LOCOMOTION OF SERPENTS. 259 



cut off, the fish reels to the right and left. If the pectorals be cut 

 off in a Perch or other big-headed fish, the head sinks ; if one 

 pectoral be cut off, the fish leans to that side ; if the ventral of 

 the same side be also removed, the fish loses its equilibrium ; 

 if the tail be cut off, the locomotive power is abrogated. 



§ 49. Locomotion of Serpents. — The sole locomotive organs in 

 serpents are the vertebral column, with its muscles, and the large 

 stiff erectile epidermal scutes crossing the under surface of the body. 



Although the vertebra? have synovial cup-and-ball terminal joints, 

 their reciprocal movements are greatly restricted by the ' tenon- 

 and-mortice ' articulations of the double zygapophyses at each end, 

 of which the inferior have flat horizontal surfaces, the superior 

 slightly oblique planes. But as a single segment of the back- 

 bone may be but ^^ part of the length of the body, the sum of 

 the small movements between two vertebras becomes considerable 

 in a certain extent of the long trunk. 



A serpent may, however, be seen to progress without any 

 inflections, gliding slowly, with a ghost-like movement, in a 

 straight line. If the observer have the nerve to lay his hand flat 

 in the reptile's course, he will feel, as the body glides over the 

 palm, the surface pressed, as it were, by the edges of a close-set 

 series of paper-knives, successively falling flat after such applica- 

 tion. The skin of the hand has been seized, so to speak, by the 

 edges of the stiff, short, but broad, transverse, horny, ventral 

 scutes, erected or made vertical for that purpose, and folding flat 

 upon the body when the effect of the resistance has been gained. 

 Each scute having secured a fulcrum in the plane of motion, the 

 ribs connected with it rotate, and transmit the movement upon 

 the trunk ; it is, in fact, a. step whose length depends on the arc 

 through which the pair of ribs may oscillate and on the distances of 

 the scutes from the axes of motion. As both these are small, and 

 the motion has to be transmitted by the succession of short scutal 

 steps through the whole length of the body, this first kind of 

 progression is slow and gliding. 



A second and swifter mode of locomotion on laud is by succes- 

 sively bending and straightening portions of the body. Extension 

 will carry the straightened part forward in the direction of least 

 resistance. If most resistance be made by the point of the 

 tail, fig. 160, e, or by the application to the ground of the edges 

 of the erect scutes, between d and e, the extension of a c will 

 carry the head to h, the smooth overlapping unerected scutes 

 between a and d favouring the forward movement ; and this being 

 effected, and the ground grasped by the erection of the scutes 



8 '2 



