376 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



The differing directions of the elements entering 

 into the articulations of the limbs of Mammalia may 

 be cited in illustration of the supposed inconsistency 

 of supposing them all to be the result of impact and 

 strain. Thus most of the condyles are directed distad, 

 but the heads of the humerus and femur, and the prox- 

 imal surfaces of the carpus and tarsus are directed 

 proximad (except the trochlear groove of the astraga- 

 lus when present). So far as regards the distal ends 

 of the radius and tibia (fore arm and leg) I have 

 pointed out that here dense layers disposed longitudi- 

 nally impinge on a similar layer disposed transversely, 

 with the natural consequence the latter has yielded to 

 the excess of impact so produced. The direct relation 

 of the sculpture of the surfaces concerned, to the 

 lengthening of the foot and increase of speed of the 

 animal, and hence increase of force of impact, leads 

 irresistibly to this conclusion. As regards the con- 

 vexity of the heads of the humerus and femur, or rather 

 the concavity of the corresponding surfaces of the 

 scapula and acetabulum the explanation has been al- 

 ready given. Henke was apparently the first to call 

 attention to the fact that the concavity and convexity 

 of the articular surfaces is directly related to the posi- 

 tions of the insertions of the muscles which move them. 

 He shows that a concave surface is developed at the 

 extremity which is nearest to the muscular insertion, 

 while the convex surface is developed on the extremity 

 which is most remote from its muscular insertion. 

 Thus is accounted for the apparently contradictory 

 evidence of the limb articulations mentioned. In some 

 cases at least, as those of the glenoid cavities of the 

 scapula, ilium, and phalanges, the muscular insertions 

 are so near to their borders, as to suggest that the 



