QUANTITY OF FOOD NECESSARY TO NUTRITION. 191 



articles of this class, which are so commonly used, with the various compound sauces 

 have no decided influence on nutrition, except in so far as they promote the secretion of 

 the, digestive fluids. Common salt, however, as we have already seen, is very important, 

 and this has been considered under the head of inorganic alimentary principles. The 

 various flavoring seeds and leaves, truffles, mushrooms, etc., have no physiological impor- 

 tance except as they render articles of food more palatable. 



Quantity and Variety of Food necessary to Nutrition. 



The inferior animals, especially those not subjected to the influence of man, regulate 

 by instinct the quantity and kind of food which they consume. The same is true of man 

 during the earliest periods of his existence ; but, later in life, the diet is variously modi- 

 fied by taste, habit, climate, and what may be termed artificial wants. It is usually a 

 safe rule to follow the appetite with regard to quantity, and the tastes, when they are 

 not manifestly vitiated or morbid, with regard to variety. The cravings of Nature indi- 

 cate when to change the form in which nutriment is taken ; and that a sufficient quan- 

 tity has been taken is manifested by a sense, not exactly of satiety, but of evident satis- 

 faction of the demands of the system. During the first periods of life, the supply must 

 be a little in excess of the actual loss, in order to furnish materials for growth ; during 

 the later periods, the quantity of nitrogenized matter assimilated is somewhat less than 

 the loss ; but, in adult age, the system is maintained at a tolerably definite standard by 

 the assimilation of material about equal in quantity to that which is discharged in the 

 form of excretions. 



Although the loss of substance by disassimilation creates and regulates the demand for 

 food, it is an important fact, never to be lost sight of, that the supply of food has a very 

 great influence upon the quantity of the excretions. As an illustration of this, we may 

 take the influence of food upon the exhalation of carbonic acid ; and this is but an 

 example of what takes place with regard to other excretions. The quantity of the 

 excretions is even more strikingly modified by exercise, which, within physiological 

 limits, increases the vigor of the system, provided the increased quantity of food required 

 be supplied. 



While a certain amount of waste of the system is inevitable, it is a conservative pro- 

 vision of Nature, that, when the supply of new material is diminished, life is preserved 

 not, indeed, in all its vigor by a corresponding reduction in the quantity of excre- 

 tions ; and, in the same way, the forces are retained after complete deprivation of food 

 much longer than if disassimilation proceeded always with the same activity. 



As regards the quantity of food necessary to maintain the system in proper condition, 

 it is evident that this must be greatly modified by habit, climate, the condition of the 

 muscular system, age, sex, etc., as well as idiosyncrasies. 



The daily loss of substance which must be supplied by material introduced from with- 

 out is very great. A large portion of this discharge takes place by the lungs, and a con- 

 sideration of the mode of introduction of gaseous principles to supply part of this waste be- 

 longs to the subject of respiration. The most abundant discharge which is compensated by 

 absorption from the alimentary canal is that of water, both in a liquid and vaporous con- 

 dition. The entire quantity of water daily removed from the system has been estimated 

 at about four and a half pounds ; and, assuming that there is no evidence of the produc- 

 tion of water in the organism, an equal quantity must necessarily be introduced. The 

 quantity which is taken in the form of drink varies with the character of the food. When 

 the solid articles contain a large proportion of water, the quantity of drink may be di- 

 minished ; and it is possible, by taking a large proportion of the watery vegetables, to 

 exist entirely without drink. There is no article the consumption of which is so much 

 a matter of habit as water, any excess which may be taken being readily removed by the 

 kidneys, skin, and lungs. Prof. Dalton estimates the daily quantity necessary for a full- 

 grown, healthy male, at fifty-two fluid ounces, or 3'38 Ibs. avoirdupois. 



