THE LIZAUm, SNAKES, AND OTHER REPTILES. 189 



gliding motion of snakes is due to the lateral bends of the 

 vertebral column, while the ribs can be moved back and 

 forth ; moreover, the large scales on the under side of the 

 body are successively advanced, the hinder edges of the 

 scales resting on the ground and acting as supports ; resting 

 on these, which are attached to one another and to the ends 



Fio. 196.— Mouth of the BuH-suake (Pityophis sayi). a, tongue-sheath; 6, epi- 

 glottis; c, rima glottidis or opening ot the windpipe. (Natural size.)-Shul;eldt. 



of the ribs, the body is then drawn or pushed rapidly for- 

 ward. 



Snakes can swallow animals much thicker than their own 

 bodies, because the bones of their skull and throat separate 

 so that the mouth and throat can be greatly extended. 

 Thus, a boa can swallow a calf whole, or a common striped 

 snake can swallow a toad or bullfrog. The bones of the 

 mouth are also armed with teeth pointing backwards, to pre- 

 vent the prey from slipping out of the mouth. Snakes oc- 

 casionally are known to hiss, the noise being caused by the 

 passage of the breath from the lungs through the wind- 



