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Anatomical Terms Of Location

Oh, you want me to… rewrite this? Wikipedia, of all things. Fine. Don’t expect sunshine and rainbows. I’ll give you the facts, but don’t mistake my precision for enthusiasm. It’s just… necessary. Like flossing.

Standard terms for unambiguous description of relative placement of body parts

To avoid the messy ambiguity that arises from casual descriptions of where things are on a body, especially when dealing with anatomy and medicine, a standardized system of terms has been established. These terms are crucial for precisely locating structures within the bodies of humans and other animals. They’re typically etched from the bedrock of Latin and Greek roots, ensuring a certain gravitas, and are applied with reference to a specific orientation known as the standard anatomical position. This position is the baseline, the neutral setting from which all descriptions of "front," "back," and so forth are derived. To further refine these descriptions, the body is conceptualized using anatomical planes and axes.

The precise meaning of these terms can shift, subtly or dramatically, depending on whether the organism in question is a biped, like us, or a quadruped. This difference is largely due to the distinct orientation of the neuraxis – the central axis of the nervous system. For invertebrates, especially those that lack a clear front or back like non-bilaterian organisms, terms like proximal (closer to the main body mass) and distal (further away) become more critical, describing relationships relative to a point of origin or attachment rather than a fixed anterior/posterior.

To maintain this clarity across disciplines, international organizations have codified these vocabularies. For humans, we have Terminologia Anatomica, Terminologia Neuroanatomica, and Terminologia Embryologica. For our animal counterparts, there's Nomina Anatomica Veterinaria. These standards ensure that anatomists, veterinarians, and medical doctors can communicate with chilling accuracy about the placement of any given structure.

Introduction

The diversity in animal body plans necessitates different terminologies, particularly concerning the neuraxis and whether an animal moves on two legs or four, or if it's even a vertebrate at all.

Standard anatomical terms of location, forged from the ancient tongues of Latin and Greek, provide a necessary precision for scientists, veterinarians, medical doctors, and anatomists to describe the positions of organs and structures with unwavering accuracy. This precision is vital, even though some terms carry a context-sensitive weight. International bodies have stepped in to standardize these terms, offering vocabularies like Terminologia Anatomica, Terminologia Neuroanatomica, and Terminologia Embryologica for humans, and Nomina Anatomica Veterinaria and Nomina Embryologica Veterinaria for animals. It’s about eliminating guesswork, which, let’s face it, is usually a bad idea in this field.[1][2][3][4][5]

The distinction between bipedal and quadrupedal vertebrates leads to different applications of these terms. The fundamental difference lies in the neuraxis and, consequently, the standard anatomical position they adopt.[1][2] Invertebrates, with their bewildering array of shapes and symmetries, often require unique sets of descriptors.[6]

Standard anatomical position

Main article: Standard anatomical position

A male and female human in the standard anatomical position

The inherent variability in an animal's posture, especially with flexible appendages like limbs or tentacles, necessitates a fixed reference point. This reference point is the standard anatomical position. It’s a theoretical posture that allows consistent description, regardless of whether the subject is currently lounging or mid-leap. For humans, this means standing erect, arms relaxed at the sides, palms facing forward.[7][8] For quadrupeds, it’s standing on all four limbs, head oriented forward.[9] Even a fish has a standard position: belly down, with fins in a neutral stance.[10] This ensures that when we talk about something being "anterior," we all understand precisely what that means, even if the creature we're describing is currently upside down.

Planes

The standard anatomical planes of both a human and a goat displaying three anatomical planes:

To dissect the body conceptually, we use three primary anatomical planes.[8] These planes are not just academic constructs; they are fundamental to medical imaging, embryology, and the analysis of movement.[11]

The three fundamental orientations are:

  • Sagittal planes: These planes slice the body vertically into left and right portions. The most central of these is the median plane, or midsagittal plane, which bisects the body precisely through the midline – think of the spinal cord and navel.[1][12][13]
  • Coronal plane (or frontal plane): This plane divides the body into front (anterior) and back (posterior) sections. In the context of quadrupeds, this is known as the dorsal plane, separating the dorsal (back) from the ventral (belly) sides.[8][14][6]
  • Transverse plane (or axial or horizontal plane): This plane cuts across the body horizontally, perpendicular to both the sagittal and coronal planes.[8]

These planes aren't just theoretical divisions. Specific transverse planes, like the transpyloric plane, subcostal plane, and transumbilical plane, are clinically relevant for dividing the torso into distinct sections.[15]

Axes

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

The fundamental axes of a vertebrate body are established early in embryonic development, even before gastrulation.[16] These axes define the body's orientation. In bilaterally symmetrical animals, three primary axes intersect at right angles:

