A Anatomical Terms Of Location

All vertebrates (including humans) have the same basic body plan – they are strictly bilaterally symmetrical in early embryonic stages and largely bilaterally symmetrical in adulthood. That is, they have mirror-image left and right halves if divided down the middle. For these reasons, the basic directional terms can be considered to be those used in vertebrates. By extension, the same terms are used for many other (invertebrate) organisms as well.

Standardized anatomical and zoological terms of location have been developed, usually based on Latin and Greek words, to enable all biological and medical scientists to precisely delineate and communicate information about animal bodies and their component organs, even though the meaning of some of the terms often is context-sensitive.

The vertebrates and Craniata share a substantial heritage and common structure, so many of the same terms are used for location. To avoid ambiguities this terminology is based on the anatomy of each animal in a standard way.

While these terms are standardized within specific fields of biology, there are unavoidable, sometimes dramatic differences between some disciplines. For example, differences in terminology remain a problem that, to some extent, still separates the terminology of human anatomy from that used in the study of various other zoological categories.

For humans, one type of vertebrate, anatomical terms may differ from other forms of vertebrates. For one reason, this is because humans have a different neuraxis and, unlike animals that rest on four limbs, humans are considered when describing anatomy as being in the standard anatomical position. Thus what is on “top” of a human is the head, whereas the “top” of a dog may be its back, and the “top” of a flounder could refer to either its left or its right side.

A.1 Anatomical Planes

Anatomical planes

A transverse plane, also known as a cross-section, divides the body into cranial and caudal (head and tail) portions.
A longitudinal plane is any plane that is perpendicular to the transverse plane. The main longitudinal planes are:
- The frontal plane or coronal plane divides the body into dorsal and ventral (back and front, or posterior and anterior) portions. For post-embryonic humans a coronal plane is vertical and a transverse plane is horizontal, but for embryos and quadrupeds a coronal plane is horizontal and a transverse plane is vertical.
- The sagittal plane is a plane parallel to the sagittal suture. All other sagittal planes (referred to as parasagittal planes) are parallel to it. The plane is a Y-Z plane, perpendicular to the ground. A special sagittal plane is the median plane or midsagittal plane in the midline of the body, and divides the body into left and right (sinister and dexter) portions. This passes through the head, spinal cord, navel, and, in many animals, the tail. The term “median plane” can also refer to the midsagittal plane of other structures, such as a digit.

A.2 Anatomical Axes

To begin with, distinct, polar-opposite ends of the organism are chosen. By definition, each pair of opposite points defines an axis. In a bilaterally symmetrical organism, there are 6 polar opposite points, giving three axes that intersect at right angles – the x, y, and z axes familiar from three-dimensional geometry.

Table A.1: The main anatomical axes.
Axis	Directional term	Directed towards
Anteroposterior	Anterior	Belly in orthograde bipeds (including humans)
Anteroposterior	Anterior	Head end in fish
Posterior	Back in orthograde bipeds
Posterior	Rear/tail end
Rostrocaudal, craniocaudal, cephalocaudal, longitudinal	Rostral, cranial, cephalad	Head end
Caudal	Rear/tail end
Caudal	Inferior in humans
Dorsoventral	Dorsal	Back, spinal column
Ventral	Belly
Left-right, dextro-sinister, sinistro-dexter	Left (sinister)	Left-hand side
Right (dexter)	Right-hand side
Mediolateral	Medial	Centre
Lateral	Left and right
Proximal/distal	Proximal	Point at which appendage joins the body
Distal	Extremity of appendage

The terms “intermediate”, “ipsilateral”, “contralateral”, “superficial”, and “deep”, while indicating directions, are relative terms and thus do not properly define fixed anatomical axes. Also, while the “rostrocaudal” and anteroposterior directionality are equivalent in a significant portion of the human body, they are different directions in other parts of the body.

