OF DIGESTION. 



19 



This elaboration of the food takes place, in animals, in a 

 cavity more or less ample, communicating with the exterior, 

 and into which the food is received, and from which the non- 

 nutrient portions are expelled. Vegetables require no such 

 apparatus. The cavity to which we allude is called the 



ive. 



41. In certain animals 

 the digestive cavity i simply 

 a pouch, having but a single 

 entrance by which the food 

 is received and the non-nu- 

 trient portion is expelled 

 (Fig. 3, a) ; and this arrange- 

 ment prevails in most of the 

 polyps, asterise or sea- stars ; 

 and many other animals 

 more complex in their struc- 

 tures also show this arrange- 

 ment. But, for the most 

 part, the digestive tube or 

 canal has two openings an 

 entrance for the food, and an 

 exit for the non-nutrient 

 part, the mouth and the 

 anus. 



The alimentary canal thus 

 forms a tube, dilated at in- 

 tervals, and with two open- 

 ings (Fig. 4). The more important of these dilatations is 

 called the stomach. This cavity is sometimes single, as in 

 the carnivora ; sometimes quadruple, or at least complex, as. 

 in the herbivora; and the reason assigned for such a com- 

 plexity is, that vegetable food, being less easy of digestion, 

 requires a longer sojourn in the stomachal cavities.* 



42. A membrane, called mucous, lines the digestive cavity 

 throughout. It is analogous to the skin, with which it is 

 continuous, but differs in structure. It is much softer, and 

 in place of an epidermis is protected on its exposed surface by 

 a reticular tissue, soft arid turgid, called epithelium. 

 Finally, it is more vascular than the other, and abounds with 



* The stomach is probably single in all animals, being simply subdivided 

 in some into different compartments. In whales, though strictly car- 

 nivorous animals, the stomach is extremely complex. R. K. 



c2 



Fig. 3. Hydra, or Fresh-water 

 Polyp. 