  • The left-right axis (also known as the horizontal or frontal axis).[16]
  • The craniocaudal axis (also called rostrocaudal or cephalocaudal axis). This runs from head to tail.[16]
  • The anteroposterior axis (also known as the dorsoventral or sagittal axis).[17][18]

Organisms that are round or possess asymmetrical body plans will have different, or perhaps no, defined axes.[6]

Main terms

Superior and inferior

In the standard human anatomical position, "superior" (from Latin super, meaning 'above') or "cranial" refers to positions closer to the head. Conversely, "inferior" (from Latin inferus, meaning 'below') or "caudal" refers to positions closer to the feet.[7] Think of the superior mediastinum versus the inferior mediastinum, or the superior colliculus and inferior colliculus in the brain.[12] In veterinary contexts, these terms are less commonly used for the body trunk, with "dorsal" and "ventral" taking precedence, except for specific structures like the eye, eyelids, and inner ear.

Anterior and posterior

"Anterior" (from Latin ante, meaning 'before') designates what is in front, while "posterior" (from Latin post, meaning 'after') indicates what is behind.[19] For instance, the gill openings in many fish are posterior to the eyes and anterior to the tail. Veterinary anatomy often reserves these terms for cranial structures, opting for "cranial" and "caudal" elsewhere.[14]

Dorsal and ventral

These terms, prevalent in veterinary anatomy, also appear in human anatomy, particularly in neuroanatomy and embryology, to denote the back (dorsal, posterior) or front (ventral, anterior) aspects of an organ or organism.[19]

The dorsal (from Latin dorsum, meaning 'back') surface refers to the back or upper side. Consider the dorsum of the hand, the dorsum of the foot, or the top of the skull.[18][12]

The ventral (from Latin venter, meaning 'belly') surface denotes the front or lower side of an organism or organ, like the underside of the tongue.[18]

In a fish, the dorsal fin is on its upper surface, while its ventral fins are on its belly.[20] These terms are also found in non-biological contexts, such as the dorsal and ventral gun turrets on a bomber.

Medial and lateral

These terms describe proximity to the median plane.[2][19] Lateral (from Latin lateralis, meaning 'to the side') refers to structures situated towards the sides of an animal, hence "left lateral" and "right lateral." Medial (from Latin medius, meaning 'middle') denotes structures close to the median plane, or closer to it than another structure.[19] For example, in humans, the arms are lateral to the torso, while the sex organs are medial to the legs. The term "temporal" is similar to "lateral" but is specifically applied to the head.

The terms "left" and "right," or sinistral and dextral, denote the two halves of a bilaterally symmetrical body divided by the median plane.

Terms derived from "lateral" include:

  • Contralateral (from Latin contra, meaning 'against'): Situated 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, meaning 'same'): Situated on the same side as another structure. For example, the left arm is ipsilateral to the left leg.[12]
  • Bilateral (from Latin bis, meaning 'twice'): Pertaining to both sides of the body.[12] For example, a bilateral orchiectomy involves the removal of testes on both sides.
  • Unilateral (from Latin unus, meaning 'one'): Pertaining to one side only.[12] For example, unilateral deafness affects only one ear.[21]

Varus (from Latin varus, meaning 'bow-legged') and valgus (from Latin valgus, meaning 'knock-kneed') describe angulation of a bone or joint within the coronal plane. In varus, the distal portion deviates towards the midline; in valgus, it deviates away from it.[22]

Proximal and distal

Anatomical directional reference

Look up proximal or distal in Wiktionary, the free dictionary.

The terms "proximal" (from Latin proximus, meaning 'nearest') and "distal" (from Latin distare, meaning 'to stand away from') describe parts of a structure that are closer to or further from the main mass of the body, respectively.[23] Thus, the upper arm is proximal, and the hand is distal, relative to the chest or heart.[24]

These terms are frequently used when describing appendages like fins, tentacles, and limbs. While "proximal" always means towards the point of attachment and "distal" away from it, a structure can be proximal or distal relative to another point. For instance, the elbow is distal to a wound on the upper arm but proximal to a wound on the lower arm.[24]

This terminology has also found its way into molecular biology and chemistry, referring to the positions of atoms within a molecule or moiety.[25]

Rostral, cranial, and caudal

In the human skull, the terms rostral and caudal are adapted to the curved neuraxis of Hominidae, with the rostrocaudal axis describing the C-shaped region connecting rostral and caudal areas.

