial tissues. Applications in include assessing central pulses at proximal sites like the to evaluate , versus peripheral pulses in distal arteries to detect circulatory obstructions such as in . An additional example occurs in the visual system, where central vision arises from the macula lutea in the retina's core, enabling high-acuity, color-detailed sight for tasks like reading. Peripheral vision, mediated by the surrounding retinal regions rich in rod cells, supports motion detection and a wide but with lower resolution. These terms underscore core-to-edge gradients in organ function, distinct from superficial-to-deep layering that describes surface versus internal depth.

Combined and modified terms

Combined directional terms

Combined directional terms in anatomy are formed by merging primary directional adjectives to specify more precise locations of structures relative to the . These compounds enhance descriptive accuracy, allowing anatomists and clinicians to denote positions that involve multiple axes simultaneously, such as both anterior-posterior and medial-lateral orientations. For instance, "anteromedial" describes a location that is both anterior (toward the front) and medial (toward the midline), as seen in the medial aspect of the anterior . The rules for combining these terms follow conventions rooted in anatomical Latin and Greek nomenclature, where prefixes or suffixes from the individual terms are adjoined to form compound adjectives. Typically, the primary direction (e.g., "antero-" for anterior) precedes the secondary one (e.g., "-medial" for medial), without additional linking vowels unless required for euphony, resulting in terms like "dorsolateral" (dorsal and lateral, toward the back and side) or "superolateral" (superior and lateral, upper and away from the midline). This prefix-based structure ensures clarity and consistency in scientific communication. In practical usage, these terms describe organ surfaces or features relative to the body's standard position, where the subject stands upright with palms facing forward. For example, the visceral (inferior) surface of the liver is oriented posteroinferiorly, facing both posteriorly (toward the back) and inferiorly (toward the feet), which aids in identifying its relations to adjacent structures like the and kidneys. Such descriptions prioritize the anatomical position to minimize ambiguity, as body orientations can vary; without this reference, terms like "posterolateral" (posterior and lateral) might be misinterpreted in non-standard postures. Clinically, combined terms are essential in fields like orthopedics for precise documentation. In distal , which often involve the distal radioulnar , descriptions may specify a "volar ulnar" (anterior medial) involvement of the line, guiding surgical approaches and reducing errors in treatment planning. Similarly, "dorsolateral" displacements in injuries indicate both posterior and lateral shifts, informing imaging and intervention strategies.

Directional modifiers

Directional modifiers are qualifiers used in anatomical terminology to refine or specify the application of primary directional terms, providing greater precision in describing locations, conditions, or structures relative to the body's midline or sides. These terms help distinguish relationships between body parts or phenomena, particularly in clinical and pathological contexts, by indicating sidedness or surface orientation. Ipsilateral refers to structures or conditions on the same side of the body relative to a reference point, such as the midline, while contralateral denotes those on the opposite side. For instance, the left arm and left leg are ipsilateral to each other, whereas the left arm and right leg are contralateral. In , contralateral neglect is a common example, where damage to one hemisphere of the , often the right following a , leads to impaired attention and perception of stimuli on the opposite (contralateral) side of space, despite intact vision. These terms are frequently combined with core directional descriptors, such as ipsilateral medial, to specify locations more accurately. Bilateral describes involvement or symmetry on both sides of the body, in contrast to unilateral, which indicates occurrence on only one side. Bilateral symmetry, a key feature in many animals including humans, means the body can be divided into mirror-image halves along the , facilitating balanced locomotion and organ function. In , bilateral conditions like affecting both hemispheres contrast with unilateral cases limited to one side. Palmar and plantar are surface-specific modifiers applied to the extremities: palmar pertains to the palm-facing surface of the hand (or forelimb in animals), while plantar refers to the sole-facing surface of the foot (or hindlimb). These terms orient structures relative to the ground in the anatomical position, such as the palmar aspect of the hand facing forward. In clinical usage, they describe pathologies like , involving unilateral or bilateral of the foot's sole. In , ipsilateral pain referral illustrates these modifiers' utility, where discomfort from a structure, such as the in the neck, radiates to the same-side rather than crossing the midline. This pattern aids by linking symptoms to their originating side, enhancing targeted interventions.

