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In human and zoological anatomy (sometimes called zootomy), several terms are used to describe the location of organs and other structures in the body of bilateral animals. These terms are listed and explained here. In some cases, the terminology in human anatomy may differ from that in general anatomy (see below). Some specific details of human anatomy are described under anatomical position.
Anatomical directions and planes shown on a kangaroo.
Animals typically have one end with a head and mouth, with the opposite end often having the anus and tail. The head end is the cranial end; the tail end is the caudal end. Within the head itself, rostral refers to the direction toward the end of the nose, and caudal is still used to refer 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 chord is located on the dorsal side of the organism. A cow's udder is on the ventral side. A dolphin's dorsal fin is, unsurprisingly, on the dorsal side.
On the limbs or other appendages, a point closer to the main body is "proximal"; a point farther away is "distal".
The right and left side (sometimes in Latin: dexter - right, and sinister - left) are given as viewed from the animal that is described. Ipsilateral means on the same side, while contralateral means on the other side.
Usage in human anatomy
In human anatomy, the body and its parts are always described using the assumption that the body is in anatomical position, i.e. standing upright.
Portions of the body which are closer to the head end are "superior" ("upper"); those which are farther away are "inferior" ("lower") -- superior corresponds to cranial, and inferior to caudal. Objects near the front are "anterior"; those near the rear are "posterior" -- these correspond respectively to "ventral" and "dorsal".
The terms "anterior" and "posterior" should not be used when referring to most animals however, and are particularly incorrect for quadrupeds.
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. For example, your cheeks are lateral to your nose and the tip of the nose is in the median line.
Structures that are close to the center of the body are proximal or central, while ones far removed 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.
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 close to or attached to the organ, while parietal is more distant. For example, the visceral pleura is attached to the lung and the parietal pleura is attached to the chest wall.
Relative directions in the limbs
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. This usage is not common in human antomy, however.
Distal to the carpal joint, the term dorsal replaces cranial and palmar replaces caudal. Similarly, distal to 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, and structures closer to the ulna are ulnar. Similarly, in the lower leg, structures near the tibia (shinbone) are tibial and structures near the fibula are fibular (or peroneal).
Three basic reference planes are used in zoological anatomy. The sagittal plane divides the body into left and right portions. A coronal plane divides the body into dorsal and ventral portions. A transverse plane divides the body into cranial and caudal portions.
Usage in human anatomy
Sometimes the orientation of certain planes need to be distinguished, for instance in medical imaging techniques such as CT scans, MRI scans or PET scans. One imagines a human in anatomical position (standing, arms hanging down with palms to the front) and an X-Y-Z coordinate system with the X-Y plane parallel to the ground, the X-axis going front and back, the Y-axis passing left and right, and the Z-axis going up and down.
- A transverse or axial 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 or 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 a X-Z plane, perpendicular to the ground and to the coronal plane, which separates left from right. The midsagittal plane is the specific sagittal plane that is exactly in the middle of the body.
Flexion means approximating adjacent parts of the body (usually at a joint) and extension means separating them. For example, the legs are flexed at the knee joints when sitting down, and extended when standing up. Generally, flexion produces an acute angle between adjacent parts, with its vertex at the joint, and extension produces an obtuse angle. One exception to this rule is in the ankle joint where moving the foot such that the toes move upwards is dorsiflexion and moving the foot such that the toes move downwards is plantar flexion.
Adduction means moving a part of the body toward or past its median line or toward the long axis of a limb. Abduction means moving a part of the body away from its median line or away from the long axis of a limb. For example, adducting the thighs brings the legs together, and abducting the thighs spreads the legs apart. Similarly, adducting the fingers brings them into contact with one another, and abducting the fingers spreads them apart.
Rotation means moving a part about its long axis, for example, in turning the neck. Supination means rotation of the forearm such that the palm of the hand faces forward or upward, and pronation means rotation of the forearm such that the palm of the hand faces backward or downward; the forearm with the hand is supinated or pronated at the elbow. Similar movements may be accomplished at the ankle, where supination results in the foot tipping inward relative to its long axis, and pronation results in the foot tipping outward; overpronation may contribute to the condition flatfoot.
An anterograde motion is in the normal direction of flow, while retrograde means reversed flow. For example, passage of food from the mouth to the stomach is in an anterograde direction, and gastric reflux is in a retrograde direction.