OPHIDIANS. 509 



is seen in the millipede ; the ribs moving in pairs, and each pair, 

 as advanced, carrying forward with themselves, the scales to 

 which their extremities are fixed, and which serve as so many 

 points of resistance to a backward movement. But the reptile 

 makes a more rapid progress by throwing the body into large 

 curvatures, the fore part being fixed, and the rest brought up by 

 the action of the muscles and ribs; the hind part of the body 

 being next fixed, and the fore part thrown forwards, and so on, 

 alternately. 



The most rapid movements are probably made when the entire 

 body is gathered up into one vertical loop, like a bent spring, the 

 head and tail being more or less approximated ; the sudden 

 straightening of this loop or spring, with the tail as the fulcrum 

 or point of resistance, may enable the animal to spring forward 

 at one operation, to a distance greater than the length of the 

 body. 



The exceeding flexibility of their bodies enables many spe- 

 cies to climb trees in pursuit of their prey, and on these, some 

 are habitually found ; others are constant inhabitants of the 

 water. 



The bones of the face, excepting in a few species, have a high 

 degree of mobility, and on account of their peculiarities, deserve 

 special notice. 



The lower jaw is not directly articulated with the upper, as in 

 other animals, but connected with it by two bones which are 

 movable upon each other. The extremities of the lower jaw 

 also, instead of being anchylosed, or immovably fixed, are con- 

 nected by an extensile ligament. Thus, unlike what is seen in 

 the Mammals, where the bones forming the jaws and face are 

 firmly locked together, those of the snake have no connection 

 but that which is made by ligaments and skin. Hence the ser- 

 pent tribe can swallow their food undivided, and many times 

 larger in bulk than the circumference of the body. In this pro- 

 cess they are aided by the expansibility of the skin, the gullet, 

 and the stomach ; but something additional was needed to com- 

 plete their capability of swallowing such enormous masses of 

 food ; how shall such masses be made to pass through the jaws? 

 Here is a difficulty which, without some peculiarity of structure 

 must have been insuperable, but which, in the case of these rep- 

 tiles, is fully met by the attachment of the lower jaw to bones 

 movable upon each other, and allowing of a sort of natural dis- 

 location, so that the jaw gives way in the act of swallowing, and 

 recovers itself when the prey is fairly engulphed. The head 

 of the snake may hence be pointed to as exhibiting one of the 



