CHAP. XIIL] ALIMENTATION. 147 



innutritious portion, usually by far the smaller of the two divis- 

 ions, 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 : 



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 " 



with usually a little sulphur and sometimes a trace of phos- 

 phorus and iron. They are the only food-stuffs that contain 

 nitrogen in any appreciable quantity, and are sometimes 

 classed as " nitrogenous " food-stuffs. Proteids occur in the 

 form of albumin in the white of egg (egg-albumin), in milk, 

 in blood and lymph (serum-albumin); in the form of casein in 

 milk and cheese ; of myosin and syntonin in muscle ; of vitellin in 

 the yolk of eggs ; of glutin in flour. Allied to proteids are the 

 chondrin, obtained from cartilage, and the gelatin, obtained 

 from other varieties of connective tissue, by boiling. 



All proteids yield peptones very readily at the temperature 

 of the body under the action of the acid gastric, and alka- 

 line pancreatic juice. These peptones are highly soluble bodies 

 and readily absorbed. 



The foods that are most rich in the various forms of proteids 

 are meat, milk, eggs, cheese, all kinds of fish, wheat, beans, 

 and oatmeal. 