Specific terms are used to denote proximity to the head or tail of an animal. To describe how close something is to the head, three terms are employed:

  • Rostral (from Latin rostrum, meaning 'beak, nose'): Situated toward the oral or nasal region, or in the brain, toward the tip of the frontal lobe.[12][19]
  • Cranial (from Greek kranion, meaning 'skull') or cephalic (from Greek kephalē, meaning 'head'): Describes proximity to the head of an organism.[12]
  • Caudal (from Latin cauda, meaning 'tail'): Describes proximity to the trailing end of an organism.[19]

These terms are more common in veterinary medicine than 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" refer to the skull, with "cranial" being more frequent. "Rostral" is seldom used in human gross anatomy, typically referring to the anterior face rather than the superior aspect of the organism. It finds more use in embryology and neuroanatomy. Similarly, "caudal" is more common in embryology and neuroanatomy, appearing only occasionally in human gross anatomy.[2] The "rostrocaudal axis" describes the curved line of the neuraxis from the forehead (rostral) to the tail end (caudal).

Central and peripheral

"Central" and "peripheral" refer to the distance from or towards the center of something – be it an organ, a body region, or an anatomical structure. The most obvious examples are the central nervous system and the peripheral nervous system.

Central (from Latin centralis) means at or close to the center.[28] The major blood vessels, for instance, run centrally, with smaller vessels branching off.

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

Superficial and deep

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

Deep (from Old English) signifies something further away from the organism's surface.[30] For example, the external oblique muscle of the abdomen is deep to the skin. It’s notable that "deep" is one of the few anatomical terms of location derived from Old English rather than Latin; the anglicised Latin equivalent would be "profound" (from Latin profundus, meaning 'due to depth').[1]

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

Combined terms

Dorsolateral prefrontal cortex highlighted in dark green

Many anatomical terms can be combined to specify a position in two axes simultaneously or to describe the direction of movement. For example, "anterolateral" indicates a position that is both anterior and lateral relative to the body axis, such as the bulk of the pectoralis major muscle, or a specific organ like the [anterolateral tibial tubercle].[31] This term can also describe the path of something entering or traversing the body, like the anterolateral system in the spinal cord, or the [anterolateral central arteries].[32] "Anteromedial" is another such term, used for example with the [anteromedial central arteries].[33]

Within the more internal structures of the brain and spinal cord of the central nervous system, "dorsal" and "ventral" and their combinations are frequently used instead of "anterior" and "posterior." Many references are needed in these organs. For instance, the prefrontal cortex is divided into the dorsomedial prefrontal cortex and the dorsolateral prefrontal cortex. The dorsomedial region itself has subcompartments that incorporate other terms, such as the anterior cingulate cortex and the infralimbic cortex. Structures like the anterior cingulate cortex can be anatomically divided based on cognitive (dorsal) and emotional (ventral) components.[34]

"Proximodistal" describes the axis of an appendage, such as an arm or leg, running from its tip (distal part) to where it attaches to the body (proximal part).[17]

In radiology, various X-ray views utilize terminology based on the path of the X-ray beam through the body. This includes anteroposterior (front to back), posteroanterior (back to front), and lateral (side views).[35] Combined terms were historically hyphenated, but the hyphen is now often omitted.[36]

Modifiers

Terms can be modified with prefixes and suffixes. In this image showing the jellyfish species Chrysaora, the prefix 'ab-' indicates something 'away from' the mouth, as in "aboral." Other terms are combined to denote axes, such as the proximodistal axis.

Several prefixes are commonly used:[37]

  • Sub- (from Latin sub, meaning 'preposition beneath, close to, nearly etc.'): Indicates something beneath, subordinate, or lesser.[37] For example, subcutaneous means beneath the skin.
  • Hypo- (from Ancient Greek hupo, meaning 'under'): Indicates something beneath.[37] For example, the hypoglossal nerve innervates the muscles beneath the tongue.
  • Infra- (from Latin infra, meaning 'under'): Indicates something within or below. For example, the infraorbital nerve runs within the orbit.
  • Inter- (from Latin inter, meaning 'between'): Indicates something situated between.[37] For example, the intercostal muscles are located between the ribs.
  • Super- or Supra- (from Latin super, supra, meaning 'above, on top of'): Indicates something above something else.[37] For example, the supraorbital ridges are located above the eyes.
  • Ab- (from Latin ab, meaning 'away'), and ad- (from Latin ad, meaning 'towards'): Indicate direction away from (ab-) or towards (ad-) something else.[37] For example, abduction and adduction refer to muscular movements away from, and towards the midline of the body, respectively.

