260 ANATOMY OF VERTEBRATES. 



between a and c, flexion of the rest of the body will draw forward 

 the tail, as from h to e. As the extent of the flexion of, say a 

 fourth part of the body, exceeds the space through which a single 

 scute is moved in erection, so does this mode of motion greatly 

 exceed in swiftness the preceding. And this swiftness is accele- 

 rated when the serpent raises the body, in arched curves, from the 

 ground, increasing their span, and progressing in a vertically, 

 instead of a horizontally, undulating course ; when, by augmented 

 vigour of the muscular actions, the whole trunk may be raised 

 into a single arc, and the movement acquire the character of a 

 leap. Thus the body being bent, whilst the neck-scutes fix the 

 head, as at h, the tail will advance from a to e, fig. 161 ; when, 

 being fixed there by the subcaudal scutes, extension will carry 

 the head forward to d, and the serpent will have advanced by the 

 two actions of flexion and extension through a space eqvial io a e 

 or h h. But, if the act of extension be vigorous and sudden, 

 and an equivalent fulcrum be afforded by the tail, the whole body 

 may be carried forward, as by a leaj], farther than its own length. 

 For the saltatory motion, however, the mechanism of a spiral 

 spring is commonly simulated ; the whole body is bent into a 

 series of close-set coils, the svidden extension of which, reacting 

 upon the point of earth against which the tail presses, throws the 

 serpent obliquely forward into the air. In all these movements 

 the curve is essentially lateral ; the amount of rotation between 

 the smaller vertebras, at the two extremes of the body, permits 

 the flexion of the intermediate joints to assume, as in fig. 161, the 

 vertical position. There is no natural undulation of the body 

 njjward and downward — it can take j^lace oidy from side to side. 

 So closely and compactly do the ten pairs of joints between each 

 of the two hundred or tliree hundred vertebr* fit together, that 

 even in the relaxed and dead state the body cannot be twisted. 

 If the attempt at rotation be made at the end of the tail on a 

 dead snake outstretched, the part grasped may be half-twisted ; 

 but the rest of the trunk will turn over, rigid, like a stick. 



Serpents derive the same advantage from their lungs in water 

 as eels ironi their swim-bladder, the air-receptacles in both being- 

 much alike, and placed above the centre of gravity. They lu'o- 

 grcss by a similar scries of successive lateral undulations, gene- 

 rating a surplus force in the moving body equal to the difference 

 between the force of the locomotive organs and the resistance 

 of the medium. In Avatcr-snakes this resistance is made more 

 effective by the lateral flattening (u- compression of tlie tail, which 

 can be drawn forward edgewise, and flapped back breadthwise. 



