92 MANUAL OF PHYSIOLOGY. 



II. INORGANIC. 



1. Salts mixed with all kinds of food. 



2. Water mixed with the foregoing or alone. 



The nutritive value of any kind of food depends upon a variety 

 of circumstances, which may be thus summed up: 



I. Chemical composition, of which the main points are 



(1.) The proportion of soluble and digestible matters (true 

 food-stuffs) to those which are insoluble and indi- 

 gestible (such as cellulose), etc. 



(2.) The number of different kinds of nutrient stuffs 

 present in it. 



(3.) The relative proportion of each of these chemical 

 groups. 



II. Mechanical Construction. The relation of the nutrient to 

 the non-nutrient parts is of the greatest importance, as is seen 

 where the nutritious starch of various vegetables is inclosed in 

 insoluble cases of cellulose, which, if not burst by boiling, prevent 

 the digestive fluids from reaching the starch. 



III. Digestibility. This depends partly upon how the sub- 

 stances affect the motions of the intestines, and partly upon their 

 construction. Thus, some substances, such as cheese, though 

 chemically showing evidence of great nutritive properties, by their 

 impermeability resist the digestive juices, and are poor aliments. 



IV. Idiosyncrasy. In different animals and in different indi- 

 viduals, and even in the same individuals under different circum- 

 stances, food may have a different nutritive value. 



Chemically, then, foods are composed of a limited number of 

 elements similar to those found in the animal tissues, viz., carbon, 

 oxygen, nitrogen, and hydrogen, together with some salts. If 

 nothing more were needed by the economy than a supply of these 

 elements and salts in a proportion like that in which they exist 

 in the tissues, such could be easily obtained from inorganic sources; 

 but, as has already been stated, it is necessary that an animal ob- 

 tain these elements associated in the form of organic materials of 

 complex construction (namely, proteids, etc.) ready made. Al- 

 lowing the necessity of organic food, it might be supposed that 

 since the elements exist in proper proportion in the proteids, an 



