150 THE BLOOD 



customary to take blood plasma as a standard. A solution of o . 64 per cent, 

 sodium chloride is isotonic for the blood plasma of the frog, and a 0.9 per 

 cent, solution for that of man. Further, any solution which is of a lower 

 osmotic pressure than the standard solution is said to be hypoisotonic (Jiypo- 

 tonic] in relation to that solution. A solution of a higher osmotic pressure 

 is said to be hyperisotonic (hypertonic). 



Water passes in the Direction of the Arrows. 

 Hypertonic saline solution (2 per cent.) 



t 



Blood-plasma 



_U 



Isotonic saline solution (o . 64 per cent.) 



I. 



Hypotonic saline solution (-3 P er cent.) 



If a hypotonic solution be mixed with blood, water from the hypotonic 

 solution passes through the cell membrane of the red corpuscles into the 

 stroma and causes it to swell. The hemoglobin at the same time passes 

 out and goes into solution in the diluted plasma. On the other hand, the 

 addition of a hypertonic solution to the plasma causes the red corpuscles 

 to lose their water and become crenated. The principles of osmosis have 

 been derived from the action of substances separated by dead animal or 

 plant membranes. It must be, however, remembered that in the applica- 

 tion of these principles to processes occurring in the living organism, the 

 cells, forming the various membanes, are an important modifying factor. 

 It is probable that physico-chemical processes, occurring in the protoplasm 

 of the cell, may change its permeability to the same substance at different 

 times. 



THE CHARACTER AND COMPOSITION OF LYMPH. 



The lymph is the fluid which immediately surrounds the tissue cells of 

 the living body. It fills up the spaces between the cells themselves and 

 between the cells and the blood vessels which ramify among the cell masses. 

 The lymph, therefore, is an intermediate fluid between blood plasma on the 

 one hand and the tissue cells on the other, receiving its ingredients by the 

 passage of fluid from the plasma through the walls of the finer blood vessels 

 in the one direction and by the discharge of the substances from the cells 

 themselves in the other. 



The Chemical Composition of the Lymph. Since the chief source 

 of the lymph is the blood plasma, one would naturally expect that its chemi- 

 cal composition would be very similar to that of plasma, which is in fact the 

 case. The variations that are noted in lymph taken from definite sources no 

 doubt have their, origin in the fact that the lymph passes through these 



