Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its

standard anatomical position The standard anatomical position, or standard anatomical model, is the scientifically agreed upon reference position for Anatomical terms of location, anatomical location terms. Standard anatomical positions are used to standardise the position of ...

. This position provides a definition of what is at the front ("anterior"), behind ("posterior") and so on. As part of defining and describing terms, the body is described through the use of anatomical planes and

anatomical axes Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Th ...

. The meaning of terms that are used can change depending on whether an organism is bipedal or quadrupedal. Additionally, for some animals such as invertebrates, some terms may not have any meaning at all; for example, an animal that is radially symmetrical will have no anterior surface, but can still have a description that a part is close to the middle ("proximal") or further from the middle ("distal"). International organisations have determined vocabularies that are often used as standard vocabularies for subdisciplines of anatomy, for example, Terminologia Anatomica for humans, and Nomina Anatomica Veterinaria for animals. These allow parties that use anatomical terms, such as anatomists, veterinarians, and

medical doctor A physician (American English), medical practitioner (Commonwealth English), medical doctor, or simply doctor, is a health professional who practices medicine, which is concerned with promoting, maintaining or restoring health through th ...

s to have a standard set of terms to communicate clearly the position of a structure.

Introduction

Standard

anatomical Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having it ...

and zoological terms of location have been developed, usually based on Latin and Greek words, to enable all biological and medical scientists, veterinarians,

doctors Doctor or The Doctor may refer to: Personal titles * Doctor (title), the holder of an accredited academic degree * A medical practitioner, including: ** Physician ** Surgeon ** Dentist ** Veterinary physician ** Optometrist *Other roles ** ...

and anatomists to precisely delineate and communicate information about animal bodies and their organs, even though the meaning of some of the terms often is context-sensitive. Much of this information has been standardised in internationally agreed vocabularies for humans ( Terminologia Anatomica) and animals ( Nomina Anatomica Veterinaria). For humans, one type of vertebrate, and other animals that stand on two feet (

bipeds Bipedalism is a form of terrestrial locomotion where an organism moves by means of its two rear limbs or legs. An animal or machine that usually moves in a bipedal manner is known as a biped , meaning 'two feet' (from Latin ''bis'' 'double' a ...

), terms that are used are different from those that stand on four ( quadrupeds). One reason is that humans have a different

neuraxis The neuraxis or sometimes neuroaxis is the axis of the central nervous system. It denotes the direction in which the central nervous system lies. During embryological development, the neuraxis is bent by various flexures, contributing to the ma ...

and another is that unlike animals that rest on four limbs, humans are considered when describing anatomy as being in the

, which is standing up with arms outstretched. Thus, what is on "top" of a human is the

head A head is the part of an organism which usually includes the ears, brain, forehead, cheeks, chin, eyes, nose, and mouth, each of which aid in various sensory functions such as sight, hearing, smell, and taste. Some very simple animals may ...

, whereas the "top" of a dog may be its back, and the "top" of a

flounder Flounders are a group of flatfish species. They are demersal fish, found at the bottom of oceans around the world; some species will also enter estuaries. Taxonomy The name "flounder" is used for several only distantly related species, thou ...

could refer to either its left or its right side. Unique terms are used to describe animals without a backbone ( invertebrates), because of their wide variety of shapes and symmetry.

Standard anatomical position

Because animals can change orientation with respect to their environment, and because

appendage An appendage (or outgrowth) is an external body part, or natural prolongation, that protrudes from an organism's body. In arthropods, an appendage refers to any of the homologous body parts that may extend from a body segment, including anten ...

s 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

. This means descriptions as if the organism is in its standard anatomical position, even when the organism in question has appendages in another position. This helps avoid confusion in terminology when referring to the same organism in different postures. In humans, this refers to the body in a standing position with arms at the side and palms facing forward, with thumbs out and to the sides.

Combined terms

Holotype of Mantophryne insignis, dorsolateral view

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 The pectoralis major () is a thick, fan-shaped or triangular convergent muscle, situated at the chest of the human body. It makes up the bulk of the chest muscles and lies under the breast. Beneath the pectoralis major is the pectoralis minor, a ...

muscle). In radiology, an X-ray image may be said to be "anteroposterior", indicating that the beam of X-rays passes from their source to patient's

anterior Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...

body wall through the body to exit through posterior body wall. Combined terms were once generally hyphenated, but the modern tendency is to omit the hyphen.

