Chap. XIV.J 



ALIMENTATION. 



169 



brane, covered with epithelium, and containing blood-vessels 

 and lacteals, and are favourably arranged for absorbing nutri- 

 tive matters from the intestines. 



Section II. Food. — Under the term 

 " food " we include all substances, solid or 

 liquid, necessary for nutrition. The ques- 

 tion at once arises : What are these sub- 

 stances, and how are they obtained ? 



If we analyze the food we daily take into 

 our mouths and introduce into the aliment- 

 ary canal, we find it separable into two 

 divisions ; viz. that which is nutritious, 

 and that which is innutritions. The nutri- 

 tious portion, that which can be digested, 

 absorbed, and made use of by the body, 

 is generally spoken of under the name 

 of food-stuffs or food-principles : the innutri- 

 tious portion, usually by far the smaller of 

 the two divisions, never enters the body 

 at all, properly speaking, but passes through 

 the alimentary canal and is excreted in the 

 form of feces. 



Food-stuffs are composed mainly of the elementary chemical 

 substances, — oxygen, carbon, hydrogen, nitrogen, — and may, 

 according to the varying proportions in which these chemical 

 elements combine, form five distinct and different classes of 

 food-stuffs. These are : — 



Fig. 111. — An Intes- 

 tinal Villus. «, a, a, 

 columnar epithelium ; 

 h, b, capillary network; 

 c, c, longitudinal muscle 

 fibres ; d, lacteal vessel. 



1. Proteids. 



2. Fats. 



3. Carbo-hydrates. 



4. Water. 



5. Saline or mineral matters. 



Proteids. — Proteids form a large proportion of all living 

 bodies, and are an essential part of all living structures. They 

 contain on an average in every 100 parts about : — 



Carbon 53 parts 



Hydrogen 7 " 



Oxygen 24 " 



Nitrogen 16 " 



