The scaphoid (B.N.A. navicular) bone (figs. 428, A and B) is named from its fancied resemblance to a boat. It is the largest bone in the proximal row and lies with its long axis directed downwards, laterally and slightly forwards. The tubercle forms a rounded elevation on the lower part of the anterior surface and is directed slightly to the lateral side. It gives attachment to the flexor retinaculum and a few fibers of the abductor pollicis brevis, and is crossed by the tendon of the flexor carpi radialis, which must be relaxed when the bony prominence is being examined. The posterior surface is rough and slightly grooved, and is narrower than the anterior surface. The lateral surface, also narrow and roughened, gives attachment to the lateral ligament of the wrist-joint. The remaining surfaces of the bone are articular. The radial surface is convex and is directed upwards and laterally. The lunate surface forms a flattened, narrow semilune, directed medially. The capitate surface, large and concave, is directed medially and downwards. The surface for articulation with the trapezium and the trapezoid bones form a continuous convex surface, directed downwards.
The lunate bone (figs. 429, A and B), distinguished by its crescentic outline, is placed between the scaphoid and the triquetral bones in the middle of the proximal row of the carpals. The rough anterior surface, almost triangular in outline, is larger and wider than the rough posterior surface. The smooth, convex proximal surface articulates with the radius and the articular disc of the inferior radio-ulnar joint. The lateral surface is narrow and presents a flat, semilunar facet for articulation with the scaphoid. The medial surface articulates with the triquetral and is almost square. It is separated from the distal surface by a curved ridge, which is usually somewhat hollowed out for articulation with the apex of the wedgeshaped hamate bone. The distal surface, is deeply concave to accommodate the medial part of the head of the capitate bone.
The triquetral bone (fig. 431), usually pyramidal in shape, is distinguished by an oval, isolated, smooth facet for articulation with the pisiform, which marks the distal part of its rough anterior surface. The medial and dorsal surfaces are confluent. Rough distally for the attachment of the medial ligament of the wrist-joint, this aspect is smooth in its proximal part, which articulates with the articular disc of the inferior radio-ulnar joint in full adduction of the hand. The hamate surface, directed laterally and downwards, forms a concavo-convex area, broad proximally and narrow distally. The hamate surface, almost square, is directed upwards and laterally.
The pisiform bone (fig. 431), shaped somewhat like a pear with one flattened surface, can be distinguished by the fact that it possesses only one articular surface. This is placed on the posterior aspect of the bone to articulate with the triquetral, and its long axis runs downwards and laterally. The non-articular part of the bone tends to project distally beyond the articular surface, and the lateral aspect is somewhat flattened, while the medial aspect is convex. The flexor carpi ulnaris is inserted on the anterior aspect and its true continuation, viz. the pisometacarpal ligament, to the distal aspect of the bone. The flexor retinaculum (transverse carpal ligament) is attached to the anterior part of the lateral aspect, while the abductor digiti minimi and the extensor retinaculum (dorsal carpal ligament) are attached to the medial aspect. The area surrounding the articular facet is slightly constricted and gives attachment to the capsular ligament or the pisotriquetral ,joint.
The trapezium (greater multangular) bone (fig. 432) can be identified by the crest and groove which mark its rough anterior surface. The groove, which lies on the medial side of the crest, lodges the tendon of the flexor carpi radialis, and its margins give attachment to the two layers of the flexor retinaculum (fig. 634). The crest is obscured to a large extent by the origin of the muscles of the thenar eminence. The opponens pollicis arises from its middle part, the flexor pollicis brevis from its distal part and the abductor pollicis brevis from its proximal part (fig. 425). The elongated, rough, dorsal, surface is closely related to the radial artery, just before it passes forwards into the palm to become the deep palmar (volax) arch. The lateral surface is also large and rough for the attachment of the lateral ligament of the wrist-,joint and the capsular ligament of the carpoxnetacarpal joint of the thumb. A large saddle-shaped surface is directed downwards and laterally for articulation with the base of the metacarpal bone of the thumb. The most distal part of the bone projects slightly between the bases of the first and second metacarpal bones and is covered by a small, quadrilateral facet, which is directed downwards and medially to articulate with a corresponding facet on the posterior part of the lateral aspect of the base of the second metacarpal bone. The medial aspect is covered by a large, gently concave facet for articulation with the trapezoid bone. The proximal surface is occupied by a small, slightly hollowed out. facet which articulates with the scaphoid bone.
The trapezoid (lesser multangular) bone (fig. 433) is small and very irregular in shape. The anterior surface is rough, narrow and considerably smaller than the rough dorsal surface. It is continued for a short distance on to the inferolateral aspect. The distal surface articulates with the grooved base of the second metacarpal bone. Triangular in outline, it is convex from side to side and concave from before backwards. The medial aspect articulates with the distal part of the capitate bone, by means of a slightly concave facet. The dorsal part of this area, is often rough for the attachment of an interosseous ligament. The lateral aspect articulates with the trapezium, and the distal aspect with the scaphoid bone.
The capitate bone (fig. 434) is the largest of the carpal bones and is placed opposite the base of the third metacarpal bone, so that it is more or less central in position. The distal surface is roughly triangular and, forms a concavoconvex facet fox articulation with the base of the third metacarpal bone. Its lateral border is marked by a concave strip, which articulates with the medial side of the base of the second metacarpal bone. and its posteromedial angle usually bears a small facet for the fourth metacarpal bone. The convex head is received into the concavity formed by the lunate and scaphoid bones. Its proximal surface articulates with the lunate and its lateral aspect with the scaphoid. The facet for the scaphoid is usually continuous with the facet for articulation with the trapezoid on the lower part of the lateral surface of the bone, but the two may be separated by a rough interval. The medial surface presents a large facet for the hamate bone. which is deeper above than below, where a part of the surface is non-articular and gives attachment to a strong interosseous ligament. The anterior and posterior surfaces are roughened; the latter is the larger of the two.
The hamate bone (fig. 435) can be identified easily, by its wedge-shaped form and the hook-like process which projects from the distal part of its rough, anterior surface. The concavity of the hook is directed to the lateral side, and takes part in the formation of the carpal tunnel. Its tip gives attachment to the flexor retinaculum (transverse carpal ligament), and more medially to the flexor and the opponens digiti minimi. The distal aspect of the base of the boot, occasionally shows a slight transverse groove for the deep terminal branch of the ulnar nerve. The rest of the anterior surface, like the posterior surface, is rough for the attachment of ligaments. The distal surface is divided into two articular surfaces by a faint ridge : of these, the smaller. lateral facet articulates with the base of the fourth, and the larger, medial facet with the base of the fifth metacarpal bone. The proximal aspect forms the cutting edge of the wedge and usually bears a narrow facet for articulation with the lunate bone. The medial aspect is covered by a broad articular strip, convex above and concave below, for the triquetral bone. The lateral aspect articulates with the capitate bone by means of a facet which covers all but the antero-inferior part of the surface.
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