OSTEOLOGY 151 



but there is no appreciably defined articular circumference 

 at the point of junction of this head with the ulna. There is 

 much variation in the diameter of the shaft but the ten- 

 dency is for this to be smallest in Neotoma — especially the 

 distal portion — and largest in Teonoma. Upon the medio- 

 caudal border of the shaft near the head is the slight tuber- 

 osity, equally well marked in the three subgenera, upon 

 which is inserted the biceps. The cranial ridge, upon the 

 medial side of which is inserted the pronator teres, and upon 

 the lateral, the supinator, is readily distinguishable in some 

 animals, but in others considerably older it may be practi- 

 cally undefined. Other muscles attached to this bone are 

 the extensor metacarpi pollicis and pronator quadratus. 

 Their positions upon the shaft are barely traceable by faint 

 ridges, in the development of which there is little or no 

 variation. The caudal border of the distal portion of the 

 shaft bears a flat facet for articulation with the ulna. The 

 distal termination is very irregular in precise shape, but the 

 only feature of especial application are the pointed, terminal, 

 stjdoid process, and the concave facet for articulation with 

 the scapholunar bone. 



Hand 



The distal extremity of the radius is considerably larger 

 than that of the ulna. In relaxed position the lateral border 

 of the latter hes a shade laterad to that of the former, when 

 the shaft of the humerus is vertical. This, however, throws 

 the medial border of the distal extremity of the radius 

 considerably mediad to that of the ulna. The palm of the 

 hand is therefore inclined with the thumb inward at ten to 

 fifteen degrees from vertical. Supination is possible only 

 to 90 degrees, while pronation can throw the palm to but 

 slightly beyond the horizontal, thus making the possible 

 movement of this member about 100 degrees, instead of some 

 180 as is usual in man. 



