HOW THE ANIMAL LIVES 213 



restricted. The intestine of the ox is about 160 

 feet long, that of the horse 90 feet, and that 

 of the dog only 12 to 14 feet. 



26. Food constituents 



369. All foods must contain chemical con- 

 stituents which will serve to repair the waste of 

 the body, to develop growing tissue, and to sup- 

 ply materials for the different sec, ret ions. 



370. Aside from mineral matters, all food 

 constituents which can build up the tissues must 

 contain nitrogen, the element which forms four- 

 fifths of the atmosphere, and which is an essen- 

 tial part of all body tissues. As familiar ex- 

 amples of such nitrogenous foods or aliments 

 may be named white of egg (albumin), milk 

 curd (casein), and one of the soluble parts of 

 flour (gluten). 



371. As common forms of foods that eontaii? 

 no nitrogen, and which cannot form tissues, ar& 

 starch, sugar and fats. These are used up or 

 burned in the system to produce body heat, to 

 stimulate the contraction of muscles, and to fur- 

 nish secretions which are free from nitrogen, 

 such as sugar and butter -fat in milk, and sugar 

 (more properly glycogen or sugar- former) in the 

 liver. 



372. Both sugar and fat, however, can be 

 formed in the body from nitrogenous food, as 



