CLASS REPTILIA. 



91 



which were previously directed backwards along the roof 

 of the mouth, are now erected, by the elevation of the 



Fig. 42. — The head of the Rattlesnake, dissected to show the poison-gland («) 

 and poison-fangs (/). After Duvemoy. 



upper jaw-bone, to which they are attached. The fangs 

 are then plunged with great force into the body of the 

 animal which the snake wishes to kill. In performing 

 this act, the miiscles which cover the poison-gland com- 

 press it strongly, so that as the poison-fangs enter the 

 flesh, a drop of the poisonous fluid is driven into the 

 wound through the little perforation in the point of the 

 poison-fang. The poison itself is a colourless, greasy 

 fluid, and it produces death, at any rate in the majority 

 of cases, with a rapidity proportioned to the size of the 

 wounded animal. 



The mouth opens into a comparatively simple digestive 

 tube, which terminates in a vent placed on the under sur- 

 face of the tail. The breathing-organs are in the form of 

 voluminous lungs, of which one is rudimentary. It is to 

 be remarked, therefore, that the animal is strictly an air- 

 breather, and that at no time of its life does it possess 

 gills, or organs adapted for breathing air dissolved in 

 water. The heart consists of three chambers, and is so 

 constructed that the impure (venous) blood, mixes to 

 a greater or less extent ^vith the pure blood which has 

 come from the lungs ; and the body is then supplied with 



