KINE TOGENESIS. 



295 



separates the ulnar and radial surfaces of the humerus 

 (Fig. 74). This keel occupies the groove or interval 

 which separates the head of the radius from the coro- 

 noid process of the ulna. It is plain that we have here 

 another tongue and groove-joint, produced by the mu- 

 tual adaptation of parts under strain, pressure, and 

 impact. The other extreme of elbow-joint is found in 

 that of the diplarthrous Ungulata (Fig. 75). Here the 

 head of the radius, while retaining its normal position 

 on the inner side of the fore-arm, is extended to the 

 external side of the ulna and 

 even beyond it, adapting it- 

 self to the entire width of the 

 humeral condyles. The same 

 structure is found in the spe- 

 cialized forms of both series 

 of Diplarthra, the Perisso- 

 dactyla and Artiodactyla. 

 This expansion of the head of 

 the radius appears to be in di- 

 rect relation to the duration 



through long geologic ages Fig. 74.— Elbow-joint of chim- 



Of the impacts which have P^n^e from behind. 



affected the limbs of these, the swiftest of the Mam- 

 malia. That the head of the radius should be spread 

 so as to fit the entire surface of the humerus, under all 

 circumstances, seems to be a mechanical necessity. 

 But in addition to this we find a tongue- and-groove 

 adaptation, in which the crest (which I have called the 

 trochlear crest) articulates with a groove in the head 

 of the radius. The internal articulation of the humerus 

 with the radius has the usual form, convex and con- 

 cave distad. The trochlear crest marks the external 

 border of the olecranar groove of the humerus. But 