Special cases in human anatomy

Anatomical landmarks

Anatomical landmarks are palpable or visible surface features on the that serve as fixed reference points for describing the location of internal structures and applying directional terms in clinical and anatomical contexts. These landmarks enable precise communication among healthcare professionals, facilitating procedures such as physical examinations, surgeries, and imaging interpretations. Key examples include the (umbilicus), which marks the midline of the anterior and corresponds to the T10 dermatome, providing a central vertical reference for dividing the into quadrants or regions. The sternal notch, located at the superior aspect of the manubrium sterni, acts as an upper thoracic landmark for identifying the position of the trachea and great vessels, while the (ASIS) is a prominent bony projection on the ilium used to delineate the boundary between the and , serving as the origin for muscles like the sartorius. Planes derived from these landmarks further refine spatial orientation. The midsternal line runs vertically along the center of the from the sternal notch to the , dividing the anterior into left and right halves and aiding in the localization of cardiac and pulmonary structures. The midaxillary line extends vertically through the apex of the , parallel to the midline, and is essential for describing lateral thoracic and abdominal positions. Similarly, the midclavicular line passes vertically through the midpoint of the , approximately 7-9 cm lateral to the midsternal line, and is a critical reference for sites. These planes intersect with transverse levels, such as the (superior to the umbilicus), to create a grid for organ mapping. In practice, anatomical landmarks guide the precise location of organs and abnormalities. For instance, the apex of the heart is typically situated in the fifth left along the midclavicular line, about 9 cm from the midline, which is vital for cardiac and percussion during physical exams. However, variations can affect reliability; in obese patients, subcutaneous fat often obscures of landmarks like the ASIS or sternal notch, necessitating imaging modalities such as for accurate identification during procedures like . In children, proportional differences arise due to growth patterns—for example, the umbilicus is positioned relatively higher on the abdomen in infants (closer to the ) compared to adults, and overall body proportions shift with age, potentially altering the relative positions of thoracic landmarks. Historically, these landmarks originated from meticulous studies during cadaveric dissections, beginning in the with systematic measurements to correlate external features with internal anatomy. Pioneering works, such as those by Thomson (1894) and Anson and McVay (1936), established standardized positions through direct observation and on preserved bodies, laying the foundation for modern clinical applications and emphasizing the importance of evidence-based verification over time.

Terms for mouth and teeth

In the context of human anatomy, specialized directional terms are employed to describe locations within the oral cavity and , particularly when the mouth is oriented in the with the face forward and the dental arches aligned. These terms facilitate precise communication in dental examinations, treatments, and research, distinguishing surfaces relative to anatomical landmarks such as the , , , , midline, and occlusal planes. For the oral mucosa and surrounding structures, buccal refers to the surface or aspect facing the , as seen on the lateral sides of the teeth and . Lingual denotes the surface toward the , applicable to the medial aspects of mandibular teeth. Labial describes the forward-facing surface toward the , primarily on the . Palatal indicates the surface adjacent to the , used for the lingual side of maxillary teeth. For example, the buccal mucosa lines the inner , while the palatal mucosa covers the roof of the . These terms are essential in describing lesions or pathologies in the oral cavity. Dental terms focus on tooth surfaces and edges. Mesial pertains to the surface or direction toward the midline of the , such as the mesial surface of a facing the central . Distal refers to the opposite, away from the midline, like the distal surface of a molar. Occlusal designates the chewing or biting surface of posterior teeth (premolars and molars). Incisal applies to the cutting or biting edge of ( and ). Proximal is sometimes used as a for mesial or distal when referring to adjacent surfaces. These orientations assume the standard dental position with the occlusal plane horizontal and the midline sagittal. In clinical practice, these terms are integral to for assessing alignment and bracket placement—e.g., evaluating buccal inclination or mesial drift—and in for charting gingival health around specific surfaces, such as probing depths on mesial and distal aspects to diagnose periodontitis. Their standardized use ensures consistency in treatment planning and interdisciplinary communication.