Planes

Anatomical terms describe structures with relation to four main anatomical planes: # The median plane, which divides the body into left and right. This passes through the

, spinal cord, navel, and, in many animals, the tail. # The sagittal planes, which are to the median plane. # The frontal plane, also called the coronal plane, which divides the body into front and back. # The horizontal plane, also known as the transverse plane, which is perpendicular to the other two planes. In a human, this plane is parallel to the ground; in a quadruped, this divides the animal into anterior and posterior sections.

Axes

The axes of the body are lines drawn about which an organism is roughly symmetrical. To do this, distinct ends of an organism are chosen, and the axis is named according to those directions. An organism that is symmetrical on both sides has three main axes that intersect at

right angle In geometry and trigonometry, a right angle is an angle of exactly 90 Degree (angle), degrees or radians corresponding to a quarter turn (geometry), turn. If a Line (mathematics)#Ray, ray is placed so that its endpoint is on a line and the ad ...

s. An organism that is round or not symmetrical may have different axes. Example axes are: * The anteroposterior axis * The cephalocaudal axis * The dorsoventral axis Examples of axes in specific animals are shown below. Anatomical axes.svg, Anatomical axes in a human, similar for other orthograde bipedal vertebrates Anatomical Directions and Axes.JPG, Anatomical axes and directions in a fish Long and short axis.png, Spheroid or near-spheroid organs such as testes may be measured by "long" and "short" axis.

Modifiers

Several terms are commonly seen and used as

prefix A prefix is an affix which is placed before the Word stem, stem of a word. Adding it to the beginning of one word changes it into another word. For example, when the prefix ''un-'' is added to the word ''happy'', it creates the word ''unhappy'' ...

es: * Sub- () is used to indicate something that is beneath, or something that is subordinate to or lesser than. For example, subcutaneous means beneath the skin, and "subglobular" may mean smaller than a * Hypo- () is used to indicate something that is beneath. For example, the hypoglossal nerve supplies the muscles beneath the tongue. * Infra- () is used to indicate something that is within or below. For example, the

infraorbital nerve The infraorbital nerve is a branch of the maxillary nerve, itself a branch of the trigeminal nerve (CN V). It travels through the orbit and enters the infraorbital canal to exit onto the face through the infraorbital foramen. It provides sensory ...

runs within the orbit. * Inter- () is used to indicate something that is between. For example, the intercostal muscles run between the

rib In vertebrate anatomy, ribs ( la, costae) are the long curved bones which form the rib cage, part of the axial skeleton. In most tetrapods, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the ches ...

s. * Super-'' or ''Supra- () is used to indicate something that is above something else. For example, the

supraorbital ridge The brow ridge, or supraorbital ridge known as superciliary arch in medicine, is a bony ridge located above the eye sockets of all primates. In humans, the eyebrows are located on their lower margin. Structure The brow ridge is a nodule or crest ...

s are above the

eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

s. Other terms are used as

suffix In linguistics, a suffix is an affix which is placed after the stem of a word. Common examples are case endings, which indicate the grammatical case of nouns, adjectives, and verb endings, which form the conjugation of verbs. Suffixes can carry ...

es, added to the end of words: * -ad () and ab- () are used to indicate that something is towards (-ad) or away from (-ab) something else. For example, "distad" means "in the distal direction", and "distad of the femur" means "beyond the femur in the distal direction". Further examples may include cephalad (towards the cephalic end), craniad, and proximad.

Main terms

Superior and inferior

Superior () describes what is above something and inferior () describes what is below it. For example, in the

anatomical position The standard anatomical position, or standard anatomical model, is the scientifically agreed upon reference position for anatomical location terms. Standard anatomical positions are used to standardise the position of appendages of animals with r ...

, the most superior part of the human body is the head and the most inferior is the feet. As a second example, in humans, the

neck The neck is the part of the body on many vertebrates that connects the head with the torso. The neck supports the weight of the head and protects the nerves that carry sensory and motor information from the brain down to the rest of the body. In ...

is superior to the chest but inferior to the

Anterior and posterior

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

Medial and lateral

These terms describe how close something is to the midline, or the medial plane. Lateral () describes something to the sides of an animal, as in "left lateral" and "right lateral". Medial () describes structures close to the midline, or closer to the midline than another structure. For example, in a human, the arms are lateral to the torso. The genitals are medial to the legs. The terms "left" and "right" are sometimes used, or their Latin alternatives ( la, dexter, lit=right; la, sinister, lit=left). However, as left and right sides are

mirror images A mirror image (in a plane mirror) is a reflected duplication of an object that appears almost identical, but is reversed in the direction perpendicular to the mirror surface. As an optical effect it results from reflection off from substances ...

