CHAPTER IX. 



THE BLOOD AND LYMPH. 



The blood consists of a fluid, the plasma, in which three sorts 

 of bodies are suspended : the red corpuscles, the leucocytes or 

 white corpuscles, and the blood-plates. 



The plasma is a solution in water of albuminous and other sub- 

 stances. Some of these are of nutritive value to the tissues of the 

 body. Others have been received from those tissues, and are on 

 their way toward elimination from the body. Still other con- 

 stituents have passed into the blood from one part of the body, and 

 are destined to be of use to other parts. 



In the smaller vessels, while on its course through the circulatory 

 system, portions of the plasma make their way through the vascular 

 walls and form the fluid of the lymph. This passage appears to be, 

 in part, a simple filtration through the walls of the vessel, or the 

 result of osmosis ; in part, the result of a species of secretion 



Fig. 112. 



a 6 c 



Red corpuscles from human blood. (Bohin and Davidoff.) a, optical section of a red blood- 

 corpuscle, seen from the edge; b, surface view. (The bounds of the central depression 

 are made a little too distinct in this figure, as is evident from an inspection of a.) c, 

 rouleau of red corpuscles. When undiluted blood has remained quiescent for a few 

 moments the red corpuscles arrange themselves in such rows, probably because of the 

 attraction which they, in common with other bodies suspended in a fluid having a nearly 

 identical specific gravity, have for each other. 



effected by the endothelial cells lining the bloodvessels, these cells 



promoting the escape of certain constituents of the plasma and 



restraining or preventing that of others. In the exercise of this 



secretory function the endothelia in different parts of the vascular 



system appear to act differently, the composition of the fluid passing 



through the walls of the vessels not being exactly the same in all 



parts of the body. It is still a question, however, in what degree 

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