A.3 Main Anatomical Terms

A.3.1 Superior And Inferior

In anatomical terminology superior (from Latin, meaning ‘above’) is used to refer to what is above something, and inferior (from Latin, meaning ‘below’) to what is below it. For example, in the anatomical position 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 is superior to the chest but inferior to the head.

A.3.2 Anterior And Posterior

Anterior refers to what is in front (from Latin ante, meaning “before”) and posterior, what is to the back of the subject (from Latin post, meaning “after”). For example, in a dog the nose is anterior to the eyes and the tail is considered the most posterior part; in many fish the gill openings are posterior to the eyes, but anterior to the tail. In projectional radiography terminology, an anteroposterior (AP) projection is taken with the X-ray generator anteriorly (such as in the front of a human), and the X-ray detector posteriorly. In contrast, a posteroanterior (PA) projection is taken with the X-ray generator posteriorly.

A.3.3 Medial And Lateral

Lateral (from Latin lateralis, meaning ‘to the side’) refers to the sides of an animal, as in “left lateral” and “right lateral”. The term medial (from Latin medius, meaning ‘middle’) is used to refer to structures close to the centre of an organism, called the “median plane”. For example, in a human, imagine a line down the center of the body from the head though the navel and going between the legs— the medial side of the foot would be the big toe side; the medial side of the knee would be the side adjacent to the other knee. To describe the sides of the knees touching each other would be “right medial” and “left medial”.

The terms “left” and “right” are sometimes used, or their Latin alternatives (Latin: dexter; “right”, Latin: sinister; “left”). However, as left and right sides are mirror images, using these words is somewhat confusing, as structures are duplicated on both sides. For example, it is very confusing to say the dorsal fin of a dolphin is “right of” the left pectoral fin, but is “left of” the right eye, but much easier and clearer to say “the dorsal fin is medial to the pectoral fins”.

Terms derived from lateral include:

Contralateral (from Latin contra, meaning ‘against’): on the side opposite to another structure. For example, the right arm and leg are represented by the left, i.e., contralateral side of the forebrain.
Ipsilateral (from Latin ipse, meaning ‘same’): on the same side as another structure. For example, the left arm is ipsilateral to the left leg.
Bilateral (from Latin bis, meaning ‘twice’): on both sides of the body. For example, bilateral orchiectomy (removal of testes on both sides of the body’s axis) is surgical castration.
Unilateral (from Latin unus, meaning ‘one’): on one side of the body. For example, unilateral paresis is hemiparesis.

Terms derived from medial include:

Inferomedial (from Latin inferus, meaning ‘lower’): lower and in or near the midline. For example, the human nose is inferomedial to the eyes.
Superomedial (from Latin superus, meaning ‘above’): above and toward the midline. For example, the human nose is superomedial to the mouth.

A.3.4 Central And Peripheral

Central and peripheral are terms that are closely related to concepts such as proximal and distal, but they are so widely applicable that in many respects their flexibility makes them hard to define. Loosely speaking, they distinguish near and far, inside and out, or even organs of vital importance such as heart and lungs, from peripheral organs such as fingers, that undoubtedly may be important, but which it may not be life-threatening to dispense with. Examples of the application of the terms are the distinction between central- and peripheral nervous systems, and between peripheral blood vessels and the central circulatory organs, such as the heart and major vessels. The terms also can apply to large and complex molecules such as proteins, where central amino acid residues are protected from antibodies or the like, but peripheral residues are important in docking and other interactions. Other examples include Central and peripheral circadian clocks, and central versus peripheral vision.

A.3.5 Superficial And Deep

These two terms relate to the distance of a structure from the surface of an animal.

Deep (from Old English) refers to something further away from the surface of the organism. 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, meaning ‘due to depth’).

Superficial (from Latin superficies, meaning ‘surface’) refers to something near the outer surface of the organism. For example, in skin the epidermis is superficial to the subcutis.

