CHAPTEE VII. 



FOOD. 



UNDER the term " food" are included all substances, both solid and 

 liquid, necessary to sustain the process of nutrition. The first act of 

 this process is the appropriation from without of the materials which 

 enter into the composition of the living frame, or of others which may 

 be converted into them in the interior of the body. Like the tissues 

 and the fluids, therefore, the food contains various ingredients, both 

 organic and inorganic ; and the first important fact to be noted with 

 regard to them is that no single class of substances, by itself, is suffi- 

 cient to sustain life, but that several must be supplied, in due propor- 

 tion, in order to maintain the body in a healthy condition. 



Inorganic Ingredients of the Food. 



It is well known that inorganic substances, although they afford the 

 necessary materials for vegetation, are not sufficient for the nourish- 

 ment of animals, which depend for their support upon elements already 

 combined in the organic form. Nevertheless, it is equally true that the 

 inorganic matters are also essential to animal life, and require to be sup- 

 plied in sufficient quantity to keep up the natural proportion in which 

 they exist in the various solids and fluids. As we have found it to 

 be a general characteristic of these substances, that they are exempt 

 from alteration in the interior of the body, but are absorbed, deposited, 

 and expelled unchanged, each one, as a rule, requires to be present 

 under its own form, and in sufficient quantity in the food. This is 

 especially true of water and sodium chloride, both of which enter and 

 leave the system in abundant daily quantity; and of the calcareous 

 salts, which during the growth and ossification of the skeleton are 

 deposited in large proportion in the osseous tissue. The alkaline car- 

 bonates, phosphates, and sulphates are partly formed within the system 

 during the metamorphosis or decomposition of organic substances ; but 

 the elements of which they are composed must of course enter the 

 body in some form, in order to enable these changes to be accomplished. 



Since water enters into the composition of every part of the body, 

 it is important as an ingredient of the food. In man, it is probably the 

 most important substance to be supplied with constancy and regularity, 

 and the system suffers more rapidly when entirely deprived of fluids, 

 than when the supply of solid food only is withdrawn. A man may 

 pass eight or ten hours without solid food, and suffer little or no 

 inconvenience ; but if deprived of water for the same length of time, 



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