KINETOGErNESIS. 



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 anzeefrombe 

 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 



