To find a location one needs directions from a known point on a map. In the body, specific anatomical terms are used for directions. Bony prominences are usually used as reference points and the body’s map is the anatomical position (figure 1). In the latter, the subject stands upright, with their arms down by their sides, and head, trunk, palms and toes facing forwards.
The sagittal plane passes vertically from front to back, the midsagittal (median) plane is in the midline and parasagittal planes are to either side of it. The coronal (frontal) plane passes vertically from side to side at right angles to the sagitttal plane and the transverse (horizontal) plane passes horizontally at right angles to the other two (producing a cross sectional/axial/transaxial radiological image). Outer structures are superficial to inner, deep, structures. The median line passes vertically downwards at the junction of the midsagittal and midcoronal planes. In the anatomical position, the body’s centre of gravity is in the midline, just anterior to the second piece of the sacrum.
When relating two points in the coronal plane, the point nearest the midsagittal plane is medial to the second, and the second point is lateral to the first (in figure 1, A is medial to B and B is lateral to A). When comparing two points along a limb, the point nearer to the trunk is proximal and the one furthest away is distal. Ipsilateral refers to structures on the same side of the body and contralateral the opposite.
The front of the body in the anatomical position is termed anterior and the back posterior. Ventral is usually synonymous with anterior and dorsal with posterior (in figure 1, E is anterior to F and F posterior to E). An exception is the foot where, due to inward twisting during development, the dorsal aspect of the foot comes to lie in front and the ventral aspect behind. Later, when taking up the upright position, the ventral part becomes the sole. When relating two points in the horizontal plane, the one above is superior (cranial, rostral) to the second, and the second point is inferior (caudal) to the first (in figure 1, G is superior to H and H is inferior to G).
The direction of joint movements is also described in relation to the anatomical position. The posture of the subject in figure 2 is in the flexed position and figure 3 extended
Forward movement of the distal part of the limb from the anatomical position, is termed flexion (of the wrist in figure 4a) and backward movement extension (of the wrist and hip in figure 4b,c): note that the description relates to the anatomical position not to the position of the joint in space.
Movement of the head and the trunk to one side is termed lateral flexion (figure 5a,b), movement of a limb away from the midline is termed abduction (figure 6a) and towards the midline adduction (figure 6b). In digital movement, the midline is taken as the middle finger and the second toe: figure 7a,b show finger abduction and adduction respectively. In development, the thumb rotates inwards through approximately 90 degrees, and its movements are described in relation to this acquired position, i.e. to the plane of the thumbnail (figure 8 shows thumb extension). In addition, movement of the thumb towards the fingers is termed opposition
Rotation of the head and trunk is to the right or the left (figure 9
shows rotation of the head to the right). In the joints of the limbs, rotation that moves the thumb or the big toe, from the anatomical position towards the midline, is termed medial (internal) rotation (figure 10a) and, away from the midline, lateral (external) rotation (figure 10b). Rotation of the lower limb occurs mainly at the hip joint, but is also demonstrable in the bent knee: figure 11a shows medial rotation from the anatomical position (figure 11b) and figure 11c, lateral rotation.
Medial (internal) rotation of the forearm at the elbow is termed pronation (figure 12a) and lateral (external) rotation, supination (figure 12b). Circular movement of the distal part of a limb, e.g. the hand at the wrist, is termed circumduction.
The rotation of the lower limb during development also affects the description of movements of the knee, ankle and foot joints. Backward movement of the distal part of these joints is flexion and forward movement, extension. In the ankle, specific terms are used: upward movement of the foot is dorsiflexion (figure 13) and downward movement, plantar flexion (figure 14); turning the sole inwards is inversion (figure 15) and outwards, eversion (figure 16).