986 DIGESTION 



ical and chemical mechanisms are combined in such a way that both 

 kinds of reductions may be had within the confines of the same organ. 

 Obviously, the histological elements more directly concerned with them, 

 are the striated and smooth muscle cells and the gland cells. 



The food made use of by the higher animals, is heterogeneous m 

 its character, consisting of inorganic and organic principles, and a 

 certain amount of non-digestible and non-nutritive material, such as 

 connective tissue and the cellulose of the plants. The predigestive 

 procedures which man employs in preparing his food, cannot materially 

 alter this condition, because only very few non-assimilable mate- 

 rials can be made available thereby. This is true of the process 

 of cooking as well as of that of maceration. All that can be accom- 

 plished by these means is to free the nutritive principles from their 

 non-digestible investments, and to increase the solvent action of the dif- 

 ferent digestive juices, so that they may be more readily reduced, chemi- 

 cally as well as mechanically. Nutritive material is consumed in the 

 form of food, consisting of different foodstuffs. Consequently, a 

 food is a mixture of nutritive substances, whereas a foodstuff is a 

 single nutritive substance. The latter are grouped as water, salts, 

 carbohydrate, fat, and protein, and may be arranged as nitrogenous 

 and non-nitrogenous substances, as follows: 



Organic Inorganic 



Nitrogenous . Non-nitrogenous (Non-nitrogenous) 



Proteins Fats Carbohydrates Salts Water 



Obviously, the purpose of food is to replenish the material which 

 nas been used up by the cells during their oxidations in furnishing the 

 energy upon which the bodily machine is run. Were this waste to con- 

 tinue without being balanced by an adequate intake, the animal 

 would soon have to discontinue its activities. Moreover, since food 

 as exemplified by meat, potatoes, milk, bread, etc., is invariably 

 made up of several foodstuffs, it will, be seen that our diet usually 

 consists of several of the proximate principles just mentioned. In 

 fact, no diet can be regarded as adequate for man which does not em- 

 brace all the different foodstuffs, mixed in proper proportion, to burden 

 the body with only a minimum of labor. 



Water, salts and some carbohydrates, such as dextrose, are capable 

 of traversing the intestinal epithelium in their original form, whereas 

 the indiffusible colloidal carbohydrates, such as starch and dextrin, 

 must first be converted into soluble and diffusible sugar. This is 

 also true of the fats which must first be changed into glycerin and fatty 

 acids, and the natural proteins which must first be converted into dif- 

 fusible peptones and simpler compounds. As stated above, digestion 

 does not end here, but in addition imparts to the now diffusible end- 

 products of the different foodstuffs a form which will enable the cells 



