General features.—The metacarpals (figs. 424-427) is made up of five metacarpal bones, which are numbered from the lateral to the medial side. They are miniature long bones, and each possesses a rounded head, a shaft and an expanded base.
The head is placed at the distal end of the bone and articulates with the proximal phalanx. Its oblong, articular surface is convex, the degree of convexity being less in the transverse than in the anteroposterior direction, and extends further upwards on the palmar surface than on the dorsal surface. The prominence of the knuckles is produced by the distal aspects of the heads of the metacarpal bones. The bases of the metacarpal bones are formed by their expanded proximal ends, which articulate with the distal row of the carpals and with one another-save that the first metacarpal bone is isolated from the rest and does not articulate with the metacarpal bone of the index. The shafts are concave longitudinally on their palmar aspects, an arrangement which provides a hollow for the lodgment of the muscles of the palm. The dorsal surface of each presents a flattened triangular area in its distal part, continued proximally as a rounded ridge. These flattened areas can be felt on the back of the hand in the living subject immediately proximal to the knuckles. It should be observed that, whereas the medial four metacarpal bones lie side by side, the first metacarpal bone lies on a more anterior plane and that it is rotated medially round its long axis through an angle of 90 degrees. As a result of this rotation its morphologically dorsal surface is directed to the lateral side, its radial border forwards, its palmar surface medially, and its ulnar border backwards. By virtue of its anterior position the thumb moves medially in front of the palm when it is flexed and it can be opposed to each of the fingers in turn. The opposibility of the thumb, rendered possible by the rotation of the bone medially, is the most important factor in rendering the hand an efficient instrument of prehension, for, when an object is grasped in the hand, the fingers encircle it from one side and the thumb from the other, and the power of the grip is increased very greatly thereby.
THE CHARACTERISTICS OF THE INDIVIDUAL METACARPAL BONES
The first metacarpal bone (fig. 436) is shorter and stouter than any of the others. Its dorsal surface is directed laterally, a fact which can easily be confirmed in the living hand, and its long axis passes downwards and laterally, diverging from its neighbour. The shaft is flattened and its dorsal surface is uniformly broad and convex from side to side. The palmar surface is concave from above downwards and is subdivided by a rounded ridge into a larger anterolateral and a smaller anteromedial surface. The opponens pollicis is inserted into the radial border and the adjoining part of the anterolateral surface ; the radial head of the first dorsal interosseous muscle arises from the ulnar border and the adjoining part with the trapezium (greater multangular bone). Its lateral side is marked, as a rule, by a small tubercle for the insertion of the abductor pollicis longus ; its ulnar side gives origin to the first palmar interosseous (deep head of the flexor pollicis brevis). The head is less convex than the heads of the other metacarpal bones, and is broader from side to side than from before backwards. On its palmar aspect the ulnar and radial corners are enlarged to form two articular eminences, on each of which a sesamoid bone glides.
The shaft is prismoid in form and curved so as to be convex backwards in its long axis and concave forwards. It has medial, lateral and dorsal surfaces. The dorsal surface is broad near the bead but narrows into a ridge as it approaches the base. This surface is covered by the extensor tendons of the index finger, and its converging borders commence in two little tubercles, which are situated one on each side of its head. The lateral surface inclines dorsally at its proximal end, it gives origin to the ulnar head of the first dorsal interosseous muscle. The medial, surface also inclines dorsally at its proximal end and is divided into two nearly parallel strips by a faint ridge. Of these, the anterior gives origin to the second palmar interosseous and the posterior to the radial head of the second dorsal interosseous muscle.
The third metacarpal bone (fig. 438) can be identified by means of the short styloid process which projects upwards from the radial side of the. dorsal aspect of its base. The base articulates, proximally, with the capitate bone by means of a facet which is convex in front but concave behind. where it covers the anteromedial aspect of the styloid process. The lateral aspect of the base is marked by a strip-like facet, oonstricted in its central part. for articulation with the metacarpal bone of the index. On its medial side it articulates with the base of the fourth metacarpal bone by means of two small, discrete oval facets. Sometimes the anterior facet is absent, and less frequently the two may be connected by a narrow bridge along the medial border of the base. The palmar aspect of the base receives a. slip from the flexor carpi radialis tendon, while its dorsal aspect, just beyond the styloid process, gives insertion to the extensor carpi radialis brevis.
The shaft resembles the shaft of the metacarpal bone of the index. Its lateral surface gives origin to the ulnar head of the second dorsal interosseous and its medial surface to the radial head of the third dorsal interosseous muscle. The palmar ridge which separates these two surfaces gives origin, in its distal two-thirds, to the transverse head of the adductor pollicis. Its dorsal surface is covered by the extensor tendon of the middle finger.
The shaft resembles the shaft of the metacarpal bone of the index finger, but its lateral surface is traversed by a faint ridge, which separates an anterior strip, for the origin of the third palmar interosseous, from a posterior strip, for the origin of the ulnar head of the third dorsal interosseous muscle. The medial surface gives origin to the radial head of the fourth dorsal interosseous muscle.
The fifth metacarpal bone (fig. 440) can be identified by the fact that the medial aspect of the base is non-articular and presents a tubercle for the insertion of the extensor carpi ulnaris. The proximal surface of the base is covered by a facet, concave from side to side and convex from before backwards, for articulation with the hamate bone. Its lateral aspect presents an elongated strip-like facet for the fourth metacarpal bone.
The shaft is characterized by the fact that the triangular area on its dorsal surface reaches almost to the base and that only the lateral surface inclines dorsally at its proximal end. The medial surface gives insertion to the opponens digiti minimi ; the lateral surface is divided by a longitudinal ridge, sometimes quite sharp and distinct, into an anterior strip for the origin of the fourth palmar interosseous, and a posterior strip for the origin of the ulnar head of the fourth dorsal interosseous muscle.
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