Terms for hands and feet

In human anatomy, the directional terms for the hands and feet are adapted to describe the specific orientations of these extremities in the standard anatomical position, where the body stands erect with feet parallel, arms at the sides, and palms facing forward. For the hand, the palmar surface refers to the palm side, facing anteriorly in this position, while the dorsal surface denotes the back of the hand. Additionally, the radial side corresponds to the thumb aspect, aligned with the radius bone, and the ulnar side to the pinky aspect, aligned with the ulna bone; these terms are used regardless of hand rotation. A key example on the palmar surface is the , the fleshy mound at the base of the thumb formed by the abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis muscles, which facilitates thumb opposition. Proximal and distal terms apply along the limbs, with proximal indicating closer to the trunk and distal farther away, aiding in describing hand structures relative to the . For the foot, the plantar surface describes the sole, facing inferiorly in the anatomical position, whereas the dorsal surface refers to the top of the foot. Medial and lateral terms are retained for the foot but contextualized to its longitudinal axis, with medial toward the midline and lateral away, often used alongside plantar and dorsal for precise localization. The dorsum of the foot, for instance, features the extensor digitorum brevis muscle, the only intrinsic muscle on this surface, which assists in toe extension. Special positional terms for the foot include inversion, where the sole faces medially, and eversion, where the sole faces laterally; these describe orientations relative to the standard position and are crucial in podiatry for assessing foot alignment and deformities. In , terms like plantar highlight pathological conditions such as , an of the —a thick extending from the calcaneal tuberosity to the metatarsal heads on the sole—often causing pain due to overuse.

Rotational directions

Rotational directions in anatomical terms of location describe the static orientations of body parts following rotational adjustments, particularly for the limbs, relative to the . These terms specify positional relationships after rotation, aiding in precise descriptions of limb alignments in clinical, surgical, and imaging contexts. For the forearm and hand, pronation refers to the position where the palm faces posteriorly (or inferiorly when the arm is extended), with the radius crossing over the ulna; this is the opposite of the anatomical position. Supination is the position where the palm faces anteriorly, with the radius and ulna parallel, aligning with the standard anatomical position. These terms are essential for describing hand orientations, such as in radiology where a pronated forearm shows overlapping bones. In the foot, beyond basic medial and lateral, inversion denotes the orientation where the plantar surface faces medially, and eversion where it faces laterally. These positional terms are used to characterize foot posture, for example, in assessing varus (inverted) or valgus (everted) deformities in orthopedics. For the head and , rotational positions are described relative to the midline, with the face turned to the left or right, though specific terms like "lateroflexion" may apply in detailed cervical anatomy. These special cases ensure accurate localization in specialized fields like and orthopedics.

Application to non-human organisms

Non-bilaterian and elongated organisms

Non-bilaterian organisms, such as sponges (phylum Porifera) and , lack the bilateral symmetry characteristic of more complex animals, necessitating adapted anatomical terms that reflect their simpler body plans. In sponges, which are asymmetrical and lack true tissues or organs, no distinct anteroposterior or dorsoventral axes exist in adults; however, their free-swimming larvae exhibit an apical-basal axis, with the apical pole serving as a sensory region and the basal pole oriented toward the substrate. This larval polarity, regulated by signaling pathways like Wnt and TGF-β, provides insight into early metazoan axial organization but does not translate to fixed directional terms in the sessile adult form. Cnidarians, including and s, possess or biradial and are organized along an oral-aboral axis, where the oral end contains the and tentacles for feeding, and the aboral end is opposite the , often featuring sensory structures. For example, in the sea anemone Nematostella vectensis, the oral end develops from the blastopore, while the aboral end includes an apical organizing center that directs polarity via Wnt signaling gradients. This axis represents a primitive , predating the more elaborate anteroposterior and dorsoventral axes of bilaterians, and likely evolved through conserved mechanisms like Wnt pathway activation over 700 million years ago. Elongated organisms, such as nematodes (phylum Nematoda) and annelids like (phylum Annelida), exhibit along their length but lack appendages, relying on linear directional terms rather than limb-specific ones. In these worm-like animals, the body is divided into anterior (head or end) and posterior ( end) regions, with the anterior typically more tapered and containing sensory and feeding structures. Dorsal and ventral orientations are retained, with the dorsal surface often darker due to blood vessels in annelids and the ventral side featuring locomotor setae or genital openings; for instance, in the earthworm Lumbricus terrestris, the anterior end includes the for reproduction, positioned closer to the mouth. Nematodes, such as Caenorhabditis elegans, similarly use anterior-posterior and dorsal-ventral terms, with longitudinal nerve cords running in these directions to coordinate movement along the elongated body. The absence of limbs in these organisms eliminates the need for terms like proximal or distal, focusing instead on the primary longitudinal axis for describing internal structures, such as the digestive tract extending from anterior to posterior . Evolutionarily, these axes in and elongated forms represent foundational patterns in metazoan development, with the oral-aboral or anterior-posterior polarity emerging before the diversification of clusters that pattern bilaterian complexity.