, using these words is somewhat confusing, as structures are duplicated on both sides. For example, it is very confusing to say the dorsal fin of a fish is "right of" the left pectoral fin, but is "left of" the right

, but much easier and clearer to say "the dorsal fin is medial to the pectoral fins". Terms derived from lateral include: * Contralateral (): 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 (): on the same side as another structure. For example, the left arm is ipsilateral to the left leg. * Bilateral (): on both sides of the body. For example, bilateral

orchiectomy Orchiectomy (also named orchidectomy, and sometimes shortened as orchi or orchie) is a surgical procedure in which one or both testicles are removed. The surgery is performed as treatment for testicular cancer, as part of surgery for transgend ...

means removal of testes on both sides of the body. * Unilateral (): on one side of the body. For example, a

stroke A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop functionin ...

can result in unilateral weakness, meaning weakness on one side of the body. Varus () and valgus ( ) are terms used to describe a state in which a part further away is abnormally placed towards (varus) or away from (valgus) the midline.

Proximal and distal

Blausen 0019 AnatomicalDirectionalReferences

The terms proximal () and distal () are used to describe parts of a feature that are close to or distant from the main mass of the body, respectively. Thus the upper arm in humans is proximal and the hand is distal. "Proximal and distal" are frequently used when describing

s, 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. 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 Moiety may refer to: Chemistry * Moiety (chemistry), a part or functional group of a molecule ** Moiety conservation, conservation of a subgroup in a chemical species Anthropology * Moiety (kinship), either of two groups into which a society is ...

of a given compound.

Central and peripheral

Central and peripheral refer to the distance towards and away from the centre of something. That might be an organ, a region in the body, or an anatomical structure. For example, the central nervous system and the peripheral nervous systems. Central () describes something close to the centre. For example, the great vessels run centrally through the body; many smaller vessels branch from these. Peripheral (, originally from Ancient Greek) describes something further away from the centre of something. For example, the arm is peripheral to the body.

Superficial and deep

These terms refer to the distance of a structure from the surface. Deep () describes something further away from the surface of the organism. For example, the

external oblique muscle The abdominal external oblique muscle (also external oblique muscle, or exterior oblique) is the largest and outermost of the three flat abdominal muscles of the lateral anterior abdomen. Structure The external oblique is situated on the lateral ...

of the abdomen is deep to the skin. "Deep" is one of the few anatomical terms of location derived from

Old English Old English (, ), or Anglo-Saxon, is the earliest recorded form of the English language, spoken in England and southern and eastern Scotland in the early Middle Ages. It was brought to Great Britain by Anglo-Saxon settlement of Britain, Anglo ...

rather than Latin – the anglicised Latin term would have been "profound" (). Superficial () describes something near the outer surface of the organism. For example, in skin, the

epidermis The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water rele ...

is superficial to the

subcutis The subcutaneous tissue (), also called the hypodermis, hypoderm (), subcutis, superficial fascia, is the lowermost layer of the integumentary system in vertebrates. The types of cells found in the layer are fibroblasts, adipose cells, and ma ...

Dorsal and ventral

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

anal fin Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

, but ventral to the dorsal fin.

Cranial and caudal

Specific terms exist to describe how close or far something is to the head or tail of an animal. To describe how close to the head of an animal something is, three distinct terms are used: * Rostral () describes something situated toward the oral or nasal region, or in the case of the brain, toward the tip of the frontal lobe. * Cranial () or cephalic () describes how close something is to the head of an organism. * Caudal () describes how close something is to the trailing end of an organism. For example, in horses, the eyes are caudal to the nose and rostral to the back of the head. These terms are generally preferred in veterinary medicine and not used as often in human medicine.Hickman, C. P. Jr., Roberts, L. S. and Larson, A. ''Animal Diversity''. McGraw-Hill 2003 Miller, S. A. ''General Zoology Laboratory Manual'' McGraw-Hill, and In humans, "cranial" and "cephalic" are used to refer to the skull, with "cranial" being used more commonly. The term "rostral" is rarely used in human anatomy, apart from embryology, and refers more to the front of the face than the superior aspect of the organism. Similarly, the term "caudal" is used more in embryology and only occasionally used in human anatomy. This is because the brain is situated at the superior part of the head whereas the nose is situated in the anterior part. Thus, the "rostrocaudal axis" refers to a C shape (see image).

