Anatomical terms of location

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Anatomical-directions — Anatomical directions shown on a dog.

In human or zoological anatomy (also called zootomy), many anatomical terms describe the location of organs and other structures in the body of bilateral animals, in standard anatomical position.^[1] These terms, generally from Latin, are explained below. Terminology in human anatomy can differ from general anatomy or in species with different organs.

Directions: general usage

In bilaterally symmetrical animals anterior denotes the end with the mouth and brain, and posterior denotes the end with the anus. In animals with a head the preferred term is cranial for the head end. The opposite end of cranial animals is the caudal end (tail end), which often has an anus and tail. Within the head itself, if there is a nose, rostral is the direction toward the nose, and caudal still refers to the tail direction.

The surface or side of the body normally oriented upwards, away from the pull of gravity, is the dorsal side; the opposite side, typically the one closest to the ground when walking on all legs, swimming or flying, is the ventral side. For example: in vertebrates, the spine or nerve cord is located on the dorsal side of the organism. A dolphin's dorsal fin is, unsurprisingly, on the dorsal side. A cow's udder is on the ventral side. Scales on the belly of a snake are called ventral scales.

On the limbs or other appendages, a point closer to the main body is "proximal"; a point farther away is "distal". (For the latter terms usage in dentistry, please see here.)

The right and left side (sometimes in Latin: dexter - right, and sinister - left) are always given as viewed from the animal that is described.

Directions: human anatomy

In humans, portions of the body which are closer to the head end are "superior" (Latin "upper"); those which are farther away are "inferior" ("lower") -- "superior" corresponds to cranial ('at the skull'), or cephalic (head), and "inferior" corresponds to caudal ('at the tail'). Objects near the front are "anterior"; those near the rear are "posterior" -- these correspond to the terms "ventral", and "dorsal" used for quadrupeds.

The terms "anterior" and "posterior" are confusing when referring to most animals, however, and are particularly unsuitable for quadrupeds. In this case, "rostral/cranial" and "caudal" are more appropriate. However the word posterior is commonly used (in common slang usage) as a noun meaning the buttocks or an adjective referring to them.

In humans, anatomical terms of motion refer to changes away from the standard anatomical position, such as the position of the forearm with the palm directed anteriorly (or upwards when seated) is known as the supine position. Where the palm faces posteriorly (or downwards when seated), this is the prone position. Turning the hand from prone to supine is called supination; turning the hand from supine to prone is pronation. Pronation results in crossing of the radius (bone) with respect to the ulna. To avoid confusion with "medial" and "lateral", different terms are used when describing the sides of the forearm (see below).

Relative directions

Structures near the midline are called medial and those near the sides of animals are called lateral. Therefore, medial structures are closer to the midsagittal plane, lateral structures are further from the midsagittal plane. Structures in the midline of the body are median, or medial. For example, your cheeks are lateral to your nose and the tip of the nose is in the median line. Ipsilateral means on the same side, contralateral means on the other side and bilateral means on both sides.

Structures on or closer to the body's surface are superficial (or external) and those further inside are profound or deep (or internal).

When speaking of inner organs, visceral means attached to or associated with an organ, while parietal refers to a structure associated with or attached to the body wall (the chest wall or the abdominal wall). For example, whilst the pleura is a single structure, for convenience the term "visceral pleura" is used to refer to that part attached to the outer surface of the lung, and "parietal pleura" to refer to that part attached to the inside of the chest wall.

Also, in common usage, the segments of the digestive system closest to the mouth are termed proximal, as opposed to those closest to the anus, which are termed distal.

Relative directions in the limbs

Specialized terms are used to describe location on appendages, parts that have a point of attachment to the main trunk of the body. Structures that are close to the point of attachment of the body are proximal or central, while ones more distant from the attachment point are distal or peripheral. For example, the hands are at the distal end of the arms, while the shoulders are at the proximal ends. These terms can also be used relatively to organs, for example the proximal end of the urethra is attached to the bladder.

In the limbs of most animals, the terms cranial and caudal are used in the regions proximal to the carpus (the wrist, in the forelimb) and the tarsus (the ankle in the hindlimb). Objects and surfaces closer to or facing towards the head are cranial; those facing away or further from the head are caudal.

Nearer the carpal joint, the term dorsal replaces cranial and palmar replaces caudal. Similarly, nearer the tarsal joint the term dorsal replaces cranial and plantar replaces caudal. 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 sides of the forearm are named after its 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).

Volar (sometimes used as a synonym for "palmar") refers to the underside, for both the palm and the sole (plantar), as in volar pads on the underside of hands, fingers, feet and toes.

The terms valgus and varus are used to refer to angulation of the distal part of a limb at a joint. For example, at the elbow joint, in the anatomical position, the forearm and the upper arm do not lie in a straight line, but the forearm is angulated laterally with respect to the upper arm by about 5-10°. The forearm is said to be "in valgus". Angulation at a joint may be normal (as in the elbow) or abnormal.

