84 ELEMENTARY PHYSIOLOGY 



(2) Sugar. There are several varieties of sugar derived from 

 both plants and animals. Cane sugar, beet sugar, grape sugar, 

 and milk sugar are the chief. The solid part of honey also consists 

 largely of grape sugar. These varieties of sugar are all more or 

 less soluble in water. 



(3) Gums. These are also vegetable products, obtained from 

 various parts of plants. 



(4) Fats and Oils. These are obtained from both animal and 

 vegetable sources. Fats are solid, derived chiefly from animals ; 

 oils are liquid, and are obtained chiefly from fruits. They re- 

 semble each other in chemical composition, and are insoluble in 

 water. 



The inorganic or mineral foods consist chiefly of water, 

 common salt, calcium carbonate, and the phosphates of calcium, 

 magnesium^ potassium, and sodium ; also certain salts of iron, &c. 

 These foods are not, with the exception of water and common 

 salt, generally taken separately ; but they all exist in the animal 

 and vegetable substances we eat. 



Life cannot be sustained on mineral foods alone, although 

 these may contain all the elements required to make good the 

 waste. Neither can life be permanently sustained by any one 

 class alone of the organic foods. Hence it follows that our food 

 must consist of mixtures of these. But the various food-substances 

 we employ do generally consist of such mixtures ; for instance, 

 wheat flour contains the nitrogenous substance gluten, the non- 

 nitrogenous substance starch, and also various mineral salts de- 

 rived by the wheat plant from the soil in which it grew. Again, 

 butcher's meat contains the nitrogenous myosin, the non-nitro- 

 genous fat, together with mineral salts obtained indirectly from 

 the soil. 



We may now study the manner in which life may be best sus- 

 tained by ordinary food-substances with the greatest economy as 

 regards the work thrown on the various organs concerned. We 

 have seen that the daily waste of carbon and nitrogen from the 

 system amounts to 



Carbon ..... 4,500 grs. about 10 ozs. 

 Nitrogen. . . . * 300 grs. about oz. 



Now let us suppose a person to feed on bread only. In order 

 to obtain the necessary quantity of nitrogen to repair this waste, 

 he would have to eat nearly 4^ Ibs. daily. This weight of bread 

 contains 



Carbon 9,000 grs. 



Nitrogen .... 300 grs. 



Hence he would be compelled to take about double the quantity 

 of carbon required in order to obtain the necessary weight of 



