XIU 



spring, replacing by opposite curvatures the twists formed 

 in its back bone and in its ribs. 



These twists are projected in a consolidated action in the 

 ball and socket joint of the head, and in partial concentra- 

 tions in the various ball and socket joints connecting the 

 ribs and the spine. 



The simplest example of the action is in any one of the 

 snake's ribs, which being in appui on the ground is, by the 

 motion of the spine, turned through its length in one direc- 

 tion ; then, being still fixed at the ground, it is by another 

 motion of the spine turned in the opposite direction, and 

 the spring is liberated by the discharge of the first turn at 

 the ball and socket joints by which the rib joins the spine. 



The eyes are the centre of direction, and in the snake as 

 in other animals, are kept steady by the arrangement of a 

 principal muscle passing through a loop, in which, after 

 receiving the counter twists, the two portions into which 

 the loop divides the muscle, slip back to their original 

 condition. 



The lung of the snake filled with air presents a perma- 

 nent compensating-portion of the machinery of locomotion. 

 This, when compressed on one line at the discharge, ex- 

 pands immediately on the alternate line, and thus, having 

 aided in giving force to the discharge of one spring, aids 

 also in the formation of the next. The working of a tense 

 fluid, as the pivot of every movement, also secures the 

 smoothness of action, which, as well as its force, depends 

 much on the filling of the lung. 



The snake's ribs, when acting along the several convex 

 and concave curvatures induced by the twistings of the 

 spine, may, on each of them, be divided into Propellers and 

 Bearers, both of which discharge with the primarily formed 

 lines of torsion, but in opposite directions. The propellers, 

 which by a peculiarity in the shape of the curvature, are in 