Planes

General usage

Three basic reference planes are used in zoological anatomy.

A sagittal plane divides the body into sinister and dexter (left and right) portions.
- The midsagittal plane is in the midline, i.e. it would pass through midline structures such as the navel or spine, and all other sagittal planes (also referred to as parasagittal planes) are parallel to it.
A coronal or frontal plane divides the body into dorsal and ventral (back and front, or posterior and anterior) portions.
A transverse plane divides the body into cranial and caudal (head and tail) portions.

For post-embryo humans a coronal plane is vertical and a transverse plane is horizontal, but for embryos and quadripeds a coronal plane is horizontal and a transverse plane is vertical.

When describing anatomical motion, these planes describe the axis along which an action is performed. So by moving through the transverse plane, movement travels from head to toe. For example, if a person jumped directly up and then down, their body would be moving in the transverse plane.

Some of these terms come from Latin. Sagittal means "like an arrow," a reference to the position of the spine which naturally divides the body into right and left equal halves, the exact meaning of the term "midsagittal".

Usage in human anatomy

Sometimes the orientation of certain planes needs to be distinguished, for instance in medical imaging techniques such as sonography, CT scans, MRI scans or PET scans. One imagines a human in the anatomical position, and an X-Y-Z coordinate system with the X-axis going from front to back, the Y-axis going from left to right, and the Z-axis going from up to down. The X-axis axis is always forward (Tait-Bryan angles) and the right-hand rule applies.

A transverse (also known as axial or horizontal) plane is an X-Y plane, parallel to the ground, which (in humans) separates the superior from the inferior, or put another way, the head from the feet.
A coronal (also known as frontal) plane is an Y-Z plane, perpendicular to the ground, which (in humans) separates the anterior from the posterior, the front from the back, the ventral from the dorsal.
A sagittal plane is an X-Z plane, perpendicular to the ground, which separates left from right. The midsagittal plane is the specific sagittal plane that is exactly in the middle of the body.

The axes and the sagittal plane are the same for bipeds and quadrupeds, but the orientation of the coronal and transverse planes switch. The axes on particular pieces of equipment may or may not correspond to axes of the body, especially since the body and the equipment may be in different relative orientations.

Occasionally, in medicine, abdominal organs may be described with reference to the trans-pyloric plane which is a transverse plane passing through the pylorus.

Anatomical planes in animal brains

In discussing the neuroanatomy of animals, particularly rodents used in neuroscience research, the convention has been to name the sections of the brain according to the homologous human sections. Hence, what is technically a transverse section with respect the body of a rat (dividing anterior from posterior) may often be referred to in rat neuroanatomical coordinates as a coronal section, and likewise a coronal section with respect to the body (ie. dividing ventral from dorsal) in a rat brain is referred to as transverse. This preserves the comparison with the human brain which is rotated with respect to the body axis by 90 degrees in the ventral direction. It does mean that the planes of the rat brain are not necessarily the same as those of the body.

Surface and other landmarks in humans

In humans, reference may be made to landmarks which are on the skin or visible underneath. As with planes, lines and points are imaginary. Examples include:

The mid-axillary line, a line running vertically down the surface of the body passing through the apex of the axilla (armpit). Parallel are the anterior axillary line, which passes through the anterior axillary skinfold, and the posterior axillary line, which passes through the posterior axillary skinfold.
The mid-clavicular line, a line running vertically down the surface of the body passing through the midpoint of the clavicle.
The mid-pupillary line, a line running vertically down the face through the midpoint of the pupil when looking directly forwards.
The mid-inguinal point, which is the point midway between the anterior superior iliac spine and the pubic tubercle.
Tuffier's line, which is a transverse line passing across the lumbar spine between the posterior iliac crests.

Additionally, reference may be made to structures at specific levels of the spine (e.g. the 4th cervical vertebra, abbreviated "C4"), or the rib cage (e.g. the 5th intercostal space, abbreviated "5ICS").

Relative motions

Main article: Anatomical terms of motion

Standard anatomical position

Anatomical terms are applied to the subject (human or animal) in the standard anatomical position. This is typically a (standing) posture similar to that seen during life, rather than lying on a table. For humans, the body is standing erect, feet together and toes pointed forward, arms at the sides and palms facing forward (forearms supine). In other species, e.g. quadrupeds, the standard anatomical position is described as standing erect with the head facing forwards in a neutral position.

In humans, the anatomical position of the skull has been agreed by international convention to be the Frankfurt plane, a position where the lower margins of the orbits and the upper margins of the ear canals all lie in the same horizontal plane. This is a good approximation to the position where the subject is standing upright and facing forwards.

Reference

↑ Introduction page, "Anatomy of the Human Body". Henry Gray. 20th edition. 1918.

External links

This page uses Creative Commons Licensed content from Wikipedia (view authors).

[intro_HGray-1] Introduction page, "Anatomy of the Human Body". Henry Gray. 20th edition. 1918.

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