Other terms function as suffixes, appended to the end of words:

  • -al (from Latin al, meaning 'pertaining to, of the'): For example, femoral neck.
  • -ad (from Latin ad, meaning 'towards'), equivalent to '-ally', is a suffix creating an adverbial form to indicate movement towards something else.[38] For example, "distad" means "in the distal direction,"[39] as in "arterial blood flows distad/distally." Other examples include cephalad (towards the cephalic end), orad, craniad, and proximad. While "proximally" and "distally" are more common in human and veterinary anatomy texts, "proximad" and "distad" are frequently used in insect anatomy.[2][1][38]

Other terms and special cases

Anatomical landmarks

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

Structures can be described by their level relative to specific vertebrae, depending on the section of the vertebral column.[40] For example, structures at the level of the fourth cervical vertebra are noted as "C4," the fourth thoracic vertebra as "T4," and the third lumbar vertebra as "L3." The sacrum and coccyx, being fused bones, are less frequently used for precise location referencing.

References can also be derived from surface anatomy, pointing to landmarks visible on or just beneath the skin.[40] For instance, structures might be located relative to the anterior superior iliac spine, the medial malleolus, or the medial epicondyle.

Anatomical lines are theoretical lines, either horizontal transverse planes or vertical sagittal planes, used to pinpoint anatomical locations. For example, the mid-clavicular line is employed in cardiac examination to palpate the apex beat of the heart, and the axillary lines serve as references for the underarm region. Other anatomical lines include the curved nuchal lines on the occipital bone and the gluteal lines on the ilium (bone).

Mouth and teeth

Specialized terms are used to describe the mouth and teeth.[2] Fields like osteology, paleontology, and dentistry apply unique locational terms for the oral cavity and dentition. This is because, while teeth may align linearly within the jaw, their relationships can be complex, with rotations and other variations making terms like "anterior" or "lateral" ambiguous.[41][42] For example, "distal" and "proximal" (or "mesial") are used for tooth surfaces relative to the midpoint of the dental arch, while "medial" and "lateral" maintain their standard meanings relative to the median plane.[43] Terms like "buccal" (from Latin bucca, meaning 'cheek') and "palatal" (from Latin palatum, meaning 'palate') refer to structures near the cheek and the hard palate, respectively.[43]

Hands and feet

  • "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 specific to the hands and feet.[2] Additional terms may be employed to avoid confusion when describing the surfaces of the hand, particularly regarding what constitutes the "anterior" or "posterior" surface. While "anterior" is anatomically correct, it can be confusing when describing the palm. Similarly, "posterior" for the back of the hand and arm can be problematic because the forearm can pronate and supinate, altering the hand's orientation. For greater clarity, the directional term palmar (from Latin palma, meaning 'palm of the hand') is commonly used for the front of the hand, and dorsal for the back. The palmar fascia is palmar to the tendons that flex the fingers, and the dorsal venous arch is named for its location on the dorsal side of the foot.

In humans, "volar" can be used interchangeably with "palmar" for the palm of the hand, and also for the sole of the foot.[44] However, "palmar" is exclusively for the hand, and "plantar" exclusively for the foot.[44][45]

Similarly, for clarity in the limbs, the sides are named after the bones. In the forearm, structures closer to the radius are "radial," those closer to the ulna are "ulnar," and those related to both are "radioulnar," as in the distal radioulnar joint.[46] In the lower leg, structures near the tibia (shinbone) are "tibial," and those near the fibula are "fibular" (or "peroneal").

Rotational direction

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 the forward (ante-) or backward (retro-) rotation of an anatomical structure relative to another position. They are particularly used for the curvature of the uterus.[47][48]

  • Anteversion (from Latin anteversus) describes a structure tilted further forward than normal, whether pathologically or incidentally.[47] For example, a person's uterus is typically anteverted, meaning tilted slightly forward. A misaligned pelvis might also be anteverted, tilted forward to a relevant degree.[49]
  • Retroversion (from Latin retroversus) describes a structure tilted backward, away from a reference point.[48] A notable example is a retroverted uterus.[48]

Other directional terms

Several other terms describe location without defining fixed axes. They include:

  • Axial (from Latin axis, meaning 'axle'): Pertaining to the central axis of an organism or extremity.[50] Related terms, "abaxial" and "adaxial," refer to locations away from and toward the central axis, respectively.[51]
  • Luminal (from Latin lumen, meaning 'light, opening'): Located on the interior of a hollow organ's lumen (body cavity or tubular structure);[52][53] "adluminal" is towards, and "abluminal" is away from the lumen.[54] This is contrasted with the outermost layers like the adventitia, serosa, or the cavity's wall.[55]
  • Terminal (from Latin terminus, meaning 'boundary or end'): Located at the extremity of a usually projecting structure, forming the end of a structure such as an axon terminal.[56]
  • Visceral (from Latin viscera, meaning 'internal organs'): Associated with the innermost layer of an organ within the body. For example, the visceral pleura covers the lungs, in contrast to the parietal pleura which lines the thoracic cavity.[57]
  • Parietal (from Latin paries, meaning 'wall'): Pertaining to the wall of a body cavity, such as the parietal pleura lining the thoracic cavity, as opposed to the visceral pleura.[57]
  • Aboral: Denotes a location in an organism that is further from the mouth.