Other terms and special cases

Anatomical landmarks

The location of anatomical structures can also be described in relation to different anatomical landmarks. They are used in anatomy, surface anatomy, surgery, and radiology. Structures may be described as being at the level of a specific

spinal vertebra The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic ...

, depending on the section of the vertebral column the structure is at. 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 The sacrum (plural: ''sacra'' or ''sacrums''), in human anatomy, is a large, triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part ...

and coccyx are fused, they are not often used to provide the location. References may also take origin from

superficial anatomy Surface anatomy (also called superficial anatomy and visual anatomy) is the study of the external features of the body of an animal.Seeley (2003) chap.1 p.2 In birds this is termed ''topography''. Surface anatomy deals with anatomical features t ...

, made to landmarks that are on the skin or visible underneath. For example, structures may be described relative to the anterior superior iliac spine, the medial malleolus or the medial epicondyle. Anatomical lines are used to describe anatomical location. For example, the mid-clavicular line is used as part of the

cardiac exam In medicine, the cardiac examination, also precordial exam, is performed as part of a physical examination, or when a patient presents with chest pain suggestive of a cardiovascular pathology. It would typically be modified depending on the indi ...

in medicine to feel the apex beat of the heart.

Mouth and teeth

Special terms are used to describe the mouth and teeth. Fields such as

osteology Osteology () is the scientific study of bones, practised by osteologists. A subdiscipline of anatomy, anthropology, and paleontology, osteology is the detailed study of the structure of bones, skeletal elements, teeth, microbone morphology, funct ...

palaeontology Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...

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. For example, the terms "distal" and "proximal" are also redefined to mean the distance away or close to the dental arch, and "medial" and "lateral" are used to refer to the closeness to the midline of the dental arch. Terms used to describe structures include "buccal" () and "palatal" () referring to structures close to the cheek and

hard palate The hard palate is a thin horizontal bony plate made up of two bones of the facial skeleton, located in the roof of the mouth. The bones are the palatine process of the maxilla and the horizontal plate of palatine bone. The hard palate spans ...

respectively.

Hands and feet

Several anatomical terms are particular to the hands and feet. 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 Motion, the process of movement, is described using specific anatomical terms. Motion includes movement of organs, joints, limbs, and specific sections of the body. The terminology used describes this motion according to its direction relative ...

and flip the location of the hand. For improved clarity, the directional term palmar () is commonly used to describe the front of the hand, and dorsal is the back of the hand. For example, the top of a dog's paw is its dorsal surface; the underside, either the palmar (on the forelimb) or the plantar (on the hindlimb) surface. The

palmar fascia The palmar aponeurosis (palmar fascia) invests the muscles of the palm, and consists of central, lateral, and medial portions. Structure The central portion occupies the middle of the palm, is triangular in shape, and of great strength Its apex i ...

is ''palmar'' to the tendons of muscles which flex the fingers, and the

dorsal venous arch The dorsal venous arch of the foot is a superficial vein that connects the small saphenous vein and the great saphenous vein. Anatomically, it is defined by where the dorsal veins of the first and fifth digit, respectively, meet the great saphen ...

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 underside of the palm, but plantar is used exclusively to describe the sole. These terms describe location as ''

palmar Palmar may refer to: Anatomy, relating to the palm of the hand * Palmar aponeurosis, deep fascia connecting and within the muscles of the palm * Palmar arches (disambiguation), various combinations of arteries in the hand and arm * Palmar art ...

'' and '' plantar''; For example, ''volar'' pads are those on the underside of hands or fingers; the ''plantar'' surface describes the sole of the heel, foot or toes. Similarly, in the forearm, for clarity, the sides are named after the bones. 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. Similarly, in the lower leg, structures near the tibia (shinbone) are tibial and structures near the fibula are fibular (or peroneal).

