This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The carpal bones besides being connected by short ligaments running from one to another on their dorsal and palmar surface also have their adjacent surfaces connected by interosseous ligaments. The three bones of the first row are joined by two ligaments near their proximal surfaces which prevent any communication of the radiocarpal with the midcarpal articulations. The four bones of the second row are joined together by interosseous ligaments (fibrocartilages, Morris) which are not complete. That between the os magnum and the unciform is attached more toward the palmar surface, while that between the os magnum and trapezoid is more toward the dorsal surface. The interosseous ligament between the trapezium and trapezoid is usually lacking (Fig. 365).
Fig. 365. - The carpal bones and joints.
From the above description it will be seen that the joints of the carpus (with the exception of the pisiform) all communicate with one another and with the carpometacarpal joints, and that the synovial membrane is practically continuous; hence suppuration implicating the synovial membrane at any point can travel without hindrance between all the carpal (with the exception of the pisiform) and metacarpal bones.
While the amount of motion between the individual carpal bones is limited to a slight gliding on one another, still, when taken together, a very considerable range of movement is allowed. The hand can be flexed and extended, abducted and adducted, and circumducted, but not rotated. If the bones of the forearm at the wrist are held immovable it is impossible to rotate the hand.
The radiocarpal joint bends more freely posteriorly (extension) than anteriorly, while the midcarpal bends more freely in the opposite direction (Fig. 343, 344, p. 334), adduction (toward the ulnar side) is more extensive than abduction. The movement between the two rows of carpal bones is quite extensive.
The movements of the inner four carpometacarpal joints are both of flexion and extension, mainly toward the palmar surface, and a lateral flexion and extension which enables a person to " hollow" the hand and so grasp round objects. The palmar flexion of the fourth and fifth metacarpal bones is more marked than that of the index and middle ones. The middle metacarpal bone is the least movable. The metacarpal bone of the thumb articulates with the trapezium by a saddle-shaped joint which allows flexion, extension, abduction, adduction, and circumduction, but little or no rotation.
Abduction, adduction, and circumduction of the thumb occur at the carpometacarpal articulation and not at the metacarpophalangeal articulation. This latter is a pure hinge-joint and possesses the movements of flexion and extension only.
The metacarpophalangeal articulations of the fingers are practically saddle-shaped joints resembling somewhat the ball-and-socket joints with all their movements except that of rotation. They can be flexed to an angle of 90 degrees. The interpha-langeal joints are hinge-joints and capable only of flexion and extension. The second joint can be flexed to an angle of 150 degrees and the end joint to about a right angle.
In flexion the distal phalanx always passes under the proximal bone, thus causing the prominence of the knuckle to be formed by the proximal phalanx or metacarpal bone (Fig. 366).
Fig. 366. - Showing how, when the fingers are flexed, the prominence of the knuckles is formed by the projection of the proximal bone.