Other animals

The diversity of animal body plans necessitates different terminologies. Whether an animal is bipedal or quadrupedal, or possesses symmetry or asymmetry, influences how these terms are applied. For instance, humans, being bilaterally symmetrical, generally use the same terms as other vertebrates.[13] However, the human standard anatomical position results in the anterior/posterior and ventral/dorsal directions coinciding. For this reason, "inferior/superior" are used out of tradition rather than "cranial/caudal," which are universally applicable.[58] The term "rostral," referring to the beak or nose in some animals, is less common in humans, except in relation to parts of the brain.[19] While humans lack a visible tail (though the coccygeal vertebrae form a tailbone), "caudal" is still used to refer to the hind part of the body.[19] Flounder and other flatfish, which lie on the seabed on their side, are asymmetric, with both eyes on the "up" side, posing a challenge for anatomical nomenclature.[59]

Invertebrates present a significant challenge due to their vast array of body shapes.[6] Some terms are borrowed by analogy from vertebrate anatomy, while others are novel. Terms like "proximal" (near the point of attachment) and "distal" (far from the point of attachment) are widely applicable. Ultimately, the usage of any term depends on the specific body plan of the organism.

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

Non-bilaterian organisms

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).

For non-bilaterian organisms with variable shapes, like amoeboid organisms, most directional terms are practically meaningless due to the lack of fixed shape and distinct axes. Similarly, radially symmetrical organisms lack a defined orientation; any arbitrary set of perpendicular axes would be as useful as any other. In these cases, only terms like "superficial" and "deep," or occasionally "proximal" and "distal," remain descriptive.

Elongated organisms

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

Organisms with a fixed, elongated shape allow for at least two directional terms: "longitudinal" along the main axis, and "transverse" across the body. A third axis is often undefined unless a unique structure, like a non-terminal cytostome, provides a reference. These are often planktonic protists, viewed in two dimensions under a microscope.

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

Some elongated protists have clearly defined ends. The end with a mouth (or equivalent, like the cytostome in Paramecium or Stentor), or the end typically leading in locomotion (like the flagellum end of Euglena), is usually designated as anterior. The opposite end is then posterior.[60] This terminology technically applies to organisms that are always planktonic, though it can be extended to sessile ones.[61]

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

Organisms attached to a substrate, such as sponges and animal-like protists, also have distinct ends. The attachment point is the basal end (from Latin basis, meaning 'support/foundation'), while the furthest end is the apical end (from Latin apex, meaning 'peak/tip').

Radially symmetrical organisms

Radially symmetrical organisms include the Cnidarians (jellyfish, sea anemones, corals) and comb jellies.[62] Adult echinoderms like starfish and sea urchins, with their five-part symmetry, are also included.[63] Echinoderm larvae, however, are bilaterally symmetrical.[63]

Cnidarians have a mouth at one end, the oral end (from Latin ōrālis, meaning 'of the mouth'), and no opening at the opposite aboral end.[62] They are radially symmetric around the oral-aboral axis.[62] With only this single defined axis, terms like "lateral," "dorsal," and "ventral" are meaningless, replaced by the generic term "peripheral" (from Ancient Greek periphereia, meaning 'circumference'). "Medial" can be used, but in radiates, it refers to the central point rather than a central axis as in vertebrates. Thus, multiple radial and medio-peripheral axes are possible.[64]

Comb jellies exhibit biradial symmetry about two planes: a tentacular plane and a pharyngeal plane.[65]

Spiders

Specialized terms are used for spiders. For example, "prolateral" and "retrolateral" describe surfaces of a spider's leg. Prolateral refers to the surface closest to the anterior end of the body, while retrolateral is closest to the posterior end.[66] Spiders typically have eight eyes arranged in four pairs on the carapace of the prosoma. The size, shape, and location of these eyes are characteristic of different spider families.[67] They are usually arranged in two horizontal, symmetrical rows, designated 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.
  • Typical arrangement of eyes in the Lycosidae, with PME being the largest.
  • In the Salticidae, the AME are the largest.

See also