THE RABBIT. 15 



helps in the process of mastication, by moving the food 

 about in the mouth, and also in the process of swallowing 

 by working it into a bolus and thrusting it back. 



7. Food- and Air-passages. The roof of the mouth 

 is formed by the palate, above which lies the air passage 

 from nose to throat. At the back of the mouth or pharynx 

 the courses of food and air cross one another, and a small 

 part of the throat is common to both, though the common 

 portion is reduced to an insignificant amount. This con- 

 nexion between the respiratory and alimentary systems, 

 which is of very slight advantage to the rabbit, which does 

 not breathe through its mouth under ordinary circumstances, 

 is one of the most characteristic peculiarities of the verte- 

 brate animals, and affords the first example of a feature 

 which is inexplicable from the point of view of the par- 

 ticular type studied, but whose meaning will become clear 

 when we have examined the other vertebrate types. We 

 shall deal with respiration fully in the next chapter; at 

 present it will be sufficient to say that^on the floor of the 

 throat is an opening, the glottis, into the beginning of the 

 passage to the lungs. To prevent the passing of food down 

 into this opening, it is protected by an epiglottis, an up- 

 growth which, in the rabbit, forms a complete ring round 

 the glottis, and which during the act of swallowing is 

 apparently thrust up above the posterior end of the palate 

 into the air-passage. Thus the food has to go either right 

 or left of the epiglottis* to reach the oesophagus. 



8. The (Esophagus. The food-bolus is forced down 

 the oesophagus by the contraction of the walls of the 

 latter. Nothing new happens to it in its course down 

 the separation of stomach from mouth by a long tube has 

 no particular advantage, but is simply an anatomical 

 necessity in an animal with a neck and thorax. 



9. The Stomach and Gastric Juice. In the abdomen 

 the original tube of the alimentary canal is first expanded 



* In man the process is different. 



