USE OF THE DIGESTED FOOD 145 



the lungs is less rich in oxygen than when it was taken in, 

 and there have been added to it certain materials which 

 are noticed later. 



206. The use of food. — ^The revivified blood now 

 passes to all parts of the body and is brought into the 

 most intimate relation with the minutest portion of every 

 tissue. Several things happen in the course of time. 



In the first place, the new supply of nutritive sub- 

 stances is used by the living cells in a way we do not 

 wholly imderstand to rebuild worn-out tissue and to 

 form new growth. With the young animal, much material 

 is appropriated in the latter way. In the case of the milch 

 cow, there is furnished to the udder the nutrients out of 

 which certain constituents of milk are formed through the 

 special activities of that gland. 



207. Nutrients are oxidized. — ^Moreover, it is in the 

 tissues that the oxygen which was taken up in the lungs 

 is used to burn slowly a portion of the food. This com- 

 bustion does not take place by the mere contact of the 

 oxygen and food in the large blood vessels, but it occurs 

 by progressive steps throughout the minute divisions of 

 the muscles and other parts of the whole body. Not- 

 withstanding this oxidation may be very gradual and 

 occupy much time, its ultimate products are, for the most 

 part, similar to those which result from the rapid com- 

 bustion of fuel. In the fireplace, starch, sugar, cellulose, 

 fats, and similar bodies would be Ijurned to carbonic acid 

 and water, and this is what takes place in the animal to 

 the extent that these nutrients are not used for the forma- 

 tion of body substance. When the protein is not stored 

 as such but is broken up, the result differs somewhat in 

 the furnace and in the animal because in the latter the 

 oxidation is not complete. 



