

OF THE MOTOEY APPABATUS IN MAN. 137 



humerus merely in flexion and extension ; but it is close to 

 this that the radius rotates on the ulna, and slides over the 

 smaller head of the humerus, which movement determines 

 the rotation of the hand in supination and pronation. The 

 lower end of the ulna is styliform, and has interposed between 

 it and the carpal bones, a fibro-cartilage of a triangular shape, 

 which attaching the radius at its extremity to the ulna, 

 permits the former to rotate around the lower end of the 

 the latter, which remains. 



These two bones at their carpal extremities also move 

 readily in flexion and extension, 



The process called olecranon belongs to the ulna or cubit ; 

 to it are attached the extensor muscles of the fore-arm. 



276. In the hand we have the carpus, metacarpus, and 

 fingers, or digital portion. 



The carpus is composed of eight bones, four in each row ; 

 they are scarcely moveable, and the strength of the arch they 

 form is considerable. In the first row we find the scaphoid, 

 semilunar, pyramidal, and pisiforme ; in the second row, the 

 trapezium, trapezoides, 'magnum, and unciforme. On the 

 flexor side of the hand they form with the carpal bones the 

 anterior annular ligament of the carpus, a canal, in which 

 are lodged and protected most of the flexor tendons, and one of 

 the great nerves (median) proceeding to the palm of the hand. 



The metacarpus is composed of a single row of small long 

 bones, corresponding to the number of the fingers, all dif- 

 ferent and readily distinguishable from each other. Pour of 

 these move but little ; the first, which supports the thumb, 

 and which some view as the proximal phalanx of that finger, 

 is very moveable, corresponding to the greater mobility of 

 the thumb as compared with the other fingers. 



Finally, the fingers have each three bones, called proximal, 

 middle, and distal phalanges ; in the thumb there are onlv 

 two. The distal phalange supports the nail, and is sometimes 

 called the nail bone. 



277. When we consider the arm as a seriesof broken levers, 

 we observe that the arm is longer than the fore-arm, and this, 

 longer than the hand ; or, in other words, that the mobility 

 of the structures and their flexibility and power of adaptation 

 increase as we approach the extremity, properly so called. 



278. The structure of the inferior or pelvic extremities 

 has the strongest analogy to that of the superior limbs, 

 and the principal differences to be observed, have a neces- 

 sary relation to their functions ; to make of them, in fact, in- 