A.3.6 Dorsal And Ventral

These two terms, used in anatomy and embryology, refer to back (dorsal) and front or belly (ventral) of an organism.

The dorsal (from Latin dorsum, meaning ‘back’) surface of an organism refers to the back, or upper side, of an organism. If talking about the skull, the dorsal side is the top.

The ventral (from Latin venter, meaning ‘belly’) 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, but ventral to the dorsal fin.

A.3.7 Cranial And Caudal

Figure A.1: In the human skull the terms rostral and caudal are adapted to the curved neuraxis of Hominidae

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, meaning ‘beak, nose’), meaning situated toward the oral or nasal region, or in the case of the brain, toward the tip of the frontal lobe.
Cranial (from Greek κρανίον, meaning ‘skull’) or cephalic (from Greek κεφαλή, meaning ‘head’).
Caudal (from Latin cauda, meaning ‘tail’) is used to describe how close something is to the trailing end of an organism, For example, in the horse, the eyes are caudal to the nose and rostral to the back of the head.

These terms are generally preferred in veterinary medicine and not used as often in human medicine. In humans, “cranial” and “cephalic” are used to refer to the skull, with “cranial” being used more commonly. The term “rostral” is rarely used in human anatomy, apart from embryology, and refers more to the front of the face than the superior aspect of the organism. Similarly, the term “caudal” is 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).

The location of anatomical structures can also be described with relation to different anatomical landmarks.

Structures may be described as being at the level of a specific spinal vertebra, 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 and coccyx are fused, they are not often used to provide location.

Directional and locational prefixes can modify many anatomical and morphological terms, sometimes in formally standard usage, but often attached arbitrarily according to need or convenience.

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, meaning ‘axle’): around the central axis of the organism or the extremity. Two related terms, “abaxial” and “adaxial”, refer to locations away from and toward the central axis of an organism, respectively
Parietal (from Latin paries, meaning ‘wall’): pertaining to the wall of a body cavity. For example, the parietal peritoneum is the lining on the inside of the abdominal cavity. Parietal can also refer specifically to the parietal bone of the skull or associated structures.
Posteromedial (from Latin posterus, meaning ‘coming after’, and medius, meaning ‘middle’): situated towards the middle of the posterior surface.
Posterosuperior (from Latin posterus, meaning ‘coming after’ and superior): situated towards the upper part of the posterior surface.
Terminal (from Latin terminus, meaning ‘boundary or end’) at the extremity of a (usually projecting) structure, as in “…an antenna with a terminal sensory hair”.
Visceral and viscus (from Latin viscera, meaning ‘internal organs’): associated with organs within the body’s cavities. For example, the stomach is covered with a lining called the visceral peritoneum as opposed to the parietal peritoneum. Viscus can also be used to mean “organ”. For example, the stomach is a viscus within the abdominal cavity.

A.4 Prefixes

Sub- (from Latin sub, meaning ‘preposition beneath, close to, nearly etc’) appended as a prefix, with or without the hyphen, qualifies terms in various senses. Consider subcutaneous as meaning beneath the skin, subterminal meaning near to the end of a structure. Sub- also may mean “nearly” or “more-or-less”; for instance subglobular means almost globular. In many usages sub- is similar in application to “hypo-”
Hypo- (from Ancient Greek ὑπό, meaning ‘under’) Like “sub” in various senses as in hypolingual nerve beneath the tongue, or hypodermal fat beneath the skin
Infra- (from Latin infra, meaning ‘preposition beneath, below etc’) Similar to “sub”; a direct opposite to super- and supra-, as in Infratemporal space or infraorbital.
Inter- (from Latin inter, meaning ‘between’): between two other structures. For example, the navel is intermediate to the left arm and the contralateral (right) leg. The intercostal muscles run between the ribs.
Super- or Supra- (from Latin super, supra, meaning ‘above, on top of, beyond etc’) appended as a prefix, with or without the hyphen, as in superciliary arches or supraorbital