172. The carpus (fig. LXXXII. 4) consists of eight small wedge-shaped bones, placed in a double row, each row containing an equal number, and the whole disposed like stones in an arch (fig. LXXXII. 4). They do in fact form an arch, the convexity of which is upwards, on the dorsal surface (fig. LXXXIII. 4); and the concavity downwards, on the palmar surface (fig. LXXXII. 4). But they differ from the stones of an arch in this, that each bone is joined to its fellow by a distinct moveable joint, each being covered with a smooth articulating cartilage. At the same time all of them are tied together by ligaments of prodigious strength, which cross each other in every direction (fig. LXXVII. 6), so that the several separate joints are consolidated into one great joint. The consequence of this mechanism is that some degree of motion is capable of taking place between the several bones, which, when multiplied together, gives to the two rows of bones such an extent of motion, that when the wrist is bent the arch of the carpus forms a kind of knuckle. By this construction a facility and ease of motion, and a power of accommodation to motion and force, are obtained, such as belong to no arch contrived by human ingenuity. [Illustration: Fig. LXXXII. 1. Lower extremity of the radius; 2. lower extremity of the ulna; 3. styloid process of the ulna; 4. bones of the carpus or wrist; 5. metacarpal bones; 6. first phalanges of the fingers; 7. second phalanges of the fingers; 8. third phalanges of the fingers.] 173. The metacarpus (fig. LXXXII. 5), the middle portion of the hand, interposed between the wrist and the fingers, is composed of five bones, which are placed parallel to each other (fig. LXXXII. 5). They are convex outwardly, forming the back (fig. LXXXIII. 5), and concave inwardly, forming the hollow of the hand (fig. LXXXII. 5). They are large at each end, to form the joints by which they are connected with the wrist and fingers (figs. LXXXII. and LXXXIII.): they are small in the middle, in order to afford room for the lodgment and arrangement of the muscles, that move the fingers from side to side (fig. LXXXVI. 2). Their ends, which are joined to the carpus, are connected by nearly plane surfaces (figs. LXXXII. and LXXXIII.): their ends, which support the fingers, are formed into rounded heads, which are received into corresponding cup-shaped cavities, excavated in the top of the first bones of the fingers (fig. LXXXII. 5.). The powerful ligaments that unite these bones pass, both on the dorsal and the palmar surface, from the inferior extremity of the second row of the carpal to the bases of the metacarpal bones (fig. LXXVII, 7, 8). The ligaments are arranged in such a manner as to limit the motions of the joints chiefly to those of flexion and extension, allowing, however, a slight degree of motion from side to side.