THE BLOOD 111 



the red marrow found in the cavities in the 

 bones. It also receives oxygen from the air 

 by the process of respiration. The blood also 

 receives matters that may be absorbed from 

 many surfaces, both internal and external, 

 such as the pleural and peritoneal cavities. 

 The blood is therefore a highly complex 

 fluid. It must be regarded not so much as 

 a fluid as a complex tissue, and as the physical 

 condition of its cellular constituents, the blood 

 corpuscles, as well as the nutrition of the tissues 

 to which it supplies lymph, depends on it, 

 we find that it varies very little either in its 

 physical characters or in its chemical constitu- 

 tion. Thus its specific gravity and its 

 viscosity vary within very narrow limits. 

 Matters coming to it by the channels above 

 indicated are constantly being used up by 

 the tissues ; so that, although a considerable 

 amount of these matters enters the blood daily, 

 the percentage amount of any one of them is, 

 as a rule, small. Again, matters that are waste 

 products and which would be injurious to 

 living tissues, if they accumulated above a 

 small percentage are continually being elimi- 



