326 



HUMAN ANATOMY. 



Radius 



Disk 



Semilunar 



as has been described. Interosseous metacarpal ligaments connect their sides distally 

 from this. These complete the capsules, which are imperfect only on the carnal 

 side. 



The articulation of the thumb (Fig. 342) differs from the others in being 

 complete in itself. It is a saddle-shaped joint. The hand lying supine, the long 



axis of the joint slants down- 

 FiG. 346. ward and inward. In this 



direction the trapezium is con- 

 cave ; at right angles to it con- 

 vex. The joint is surrounded 

 by a capsule, which is strongest 

 on the dorsal and palmar sides, 

 where the direction of the 

 fibres is longitudinal ; it is 

 weak at the outer anterior end, 

 where it is strengthened by^ 

 the tendon of the extensor of 

 the metacarpal bone. 



The motions are flexion, 

 extension, adduction, abduc- 

 tion, and circumduction. Ro- 

 tation in the flexed position 

 may be possible from the im- 

 perfect adaptation of the ar- 

 ticular surfaces, but can hardly be of practical importance. Flexion is limited by 

 the locking of the palmar projection of the metacarpal against the trapezium ; the 

 other angular motions by the tension of the ligaments. 



Movements and Mechanics of the W^rist and Carpo-Metacarpal 

 Articulations. — It is convenient in studying these movements to imagine that the 

 metacarpus follows the motions of the second row of carpal bones. This is true of 

 the index- and middle fingers, but not of the others. The motions of the wrist in 

 the widest sense are flexion, extension, adduction, abduction, and circumduction. 

 The joint -is a compound one, egg-shaped above, the scaphoid, semilunar, and 

 cuneiform acting as a meniscus. The motions are best studied by removing the 

 skin and tendons on the dorsal aspect and inserting long pins into the radius, semi- 

 lunar, and OS magnum, and, for some purposes, the scaphoid. The Rontgen r^ys 

 have been useful chiefly as 



Os magnum Third metacarpal 



Same as Fig. 345, the hand being flexed. 



Fig. 



347- 



Radius 



Semilunar 



corroboratory evidence. In 



flexion the motion begins in 



the upper joint, where it is 



most extensive ; as it goes 



on the lower takes part. In 



extension, starting with the 



arm straight, more than half 



occurs in the lower joint. 



Addtictio7t (ulnar flexion) 



(Fig. 348, B), owing to the 



lesser prominence of the ulna, 



is more free than abduction. 



The meniscus glides towards 



the radial side, and in so 



doing assumes the relation to 



the radius that it has in 



extension. The scaphoid 



touches the radius only by one end, so that its long axis approaches the direction of 



that of the forearm, and the semilunar leaves the triangular cartilage. The curve of 



the meniscus broadens, increasing the distance between the ends of tlje Cuneiform 



and the scaphoid. A small part of the motion occurs in the mid-carpal joint. The 



unciform, moving with the os magnum, comes nearer to the semilunar. The space 



Third metacarpal Os magnum 



Same as Fig. 345, the hand being overextended. 



