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 are in two rows. The upper row is convex above and the lower row is convex below.
The upper row, beginning on the radial side, is composed of the navicular (scaphoid), lunate, cuneiform, and pisiform. The three first-named articulate with the radius and triangular cartilage, forming the radiocarpal joint, but the pisiform is separate. It is perched on the cuneiform bone and is practically a sesamoid bone developed in the tendon of the flexor carpi ulnaris muscle. The anterior end of the navicular (scaphoid) has on its palmar surface a tuberosity which can be felt immediately below the flexor carpi radialis tendon at the wrist; this tendon passes along the palmar surface to insert in the base of the second metacarpal bone (Fig. 360).
The lower row, beginning on the radial side, is composed of the trapezium, trapezoid, os magnum, and unciform. The first three articulate with the first three metacapal bones but the unciform, like the cuboid in the foot, articulates with two metacarpal bones - the fourth and fifth.
The trapezium articulates with the first metacarpal bone by a saddle-shaped joint and has on its palmar surface a ridge. Along the inner side of this ridge runs the flexor carpi radialis tendon. The ridge of the trapezium and tuberosity of the navicular (scaphoid) give attachment to the radial side of the anterior annular ligament.
The unciform bone has a hook-like (unciform) process on its palmar surface. It can be felt by deep pressure 2 cm. (about 3/4 in.) below and to the radial side of the pisiform bone. This process and the pisiform bone give attachment to the ulnar side of the anterior annular ligament.
The metacarpal bones have their bases at the carpus and their heads toward the phalanges. The shafts are small as compared with the extremities, and hence are not infrequently fractured. On each side of the head is a small projecting tubercle, which, when the bone becomes luxated, catches in the tissues and hinders reduction.
On the palmar surface of the base of the second metacarpal bone is inserted the flexor carpi radialis and into the base of the fifth the flexor carpi ulnaris, which is continued onward from the pisiform bone. On the dorsal surface, into the base of the second, is inserted the extensor carpi radialis longior; into the base of the third (and part of the second) is inserted the extensor carpi radialis brevior, and into the base of the fifth, the extensor carpi ulnaris (Fig. 361).
Thus it will be seen that all the flexors and extensors of the wrist have their ultimate insertion into the metacarpal bones.
The metacarpal bone of the thumb has inserting into its base the extensor ossis metacarpi pollicis tendon. As this tendon has its origin in the forearm it also acts as a carpal extensor, but owing to the movability or the thumb it acts especially as an extensor of the latter, the carpus remaining immovable.