1892.] ^(i'J [Cope. 



the knee forwards. This diversity is clearly due to the diverse positions 

 of the functioning regions. The opposite extremities of the alimentary 

 canal, the posterior including the exits of the urogenital organs, requires 

 that the fore limbs should bend forsvards, and the posterior limbs back- 

 wards. And the constantly recurring necessity lor the exercise of these 

 flexures must necessarily have developed the appiopriate articulations 

 in preference to all others. The terminal flexure, that of the wrist or 

 ankle, has been evidently due to a similar mechanical cause; viz., the 

 flexure due to pressure of the weight of the body on the terminal segments 

 when in contact with earth. The distal segments are the most 

 slender in all types, and least able to maintain a linear direction under 

 pressure, hence, they have flexed easily and thus the line of separation be- 

 tween leg and foot had its origin. 



II. The Origin of the Crests of the Humeral Condyles. 



I have already pointed out (op. cit.) the kinetogenetic origin of the tongue 

 and groove articulations in the Mammalia. 



An excellent example is furnished by the elbow joint of the Quadri.:- 

 mana and Diplarthra. In the lower Mammalia, including the Carnivora 

 {op. cit., PI. ix, fig. A), the distal end of the humerus presents a submedian 

 groove which receives the ulna, and on the inner side of it, a more or less 

 convex surface, which is applied 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 the head of the radius on the 

 humeral condyle has been similar and in the same direction, i. e., up- 

 wards. The dense edges of the former have impressed themselves on the 

 latter, while the unsupported middle portion has yielded in the direction 

 of gravity, and the result is what we find, *. e., a cup-shaped surfice 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 

 the 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 con- 

 dyle of the humerus. The concavity 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, separates the ulnar 

 and radial surfaces of the humerus. (Fig. B). 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 mutual adaptation of parts, under 

 strain, pressure and impact. The other extreme of elbow joint is found in 



