When discussing the anatomy of the human body, it is useful to first define a standard position in which the body will be presented. This standard allows the relative position of parts of the body to be described accurately and with less confusion. Anatomists use the anatomical position. A person in the anatomical position is standing up straight, with arms at the sides and palms facing forwards with the fingers extended. The feet are facing forward, as are the head and eyes (see the figure to the right).
Planes of the Body
Three planes through the body are used to describe the position and orientation of parts of the body. These are:
- Median or Sagittal plane - This plane runs through the body from the head (cephalic end) to the feet (plantar end). A cut on the sagittal plane of humans creates a right and left bilaterally symmetrical half. Any plane parallel to this through the body is known as a para-sagittal plane.
- Coronal plane/lateral/frontal - This plane runs through the body, also from the head to the feet, and divides the body into front and rear halves.
- Transverse plane - This is any plane that runs left to right through the body and divides it into upper and lower sections.
- Oblique plane - Any plane through the body that is not parallel to one of the former three.
When discussing anatomy, the following terms are used to describe the relative positions of the parts of the body. When using these terms, it is assumed that the body is in the anatomical position.
- Lateral - More distant to the body midline. In the anatomical position, the radius is lateral to the ulna. A simpler example is the "thumb" is lateral to the "pinky."
- Medial - Closer to the midline; on the inner side. The sternum (breast plate) is medial to the clavcle (shoulder bone).
- Superior - Above, towards the cephalic (head) end. The cranial cavity (head cavity) is superior to the scapula (shoulder blade).
- Inferior - Below, towards the plantar (foot) end. The patella (knee cap) is inferior to the femur (thigh bone).
- Anterior - Towards the front of the body. The sternum is anterior to the spine. Also known as ventral, but this term is not as common in human anatomy.
- Posterior - Towards the rear/backside of the body. The fibula is posterior to the tibia. Also known as dorsal, but this term is not as common.
- "Oblique sections" - separates top from bottom at a diagonal. This term is common.
- Superficial - Closer to the skin, nearer the body surface. Skin is superficial to the organs.
- Deep - Further from the body surface.
In the limbs, the following terms are also used:
- Distal - Further from the limb's attachment to the trunk (where 'trunk' refers to the 'torso' of the body: the body minus the head, kneck, and limbs).
- proximal - Nearer to the limb's attachment to the trunk. The humerous is proximal to the radius.
Terms may also be combined to describe more complicated relative positions, for example anterolateral, meaning in front of and more lateral to something.
- The thumbs are proximal to the index fingers.
- The shoulder is superior to the hip.
- The vertebrae of the spine are posterior to the abdominal muscles.
- The wrist is distal to the elbow.
- The nose is inferior to the eyes
- The shoulder blade is posterolateral to the spine.
All acts of movement are considered to be a mixture or a single contribution of the following movement terms:
- Flexion - where there is a reduction in the angle between bones or parts of the body. This term applies only to movement along the sagittal or median plane. An example of arms flexing is lifting a dinner plate. When applied to the trunk of the body, this term means bowing forwards.
- Extension - is the opposite of flexion, and there is an increase in the angle. This term applies only to movement along the sagittal or median plane. With the trunk of the body, this movement is bowing backwards.
- Adduction - where there is a reduction in the angle between bones or parts of the body. This only applies to movement along the coronal plane.
- Abduction - the exact opposite, with an increase in the angle. Also only applies to movement along the coronal plane.An example of this is where extending arms outwards as if to fly.
- Rotation - is rotation of an entire limb laterally (away from the body) or medially (towards the midline of the body).
- Pronation - this is the rotation of the hand so that the palm faces posteriorly. This is not medial rotation as this must be performed when the arm is half flexed. Prone means the hand is facing posteriorly.
- Supination - the rotation of the hand so that the palm faces anteriorly. The hand is supine (facing anteriorly) in the anatomical position.
- Protrusion - is the anterior movement of an object. This term is often applied to the jaw.
- Retrusion - the opposite of protrusion.
- Elevation - superior movement. This term is often applied to the shoulders (eg shrugging shoulders is elevation)
- Depression - inferior movement, opposite of elevation.
- Circumduction - is a special case of movement involving adduction, flexion, extension and abduction. The resulting movement creates a circular path of movement. Circumduction can be done in the hip, shoulder, thumb and fingers.
There are several cases where the meaning of the anatomical planes and terms are slightly different. Hence new definitions for movement applies. This is apparent in the foot, the tongue, the hand, the penis, the eyes, and the brain.
Foot and hand
The palm (adj palmar) of the hand corresponds to the sole (adj plantar) of the foot, and the dorsum (back) of the hand corresponds to the dorsum (top) of the foot.
From this, the term dorsiflexion means to flex upwards (true extension) and the term plantarflexion, meaning to extend downwards (true flexion) are derived.
The term volar, used mainly in orthopaedics, is synonymous with palmar and plantar.
The foot is also capable of movement along another axis due to the flexibility of the ankle joint. These movements are
- Eversion - the movement of the sole of the foot away from the median plane.
- Inversion - the movement of the sole towards the median plane. (same as when an ankle is twisted).
The position of the hand in anatomical position is considered supine, such that rotation of the hand so that the palm faces backwards is called pronation and the reverse action, supination.
In the anatomical position, the penis is considered to be erect and pointing upwards, i.e. the dorsal side of the penis is what one would normally call its top side (meaning the side that would touch the belly).
The tongue has a dorsal side which is facing upwards (the side that tastes).