Radially symmetrical organisms

Radially symmetrical organisms, such as echinoderms, employ directional terms adapted to their pentaradial body plans, which lack the bilateral axes typical of most animals. The primary axis is the oral-aboral axis, where "oral" refers to directions or surfaces toward the mouth, and "aboral" denotes the opposite side, away from the mouth. This axis runs from the central mouth on the oral surface through the body to the aboral surface, serving as the main reference for orientation in these animals. Unlike bilaterians, radially symmetrical organisms do not possess a distinct anterior-posterior axis, as their body parts radiate equally around a central point, emphasizing symmetry over directed locomotion. In (class Asteroidea), the central disk acts as the structural reference, with arms extending radially outward. The oral surface features ambulacral grooves along each arm, housing for locomotion and feeding, while the aboral surface bears the , a sieve-like structure for water intake into the vascular system. The term "adradial" describes positions toward or adjacent to a radius (arm), particularly along the boundary between ambulacral (tube foot-bearing) and interambulacral areas. These terms facilitate precise description of structures like the oral groove, which runs along the underside of each arm. Sea urchins (class Echinoidea) exhibit variations on this system within their spherical test (shell). The oral surface encompasses the mouth and surrounding ambulacral regions with , covering much of the body, while the aboral surface forms a smaller apical disc containing the and . Spines and are distributed radially, with adradial positioning relevant for ossicle arrangements near ambulacra. The test provides a rigid framework, orienting the oral-aboral axis vertically in many species. This radial symmetry represents an evolutionary shift from bilateral ancestors, evident in the bilateral larvae of echinoderms that metamorphose into pentaradial adults, adapting to sessile or slow-moving lifestyles. The loss of strict anterior-posterior distinction reflects this transition, prioritizing central-peripheral orientations around the disk or test.

Arthropods and other bilateral variations

In arthropods, such as insects and spiders, the standard anatomical orientations of dorsal and ventral are inverted relative to those in vertebrates. The dorsal surface corresponds to the upper or back side, housing the heart within a dorsal vessel, while the ventral surface aligns with the lower or belly side, containing the primary nerve cord. This arrangement facilitates the open circulatory system, where hemolymph flows dorsally toward the heart before distribution. Proximal and distal terms are applied to appendages like legs, with proximal indicating closeness to the body and distal referring to the extremity farthest from the attachment point; for example, in insect legs, the coxa is proximal, followed distally by the trochanter, femur, tibia, and tarsus. In spiders, anterior-posterior directions are denoted by the division into prosoma (anterior tagma, equivalent to ) and opisthosoma (posterior tagma, equivalent to ). The prosoma bears the , which are the most anterior appendages and function as fang-like structures, analogous to rostral elements in positioning. The ventral opisthosoma often contains book lungs for respiration, reinforcing the ventral association with supportive structures. These terms adapt bilateral symmetry to the body plan, emphasizing tagmata over segmented regions. Among other bilateral animals, variations maintain core orientations but adapt to specific morphologies. In fish, the dorsal fin protrudes from the dorsal midline, aiding stability and propulsion, while ventral structures like the anal fin oppose it. In birds, cranial denotes the direction toward the head, aligning with rostral usage and facilitating descriptions of beak and skull features in a lightweight, fused cranium. These differences arise from evolutionary divergences between and bilaterals, where arthropods retain a ventral cord and dorsal heart from early deuterostome-protostome splits, contrasting the dorsal and ventral heart positioning. Such inversions highlight the need for context-specific application of terms to avoid confusion in .

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