294 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



side of it, a more or less convex surface, which is ap- 

 plied to the head of the radius. The coronoid process 

 of the ulna is narrow, and its dense bounding walls 

 impinge on the broad face of the humeral condyle in 

 flexion and extension, and transfers to it the force of 

 impact when the foot strikes the ground. In either 

 case strong pressure has been brought to bear on 

 the humeral condyle, and it has 

 yielded to the denser body of the 

 ulna, thus forming the groove in 

 question. In such Mammalia the 

 effect of impact of the limb on the 

 ground has been to impress the 

 head of the radius on the humeral 

 condyle upwards. The dense 

 edges of the former have im- 

 pressed themselves on the latter, 

 while the unsupported middle 

 portion has yielded in the direc- 

 tion of gravity, and the result is 

 what we find, i. e., a cup-shaped 

 surface of the head of the radius, 

 and a convexity of the humeral 

 condyle adapted to it. 



Among specializations of the 

 elbow-joint, I call attention to 

 two. In Quadrumana the head of 

 the radius, probably owing to continued supination of 

 the manus, occupies a position at the external side of 

 the coronoid process of the ulna, and impinges on the 

 outer part of the condyle of the humerus. The con- 

 cavity of its head, and the convexity of the humeral 

 condyle, are visible as before, but a prominent tongue 

 or keel, which has been called the intertrochlear crest, 



hyena) seen from behind ; h, 

 humerus ; r, radius ; , ulna. 

 Original. 



