Anatomical terms of location
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, behind 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
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. Much of this information has been standardised in internationally agreed vocabularies for humans, with Nomina Anatomica Veterinaria and Nomina Embryologica Veterinaria used for animal anatomy.Different terms are used for those vertebrates that are bipedal and those that are quadrupedal. The reasoning is that the neuraxis, and therefore the standard anatomical position is different between the two groups. Unique terms are also used to describe invertebrates, because of their wider variety of shapes and symmetries.
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. In humans, this refers to the body in a standing position with arms at the side and palms facing forward. In quadrupeds this is an animal standing upright with all four feet on the ground and the head facing forward. For a fish this is belly down with neutral appendages.Planes
Anatomical terms describe structures with relation to three main anatomical planes. Anatomical planes are useful in a number of fields including medical imaging, embryology, and the study of movement.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. 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.
- The coronal plane or frontal plane divides the body into front and back parts. In quadrupeds this plane is termed the dorsal plane and divides the body into dorsal and ventral parts.
- The transverse plane, also called the axial plane or horizontal plane, is perpendicular to the other two planes.
Axes
The three axes of a vertebrate, are formed in embryonic development before and during the gastrulation stage. 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.- The left-right axis, also known as the horizontal or frontal axis
- The craniocaudal axis, also known as the rostrocaudal, longitudinal or cephalocaudal
- The anteroposterior axis, also known as the dorsoventral, or sagittal axis
Main terms
Superior and inferior
In the standard human anatomical position, superior or cranial, describes something that is nearer to the head, and inferior or caudal describes what is below, and nearer to the feet. Examples are the superior mediastinum, and inferior mediastinum. Neuroanatomy examples are the superior colliculus, and the inferior colliculus. 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.Anterior and posterior
Anterior describes what is in front, and posterior describes what is to the back of something. 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.Dorsal and ventral
These two terms, used in veterinary anatomy, are also used in human anatomy mostly in neuroanatomy, and embryology, to describe something at the back or front of an organ, or organism.The dorsal surface, 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.
The ventral surface refers to the front, or lower side, of an organism, or organ such as the undersurface of the tongue.
In a fish, the dorsal fin is on the upper surface and its ventral fins are on the belly or undersurface.
The terms are used in other contexts, for example in dorsal and ventral gun turrets on a bomber aircraft.
Medial and lateral
These terms describe how close something is to the median plane. Lateral describes something to the sides of an animal, as in "left lateral" and "right lateral". Medial describes structures close to the median plane, or closer to the median plane than another structure. 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 : 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 means removal of testes on both sides of the body.
- Unilateral one-sided or single-sided: on one side of the body. For example, unilateral deafness is hearing impairment in one ear.
Proximal and distal
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. The main mass is taken as the center, the chest, or the heart."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.
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.
Rostral, 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.
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. The "rostrocaudal axis" refers to the curved line of the neuraxis from the forehead towards the tail end.