61 



against the ground to secure a firm resistance in? 

 appui of the gathering.* 



The twisting of the scutae also may have some- 

 influence in adjusting the bevels of the ribs " en 

 air." 



§ 67. We will now give a short resume of the 

 foregoing theory of the progressive locomotion of 

 the snake. 



There are for each diagonal spring two virtual 

 actions at the head-joint. 



First. That produced by the moving pressure of 

 the head-hall and consisting (a) of the passage of 

 the pressui'e around' the edge of the socket on the 

 side of the active anterior " point of application.'* 

 (In our example the left), {a '.) Of its passage- 

 from the rear, across the middle of the socket, and 

 (unless the opposing action have begun) to its own 

 side of the front centre. 



Second. That produced by the moving and re- 

 traction (as to the moving t side), of the socJ^et, un-^ 

 der the side of the head-ball, opposite to that just 



*That is, only the posterior edge acts (speaking of the propellers). 

 The backward bearing of the spring in perfect motion is included en- 

 tirely within the rib or limb of appui, that element of the force not com- 

 ing to the ground— were it not for this no locomotion could be obtained 

 on ice where the weight pressing forward secures the gathering, buti 

 would not secure the spring. 



1 1. e., side which leads the movement. 



