76 



INTRODUCTION. 



The hand is united to the fore-arm, consisting of two bones, the 

 one called the radius, the other the ulna (figs. 73, 74, 75). 



73 



Figs. 73, 74, 75. Three views of the human fore-arm and hand. a, The lower part of the humerus, entering 1 into the 

 elbow-joint; b, the ulna, of which the protuberance, c, at its upper end, advancing upon the humerus, as distinctly shewn 

 in figs. 73 and J5, is termed the olecranon ; d,the radius. (The same letters refer to the same part in all the above figures.) 



The hand is more immediately, though not exclusively, connected 

 with the carpal extremity of the radius ; and this articulation, thus formed, 

 is free and flexible : the movements, however, which this articulation 

 allows to the hand, are merely up and down, and, to a certain degree, 

 from side to side. But the hand is capable of pronation and supination 

 (that is, of being turned round, so as to present, alternately, the back 

 part, and the palm, uppermost). On examining the radius, it will be 



discovered that to this bone is the power in 

 question to be attributed. Its humeral ex- 

 tremity, or that part which enters into the 

 elbow-joint, is dilated into a well-defined 

 circular head, a (fig. 76), flattened, or rather concave, at the end, for the 

 reception of a tubercle of the humerus ; and upon this tubercle the 

 radius rotates on its own axis, while, at the same time, it is duly secured 

 to the ulna, b (figs. 73, 74, and 75), by ligaments. The ulna (the ole- 

 cranon process of which, c, figs. 73 and 75, forms the point of the elbow 

 on which we lean) is very strongly united to the humerus, a deep semi- 

 circular depression receiving the head of that bone, and the articulation 

 being strictly hinge-like. When, therefore, we bend the elbow-joint, the 

 radius and ulna move together ; but, when we revolve the hand upon 

 itself, the radius rotates ; its rotation, at the humeral end, being simply 

 round its axis : while, at the carpal end, it moves in a sort of cycloid, with 

 respect to the carpal end of the ulna. The hand, and, indeed, the whole 

 of the arm, derive a rotatory power, also, from the nature of the shoulder- 

 joint, which allows the head of the humerus to play freely in the glenoid 



