HUNGER AND THIRST. 169 



to physiology. According to Chossat, death from starvation occurs after a 

 loss of four-tenths of the weight of the body, the time of death being variable 

 in different classes of animals. 



Thirty to thirty-five days may be taken as the average duration of life in 

 dogs deprived entirely of food and drink. It is important to bear in mind 

 this fact in connection with observations on the nutritive value of different 

 articles of food. 



ALIMENTATION. 



Under the name of aliment, in its widest signification, it is proposed to 

 include all articles composed of or containing substances in a form which en- 

 ables them to be used for the nourishment of the body, either by being them- 

 selves appropriated by the organism, by influencing favorably the process of 

 nutrition, or by retarding disassimilation. Those substances which are them- 

 selves appropriated may be called direct aliments ; and those which simply 

 assist nutrition without contributing reparative material, together with those 

 which retard disassimilation, may be termed accessory aliments. In this 

 definition of aliment, nothing is excluded which contributes to nutrition. 

 The air must be considered in this light, as well as water and all articles 

 which are commonly called drinks. 



In the various articles used as food, nutritious substances are frequently 

 combined with each other and with indigestible and innutritions matters. 

 The constituents of the food which are directly used in nutrition are the true 

 alimentary substances, embracing, thus, only those which are capable of 

 absorption and assimilation. The ordinary food of the warm-blooded ani- 

 mals contains alimentary matters united with innutritious substances from 

 which they are separated in digestion. This necessitates a complicated 

 digestive apparatus. In some of the inferior animals, the quantity of nu- 

 tritious matter forms so small a part of the ingesta that the digestive 

 apparatus is even more complicated than in the human subject. This is 

 specially marked in the herbivora, the flesh of which forms an important part 

 of the diet of man. In addition to what are distinctly recognized as ali- 

 mentary substances, food has many constituents which exert an important 

 influence on nutrition, which have never been isolated and analyzed, but 

 which render it agreeable. Many of these are developed in the process of 

 cooking. 



Alimentary substances belong to the inorganic, vegetable, and animal 

 kingdoms. They are generally divided into the following classes : 



1. Organic iiitrogenized substances (albumin, fibrin, caseine, myosine 

 etc.), belonging to the animal kingdom, and vegetable nitrogenized substances, 

 such as gluten and legumine. 



2. Organic non-nitrogenized substances (sugars, starch and fats). 



3. Inorganic substances. 



Nitrogenized Alimentary Substances. In the nutrition of certain classes 

 of animals, these substances are derived exclusively from the animal king- 

 dom, and in others, exclusively from the vegetable kingdom ; but in man, 



