ANIMAL CHEMISTRY. 397 



stance, oxygen, is partly furnished from, the air that enters 

 the lungs. The process of digestion, by which the blood 

 is made from the food, we shall not particularly describe 

 here, but will refer you to Hooker's two works on Physi- 

 ology. It is sufficient to speak of it here very briefly and 

 generally. The food on being ground up by the teeth is 

 at the same time mixed with the saliva, which is poured 

 into the mouth by several glands, or saliva factories, as 

 they may be called. In the stomach the ground and moist- 

 ened mass is acted upon by the gastric juice, a fluid which 

 oozes out from myriads of minute glands set into the inner 

 surface of the organ. This is a chemical operation, and it 

 is promoted by a constant motion which is kept up in the 

 stomach, thus stirring up the food so that the gastric juice 

 may be well mixed with it. After the proper chemical 

 change is effected the nutritious part of the mass is ab- 

 sorbed and poured into the blood, and becomes a part 

 of it. 



562. Albumen. The protein compounds, or albuminoids, 

 are very nearly identical in composition, as you have al- 

 ready learned, and they are convertible into each other. 

 In the animal the various tissues or structures, as we have 

 stated in 560, are made chiefly of fibrin. Exactly how 

 fibrin is formed in the animal economy is an unsettled ques- 

 tion. It was formerly supposed that albumen was trans- 

 formed into fibrin, and that all other protein bodies were 

 first converted into albumen in the stomach, but this lacks 

 demonstration. Albumen is held in solution in the blood 

 by means of chloride of sodium associated with it. It does 

 not occur in the animal in the free state, jbut as an alkaline 

 albuminate. It forms about seven per cent, of blood, and 

 occurs in the brain, in the juice of the flesh, and in a greater 

 or smaller quantity in all the liquids effused from the blood- 

 vessels into different parts of the system. 



