MODIFYING EFFECTS OF FOOD. 55 



considered impossible, is afforded by the genus Dasypeltis among 

 the snakes ; the species occur in Africa and live on birds' eggs. 

 They swallow their food — the eggs — as other snakes swallow 

 frogs and fishes, snails or mammals. But the nourishment 

 contained in the egg is enclosed in a calcareous envelope — 

 the egg-shell ; if the snake, in order to get at the -contents of 

 the egg, were able to crush it by its teeth and jaws, it would 

 certainly lose the greater part of the fluid contents. The only 

 way. to lose nothing of it, therefore, is to swallow the egg 

 whole; and in point of fact it does reach the stomach xax- 



il 



<^ ^ 



Fig, 13.— Section of the oesophagus and stomach of the Pigeon, dr, the glands ; i, the 

 gizzard, enclosed in a thick brown skin. 



broken. But here organs have developed in a most marvellous 

 way, which, in all other cases without exception, are confined to 

 the bones of the mouth, namely, teeth. These occur firmly set 

 on the lower side of the vertebrse and in the forepart of the 

 stomach, and their points pierce through the coat of the 

 stomach so far that they seem to be pui-posely fitted for break- 

 ing the eggs passing through it ; in fact they must work in this 

 way, for they are the only part of the stomach strong enough to 

 be able to answer this purpose. This is, as has been said, the 

 only instance of true teeth, acting as such, occurring in any 

 other situation than on the bones which surround the cavity of 