Rotational direction

Anteversion and retroversion are complementary terms describing an anatomical structure that is rotated forwards (towards the front of the body) or backwards (towards the back of the body), relative to some other position. They are particularly used to describe the curvature of the uterus. * Anteversion () describes an anatomical structure being tilted further ''forward'' than normal, whether pathologically or incidentally. For example, a woman's uterus typically is anteverted, tilted slightly ''forward''. A misaligned

pelvis The pelvis (plural pelves or pelvises) is the lower part of the trunk, between the abdomen and the thighs (sometimes also called pelvic region), together with its embedded skeleton (sometimes also called bony pelvis, or pelvic skeleton). The ...

may be anteverted, that is to say tilted ''forward'' to some relevant degree. * Retroversion () describes an anatomical structure tilted ''back'' away from something. An example is a

retroverted uterus A retroverted uterus (tilted uterus, tipped uterus) is a uterus that is oriented posteriorly, towards the back of the body. This is in contrast to the typical uterus, which is oriented forward (slightly "Anteversion, anteverted") toward the Urinar ...

Other directional terms

Several other terms are also used to describe location. These terms are not used to form the fixed axes. Terms include: * Axial (): around the central axis of the organism or the extremity. Two related terms, "abaxial" and "adaxial", refer to locations away from and toward the central axis of an organism, respectively * Luminal (): on the—hollow—inside of an organ's lumen (body cavity or tubular structure); adluminal is towards, abluminal is away from the lumen. Opposite to outermost (the adventitia, serosa, or the cavity's wall). * Parietal (): pertaining to the wall of a body cavity. For example, the parietal peritoneum is the lining on the inside of the abdominal cavity. Parietal can also refer specifically to the parietal bone of the skull or associated structures. * Terminal () at the extremity of a usually projecting structure. For example, "...an antenna with a terminal sensory hair". * Visceral and ''viscus'' (): associated with organs within the body's cavities. For example, the stomach is covered with a lining called the visceral peritoneum as opposed to the parietal peritoneum. Viscus can also be used to mean "organ". For example, the stomach is a viscus within the abdominal cavity, and ''visceral pain'' refers to pain originating from internal organs. * Aboral (opposite to oral) is used to denote a location along the

gastrointestinal canal The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans ...

that is relatively closer to the

anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...

Specific animals and other organisms

Different terms are used because of different body plans in animals, whether animals stand on one or two legs, and whether an animal is symmetrical or not, as discussed above. For example, as humans are approximately

bilaterally symmetrical Symmetry in biology refers to the symmetry observed in organisms, including plants, animals, fungi, and bacteria. External symmetry can be easily seen by just looking at an organism. For example, take the face of a human being which has a pla ...

organisms, anatomical descriptions usually use the same terms as those for other vertebrates. However, humans stand upright on two legs, meaning their anterior/posterior and ventral/dorsal directions are the same, and the inferior/superior directions are necessary. Humans do not have a beak, so a term such as "rostral" used to refer to the beak in some animals is instead used to refer to part of the brain; humans do also not have a tail so a term such as "caudal" that refers to the tail end may also be used in humans and animals without tails to refer to the hind part of the body. In invertebrates, the large variety of body shapes presents a difficult problem when attempting to apply standard directional terms. Depending on the organism, some terms are taken by analogy from vertebrate anatomy, and appropriate novel terms are applied as needed. Some such borrowed terms are widely applicable in most invertebrates; for example proximal, meaning "near" refers to the part of an appendage nearest to where it joins the body, and distal, meaning "standing away from" is used for the part furthest from the point of attachment. In all cases, the usage of terms is dependent on the body plan of the organism. Direcoes anatomicas.svg, Anatomical terms of location in a dog Anatomical-directions-kangaroo.svg, Anatomical terms of location in a kangaroo Anatomical Directions and Axes.JPG, Anatomical terms of location in a fish. Horse Axes.JPG, Anatomical terms of location in a horse.

Asymmetrical and spherical organisms

In organisms with a changeable shape, such as amoeboid organisms, most directional terms are meaningless, since the shape of the organism is not constant and no distinct axes are fixed. Similarly, in spherically symmetrical organisms, there is nothing to distinguish one line through the centre of the organism from any other. An indefinite number of triads of mutually perpendicular axes could be defined, but any such choice of axes would be useless, as nothing would distinguish a chosen triad from any others. In such organisms, only terms such as ''superficial'' and ''deep'', or sometimes ''proximal'' and ''distal'', are usefully descriptive.

