92 



ANIMAL MECHANISM. 



direction ; such are the trochlear surfaces, of which the articu- 

 lation of the elbow, the condyles of the jaw, &c., are examples. 

 But if the movement is executed in two directions at once, 

 the surfaces will present a double curvature, and in the case 

 of an inequality in the amplitude of the movements, the radii 

 of these curvatures will be unequal. Thus, in the wrist there 

 exist movements of flexion and extension which are consider- 

 ably extensive, but the lateral movements are restricted. The 

 result of this is that in the elliptical head formed by the 

 carpal bone, there is a curvature of small radius in the direc- 

 tion of the movements of flexion and extension, while, in the 

 lateral direction, the curvature belongs to a circle of much 

 greater radius. 



It is still more interesting to observe the articular surfaces 

 of a series of animals in different classes and species. 

 Similar articulations present movements of very different kinds, 

 which must bring about no less important differences in the 

 articular surfaces. 



Let us take, for example, the head of the humerus, and 

 follow the changes of its form, in man, in the ape, the carni- 

 vora, the herbivora, the birds. We shall see that the perfect 

 equality of movement in every direction which can be exe- 

 cuted by the human arm corresponds with a perfect sphericity 

 to the head of the humerus — that is to say, a curvature of the 

 same radius in every direction. Among apes, those which in 

 walking throw a part of their weight usually on their anterior 

 limbs, have the head of the humerus flattened at the upper 

 part, as if by the weight of the body. Besides this, the 

 movements which are required in walking being more ex- 

 tended, the curvature of the head of the humerus in these 

 animals presents its least radius in the antero-posterior direction. 

 This modification is more marked still in the carnivora, and 

 above all in the herbivora, the head of whose humerus, flat- 

 tened above, presents its short radius of curvature in the 

 direction of the movements which serve for walking, and 

 which predominate in this articulation. 



Birds possess, in the articulation of the shoulder, two 

 movements of unequal extent. One, by which they spread 

 and fold their wings, and which carries the elbow sometimes 



