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. 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 that are close to the center of the body are proximal or central, while ones more distant 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 anatomy, 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, 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, a term which refers to both the palm and the sole, is sometimes used as a synonym for "palmar".

PLANES

Three basic reference planes are used in zoological anatomy. A sagittal plane divides the body into 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 are parallel to it. A coronal plane divides the body into dorsal and ventral portions. A transverse plane divides the body into cranial and caudal portions.

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 (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 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.

The anatomical position is a schematic convention for describing the relative morphology of the human body. All terms in the study of anatomy refer to when the body is in this position.

The person is oriented:

• in an erect standing position

• eyes and head facing forward

• feet forward and perpendicular to the body

• arms close to the sides and palms of the hands facing forward with fingers extended.

It should be noted that this is not the normal position that arms rest in.