The skeleton of the hand consists of three segments : (1) the carpal or wrist bones ; (2) the metacarpal bones or bones of the palm ; and (3) the phalanges or bones of the digits.
THE CARPALS (figs. 424-427)
General features.-The carpals is made up of eight short bones, which are arranged in a proximal and a distal row, each containing four bones. The carpal bones of the proximal row are named, from the lateral to the medial side, the scaphoid (navicular), lunate, triquetral and pisiform ; those of the distal row, the trapezium (os multangulum magus), the trapezoid (os multangulum minus), the capitate and the hamate. Of these, the pisiform is placed on the anterior aspect of the triquetral and is separated from the other carpal bones, all of which articulate with their immediate neighbours. The other bones of the proximal row form an arch, which articulates, on its convex aspect, with the radius and the articular disc of the inferior radio-ulnar joint. The concavity of the arch is directed downwards and forms a mortise for the upwardly projecting parts of the capitate and hamate bones. In this way the two rows are locked to each other and stability is assured.
The dorsal aspect of the whole carpals is gently convex from side to side, but the anterior surface is deeply concave, owing to the presence of certain forward projections on its lateral and medial borders. The pisiform bone lies at the base of the hypothenar eminence, and its position on the front of the triquetral makes it easy to feel through the skin. In addition, the distal part of the hamate bone bears a hooklike process on its anterior surface. The concavity of the hook is directed towards the lateral side and its tip can be identified in the living subject. It lays 2.5 cm. distal to the pisiform and in line with the ulnar border of the ring finger. The projecting lateral border of the carpal groove is formed by the tubercle of the scaphoid and the crest of the trapezium. The tubercle is placed on the distal part of the anterior surface of the scaphoid and forms a small rounded knob, which can be felt-and sometimes seen-at the base of the thenar eminence. The crest of the trapezium forms a rounded ridge, which runs vertically across the anterior surface of the bone, being slightly hollowed out on its medial side. It lies immediately below and slightly lateral to the tubercle of the navicular, and can be felt only on deep pressure. The margins of the groove give attachment to a strong fibrous retinaculum, which increases the stability of the carpals and renders the flexor muscles of the fingers more efficient by retaining their tendons within an osteofibrous carpal tunnel. The anterior and posterior surfaces of the carpal bonesapart from the triquetral and the pisiform–are rough for the attachment of ligaments(radiocarpal, intercarpal and carpornetacarpal).
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