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 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.
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 standards 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 doctors, to have a standard set of terms to communicate clearly the position of a structure.
Introduction
Standard anatomical and zoological terms of location have been developed, usually based on Latin and Greek words, to enable all biological and medical scientists, veterinarians, 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 (Terminologia Anatomica) and animals (Nomina Anatomica Veterinaria).
Different terms are used for groups of creatures with different body layouts, such as bipeds (creatures that stand on two feet, such as humans) and quadrupeds. The reasoning is that the neuraxis is different between the two groups, and so is what is considered the standard anatomical position, such as how humans tend to be standing upright and with their arms reaching forward. Thus, the "top" of a human is the head, whereas the "top" of a dog would be the back, and the "top" of a flounder may be on either the left or right side. Unique terms are also used to describe invertebrates as well, because of their wider variety of shapes and symmetry.
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. 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
Many anatomical terms can be combined, either to indicate a position in two axes simultaneously or to indicate the direction of a movement relative to the body. For example, "anterolateral" indicates a position that is both anterior and lateral to the body axis (such as the bulk of the pectoralis major muscle).
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 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 head, 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.