20 



requiring a large supply of air to be able to stay long under 

 water. In most snakes there is but one lung, the other 

 being atrophied. They breathe at considerable intervals ; 

 and many kinds can remain under water for nearly half 

 an hour at a time. During the intervals between each 

 inspiration, respiratory movements of the ribs will be 

 observed ; the lungs acting as reservoirs of air, these move- 

 ments are for the purpose of changing the air in the cells of 

 the luDg-tissue. The air breathed by the nostrils passes 

 through the trachea or windpipe, the upper part of which 

 lies on the floor of the mouth and is closed by two 

 cartilages. The vertical slit between them forming the 

 glottis is just opposite the inner orifice of the nostrils when 

 the mouth is shut ; it is the rapid expulsion of air through 

 the glottis which produces the hiss of some snakes when 

 they are angry (the noise, is something between a hiss and 

 the spit of an angry cat). 



The heart is situated at about one-sixth of the distance 

 down the body. It is composed of one ventricle incom- 

 pletely divided, and of two auricles (atria). The division 

 of the ventricle is suflScient to enable the pulmonary and 

 the systemic circulation to be carried on in very much the 

 same way as in the higher classes of vertebrata. 



The stomach appears to be merely the distended part of 

 the gullet ; there is little difference perceptible between 

 them. The combined organ is well lubricated by the 

 secretions of the jaws and its own proper secretions and 

 is capable of great distension. Digestion appears to go on 

 principally at the lower end, where that part of the animal 

 which was swallowed first passes into a state of solution and 

 the rest gradually comes down as the space becomes vacant. 

 The intestinal canal, very little convoluted, occupies the 

 hinder half of the abdomen ; the mesentery is plentifully 

 loaded with fat, which becomes a reserve of nutriment for 



