158 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



reveals the wants of the organism and creates a demand for food. If 

 these demands are not attended to, other more serious phenomena 

 result. These, as well as the sensations of hunger and thirst, will be 

 described at a later point. 



Not only must the aliments taken to repair waste have a certain 

 weight, but they must also have a definite quality, since nitrogenous 

 and non-nitrogenous material, water and inorganic substances, all escape 

 through the various excretions, and their losses must be supplied by 

 analogous substances in quantities in proportion to the amounts lost 

 by excretion. Under all circumstances the foods of animals are or- 

 ganic, and these foods for the most part contain those inorganic sub- 

 stances already prepared which form the direct constituents of the 

 animal body ; they are therefore analogous and equivalent to what they 

 replace. The animal economy does not, as does the plant, supply its 

 nutritive wants by synthesis and condensation of the substances con- 

 tained in its food ; but it requires the constituents of its flesh and 

 blood to be already formed in its food. In the flesh of the herbivora 

 the carnivora consume flesh similar to their own. In plants the her- 

 bivora obtain readj'-formed constituents of their flesh and blood. The 

 end products of the activity of plant life, vegetable albumen, and other 

 constituents of vegetable tissue, serve directly and without further exten- 

 sive chemical modification to supply the waste in the animal economy ; 

 consequently, plants act as the food-preparing organisms in the general 

 circle of life. Vegetable life must, therefore, first have appeared on the 

 earth, for it is a necessary condition for the existence of animal life ; 

 both herbivora and carnivora are dependent upon the vegetable kingdom 

 for food. In vegetable matters are repeated the most complex ingre- 

 dients of animal tissues. Chemists have been so struck by the similarity 

 of such bodies that they have designated them by the same names. 

 Thus, we have vegetable albumen, vegetable fibrin, vegetable casein, 

 representing the albuminous group. Among the carbo-hydrates we 

 . have starches and sugars, and it is well known that fats are abundant 

 in the vegetable kingdom. Animals, therefore, find their tissue-con- 

 stituents ready-made in their food, whatever be its nature. 



The principles of food, whether derived from the animal or vegetable 

 kingdom, and whether appropriated by the herbivora or carnivora, are 

 not retained in the organism in the form in which they are taken as food. 

 They must first be subjected to certain modifications before they can 

 become constituents of the animal tissue or juices ; in other words, they 

 cannot fulfill their nutritive purposes until they have been subjected to 

 preparatory modifications in the digestive tube. These modifications are 

 not in general very profound, and usually consist in reducing foods to a 

 soluble form, if not already so, or in reducing them to a state in which 



