87 



double - twisting action shown for the internal 

 oblique muscle of the eye (§ 77), although with 

 them there is no necessity for a pulley attachment, 

 the support against the neck-spine answering the 

 same purpose. 



It is easily seen that the two sterno-mastoids ap- 

 proach each other as they descend in fi-ont of the 

 neck. Thus their separate act ion must be diagonal, 

 ending, in fact, for each at the diagonal hip-joint, 

 so soon as the intervening ophidian S has placed 

 the points in the requisite position, by moving its 

 substitute ground, the breast-bone. 



§ 88. At its posterior end the breast-bone is stead- 

 ied against tlie pelvis by a combination of muscles, 

 which, in relation to the pelvis, carry out from the 

 breast-bone a working in harmony with, but sub- 

 ordinate to that proceeding from the breast-bone, 

 through the sterno-mastoids toward the head. 



§ 89. In the snake a simple ball and socket joint 

 at the head answers every purpose, for allowing 

 the working of the spine over the ribs, these latter 

 being the simple and only appuis ; but in the 

 higher animals another arrangement becomes nec- 

 essary. 



This consists in dividing the simple ball and 

 socket articulation of the snake from behind for- 



