OF DIGESTION. 



99 



every animal where they are found, in an invariable order ; 

 first, the stomach, (s,) then the intestine, which is small at 

 first, but often enlarged 

 towards its termination. 

 This arrangement may 

 be seen by the follow- 

 ing diagrams from a bee- 

 tle and a land mollusk, 

 where the same letters 

 indicate corresponding 

 parts, (Figs. 51, 52.) 



207. From the mouth, 

 (m,) the food passes into 

 the stomach through a 

 narrow tube in the neck, 

 called the oesophagus or 



gullet, (o.) This is not Fi S- 51 - Fi s- 52 - 



always a direct passage of uniform size ; but there is some- 

 times a pouch, the crop, (c,) into which the food is first intro- 

 duced, and which sometimes acquires considerable dimen- 

 sions, especially in birds, and in some insects and mollusks, 

 (Fig. 51.) In the stomach, the true digestive process is be- 

 gun. The food no sooner arrives there than changes com- 

 mence, under the influence of a peculiar fluid called the gas- 

 tric juice, which is secreted by glands lining the interior of 

 the stomach. The digestive action is sometimes aided by the 

 movements of the stomach itself, which, by its strong contrac- 

 tions, triturates the food. This is especially the case in the 

 gizzard of some birds, which, in the hens and ducks, for in- 

 stance, is a powerful muscular organ. In some of the Crus- 

 tacea and Mollusks, as the Lobster and Aplysia, there are 

 even solid organs for breaking down the food within the 

 stomach itself. 



208. The result of this process is the reduction of the food 