Elongated organisms

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 Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a crucia ...

(free-swimming)

protists A protist () is any eukaryotic organism (that is, an organism whose cells contain a cell nucleus) that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor (the last eukaryotic common ancestor), the excl ...

, 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 A cytostome (from ''cyto-'', cell and ''stome-'', mouth) or cell mouth is a part of a cell specialized for phagocytosis, usually in the form of a microtubule-supported funnel or groove. Food is directed into the cytostome, and sealed into vacuole ...

or other unique structure is present. Some elongated

have distinctive ends of the body. In such organisms, the end with a mouth (or equivalent structure, such as the

in '' Paramecium'' or '' Stentor''), or the end that usually points in the direction of the organism's locomotion (such as the end with the flagellum in '' Euglena''), is normally designated as the anterior end. The opposite end then becomes the posterior end. Properly, this terminology would apply only to an organism that is always

(not normally attached to a surface), although the term can also be applied to one that is sessile (normally attached to a surface). Organisms that are attached to a

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (locomotion), the surface over which an organism lo ...

, such as

sponges Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through ...

, animal-like protists also have distinctive ends. The part of the organism attached to the substrate is usually referred to as the basal end (), whereas the end furthest from the attachment is referred to as the apical end ().

Radially symmetrical organisms

Radially symmetrical organisms include those in the group Radiata primarily jellyfish, sea anemones and corals and the comb jellies. Adult

echinoderms An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the sea li ...

, such as

starfish Starfish or sea stars are star-shaped echinoderms belonging to the class Asteroidea (). Common usage frequently finds these names being also applied to ophiuroids, which are correctly referred to as brittle stars or basket stars. Starfish ...

sea urchin Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) of ...

sea cucumber Sea cucumbers are echinoderms from the class Holothuroidea (). They are marine animals with a leathery skin and an elongated body containing a single, branched gonad. Sea cucumbers are found on the sea floor worldwide. The number of holothuria ...

s and others are also included, since they are pentaradial, meaning they have five discrete rotational

symmetry Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definit ...

. Echinoderm larvae are not included, since they are

. Radially symmetrical organisms always have one distinctive axis. Cnidarians (jellyfish, sea anemones and corals) have an incomplete digestive system, meaning that one end of the organism has a mouth, and the opposite end has no opening from the gut (coelenteron). For this reason, the end of the organism with the mouth is referred to as the oral end (), and the opposite surface is the aboral end (). Unlike vertebrates, cnidarians have no other distinctive axes. "Lateral", "dorsal", and "ventral" have no meaning in such organisms, and all can be replaced by the generic term peripheral (). 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. However, some biradially symmetrical comb jellies do have distinct "tentacular" and "pharyngeal" axesRuppert et al. (2004), p. 184. and are thus anatomically equivalent to

animals. Image:Radiate Radial Axes.JPG, ''Aurelia aurita'', another species of jellyfish, showing multiple radial and medio-peripheral axes File:Porania_pulvillus.jpg, The sea star ''Porania pulvillus'', aboral and oral surfaces

Spiders

Special terms are used for

spiders Spiders ( order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk. They are the largest order of arachnids and rank seventh in total species dive ...

. Two specialized terms are useful in describing views of arachnid legs and pedipalps. Prolateral refers to the surface of a leg that is closest to the anterior end of an arachnid's body. Retrolateral refers to the surface of a leg that is closest to the posterior end of an arachnid's body. Most spiders have eight eyes in four pairs. All the eyes are on the

carapace A carapace is a Dorsum (biology), dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tor ...

of the prosoma, and their sizes, shapes and locations are characteristic of various spider families and other taxa. Usually, the eyes are arranged in two roughly parallel, horizontal and symmetrical rows of eyes. Eyes are labelled according to their position as anterior and posterior lateral eyes (ALE) and (PLE); and anterior and posterior median eyes (AME) and (PME). File:Palystes superciliosus male anterior 2012 03 04 3452.JPG, Aspects of spider anatomy; This aspect shows the mainly prolateral surface of the anterior femora, plus the typical horizontal eye pattern of the Sparassidae File:Hogna species female Lycosidae showing eye pattern EOS 027s.jpg, Typical arrangement of eyes in the Lycosidae, with PME being the largest File:Salticidae Male Anterior annotated.jpg, In the Salticidae the AME are the largest

References

Citations

General sources

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