18 ZOOLOGY. 



is effected, is a cavity communicating with tho exterior, and 

 into which the food is introduced. 



30. Alinif-iits. We restrict the meaning of this term, 

 for the sake of clearness, to those substances which, being 

 introduced into the stomach, are absorbed only after being 

 digested. It is needless to remark that food and air are 

 essential to the support of life. 



The want of food is indicated by the painful sensation we 

 call hunger; its seat is in the stomach. This want and its 

 scns.it ion may be diminished and kept off by rest, sleep, and 

 by whatever retards the vital movements. On the contrary, 

 it is increased by activity, fresh air, and by the use of bitters. 

 Hybernating animals, which sleep during winter, eat not, so 

 Iniii4- as the lethargy continues; and cold-blooded animals, as 

 fishes and frogs, can live in despite of a long-continued absti- 

 nence. But man and other hot-blooded animals and more 

 especially, for an obvious reason, the young perish speedily 

 when food is withheld, even for a comparatively short time. 

 In Dante's celebrated episode of the destruction of the Ugolini 

 family by starvation, the youngest perished first. 



All alimentary substances are furnished by the organic 

 kingdoms animal and vegetable ; whatever be their origin, 

 they may be divided into azotized elements, amylaceous or 

 sweet, and fatty bodies. These substances differ in their nu- 

 trient qualities ; and it is a fact, proved by many curious 

 experiments, that a certain number of different substances is 

 essential for the support of life. 



Thus, rabbits fed upon only one article of food, as hay, 

 wheat, cabbage, carrots, &c., die in about fifteen days ; whilst 

 fed on these articles combined or given in succession, they 

 live and thrive. 



A hygienic law, then, is, the diversity and variety in r 

 of food; and experience and experiment agree as to this. 



It has been proved experimentally, that substances (such 

 as suirur, 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. 



