Serpents. 45 



The vertebrae have each a concavity on the 

 anterior side of each body, into which the ball or 

 convexity of the hinder surface of the foregoing body 

 fits. There are also two pairs of articular facets on 

 the processes of each vertebra, so that the entire 

 spine combines flexibility with amazing strength. 

 The ribs are capable of being moved forwards and 

 backwards, and the ventral surface of the animal's 

 body is covered with flat, horny shields, into which 

 muscles run from the tip of each rib. The rapid, 

 even, gliding motion in serpents is accomplished by 

 the successive advances of these ventral scutes, and 

 the drawing of the body forwards towards them, while 

 the slightly projecting hinder edges of the scutes 

 serve as fixed points by catching the surface of the 

 ground. The brain case is firmly built up of sin- 

 gularly united bones ; but the bones of the upper and 

 lower jaw-arches are loose, united together by means 

 of fibrous tissue, and hence capable of an extreme 

 degree of stretching during the swallowing of food, 

 which these animals bolt in large masses. 



The teeth are recurved and solidified to the jaw, 

 not set in sockets, and they can only act as organs of 

 prehension. 



The tongue consists of a long, bifid muscular organ, 

 capable of being rapidly protruded, or of being drawn 

 back into a sheath when not in use. The windpipe 

 is long and protected by complete gristly rings ; only 

 one lung is usually large and developed, the other is 

 rudimental or simply saccular, and they are never 

 symmetrical. The digestive canal is capacious and 

 short, and the cloacal opening is transverse. The 



