41 



In this case, where we have begun with the left 

 anterior and right posterior, the latter becomes the 

 right anterior line leading, and the left posterior 

 will develop on it. 



§ 43. Although the general action is the same, 

 and, on the theory of each interior point of appli- 

 cation being in a cross-hne, we might consider each 

 C from the front as replacing the one in rear, and 

 each from the rear as replacing the one in front, 

 yet we shall, for reasons which will appear when 

 the locomotion of the higher animals is taken up, 

 first consider the action of each as simple and un- 

 connected with others. 



In general action, the head is steady, and the 

 spine moves from or against it at the socket ; but, 

 since the forces are gathered against the head-ball 

 as a focus — since the ultimate result is as if the 

 head gave a final covering tvnat, and since it seems 

 to facihtate explanation — we shall suppose the head 

 to move. 



§ 44. The spine or back-bone of the snake, 

 which represents the simple elastic rod of the pre- 

 ceding discussion, is made up of a large number of 

 httle blocks of bone called vertebrae. These are 

 jointed to each other by means of a convex surface 

 on the rear of one vertebra, fitting into a concave 



