154 THE BLOOD 



be easily understood, is as follows: In figure 125 one surface of the mem- 

 brane is being bombarded by the molecules of a non-diffusible substance 

 mixed with those of a diffusible one (water) ; while the other surface is being 

 bombarded entirely by water molecules. The former condition permits 

 only a fraction of the molecules to diffuse out, since fewer water molecules 

 get to the surface of the membrane; while the latter permits all of the 

 molecules which reach it to pass through. 



Osmotic pressure can be estimated in several different ways in addition 

 to the above, viz., the determination of the freezing-point of the solution, 

 determination of the boiling-point, determination of the electrical conduc- 

 tivity. The results obtained with the various methods agree very closely. 

 The following solutions have the same osmotic pressure: Sodium chloride, 

 i . 64 per cent. ; potassium nitrate, i . 09 per cent. ; sugar 5 . 5 per cent. 



Isotonic Solutions. Solutions that have the same osmotic pressure 

 are called isotonic. The term isotonic is a relative one, implying a compari- 

 son with some other solution taken as a standard. In physiology it has been 

 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 o . 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 (hypo- 

 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.) 



I 



Blood-plasma 



Ti 



Isotonic saline solution (o . 64 per cent.) 



I. 



Hypotonic saline solution (o-3 per 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 



