SNAKES AND SNAKE-VENOMS 221 



ligament, and so can be widely separated in a horizontal 

 plane. 



In respect of the elongate body and the absence of 

 limbs, there are numerous lizards and some amphibia 

 that resemble snakes, but the means by which these and 

 certain other snake-like animals can be discriminated 

 will be considered presently. 



In the absence of limbs locomotion is effected by the 

 flexibility of the backbone, and by the mobility of the 

 ribs. Every vertebra in front of the cloacal level, except 

 the atlas, carries a pair of ribs, the free ends of which 

 are attached to the tissue underlying the large epidermal 

 scales (shields) of the belly, so that a snake crawls on 

 the ends of its ribs, acting upon the belly-shields. 



The tongue of snakes must be mentioned, as, although 

 it is merely a very protractile organ of touch, it is com- 

 monly mistaken for a weapon of offence or sting. The 

 tongue consists of a basal sheath, and two long, slender, 

 pointed, muscular cylinders separated in their distal 

 half which can be shot out of the sheath even when the 

 jaws are shut. A similar forked and ensheathed tongue 

 occurs in certain lizards. 



For the recognition of snakes it is necessary to under- 

 stand the terminology of the scales, especially of those 

 covering the head, and to know something of the skull 

 and dentition. 



The scales of snakes are horny plates of epidermis, 

 the superficial cuticular layer of which is moulted 

 periodically en masse. On the back and sides of the 

 body they are small, and are arranged in overlapping 

 rows. On the belly they are often much enlarged, and 

 are then known as shields ventral shields in front of the 

 cloacal opening, sub-caudal shields behind that point. 

 On the head also they often have the form of large 

 shields, which touch only by their edges. 



The shields of the head are best studied in a harmless 

 Colubrine snake, such as is represented in figs. 92 to 94, as 

 a type. 



