THE BLOOD. 73 



remainder. It is an interesting fact to note that while the 

 potassium salts are contained so largely in the corpuscles, 

 the sodium salts figure in a similar role in the liquid. A 

 yellow pigment of a nature not understood, and traces of a 

 substance to which the characteristic odor of blood is due, 

 finally complete the list of things which enter into the com- 

 position of blood. There are, of course, dissolved in the 

 blood certain gases, but the discussion of these is post- 

 poned to the chapter on respiration. 



In the discussion of blood, that of lymph is naturally 

 included, lymph being in fact nothing but the plasma of 

 the blood which has by osmotic processes soaked through 

 the walls of the capillaries and so bathed the tissues. To 

 use an arithmetical expression, lymph is blood minus the 

 red corpuscles. White corpuscles are present in lymph 

 owing to the fact that they are able to pass through the 

 capillary walls, and in general are able to wander among 

 the tissues. Occurring so plentifully in lymph, has given 

 to them the name by which they are frequently called, that 

 of lymph corpuscles. It is well to keep in mind that the 

 migration of these corpuscles through the walls of the capil- 

 laries, as it is normally done, does not necessarily injure 

 them. Probably they press their way through tiny open- 

 ings made between the cells which form the capillary wall. 

 In any case the opening made is so small that it is at once 

 closed up. 



THE PHENOMENON OF OSMOSIS. 



The phenomenon of osmosis referred to so frequently in 

 physiological discussion is the phenomenon of the diffusion 

 of liquids through a membrane in such a way that the com- 

 positions of the two liquids tend to become similar. Thus, 

 if a moist animal membrane, such as a stretched bladder, 

 be taken and placed so as to divide a vessel in two compart- 

 ments, and then water containing sugar be placed in one 

 compartment, and salty water in another, osmotic currents 

 are soon set up through the membrane, by means of which 



