OF, DIGESTION. 



19 



It has been proved experimentally, that substances (such 

 as sugar, oil, gum, fat) devoid of azote do not nourish, 

 however much they may be varied. The use of a certain 

 number of substances, such as the muscular flesh albu- 

 men and the gluten found in wheat, seems essential to the 

 support of life. 



40. Digestive Apparatus. The object of digestion is. 

 1st, to separate the nutrient part of the aliment from the 

 non-nutrient (faces); 2nd, to convert the nutrient part into 

 a liquid fit to mingle with the blood and thus to nourish the 

 body. 



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 

 digestive. 



41. In certain animals 

 the digestive cavity is simply 

 a pouch) having but a single 

 entrance by which the food 

 is received and the non-nu- 

 trient portion is expelled 

 (Fig. 4. 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 

 amis. 



The alimentary canal thus pig. 4> _ 

 forms a tube, dilated at in- 

 tervals, and with two open- 

 ings (Fig. 5). 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- 

 c 2 



-Hydia, or Fresh-water 

 Polyp. 



