CHAPTER XVIII.
FOOD AND FOOD-SUBSTANCES.
There are two kinds of food: 1. Those food substances that are derived from the animal kingdom; and, 2. Food substances that are derived from the vegetable kingdom.
Food is taken into the system to replace the material expended by the human body, or the waste products which are thrown off from the master tissues.
Definition: Food may be defined to be any natural substance, vegetable or animal, recognized as such, that has undergone neither the process of fermentation nor that of putrefaction.
Food may be considered in its relation to two purposes—the nutrition of the tissues, and the production of heat. Under the first of these heads will be included many other allied functions, as for example, secretion and generation; and under the second, not the production of heat only as such, but of all other forces correlated with it, which are manifested by the living body.
Foods derived from the animal kingdom are called nitrogenous substances, or azotized. They are also known by the name of proteids. These are mainly derived from meat, milk, eggs, etc. Of several we will examine the chemical composition.
It will be well to state in general terms that all food substances contain in their composition from two-thirds to three-fourths, or even more, of water—some more, some less.