42 THE HUMAN BODY. 



coats, and through the walls of these capillaries liquid 

 transudes from the blood and bathes the various tissues. 

 The transuded liquid is the lymph, and it is this which 

 forms the immediate nutrient plasma of the tissues except 

 the few which the blood moistens directly. 



Dialysis. When two liquids containing different mat- 

 ters in solution are separated from one another by a moist 

 animal membrane, an interchange of material will take 

 place under certain conditions. If A be a 

 vessel (Fig. 9) completely divided vertically 

 by such a membrane, and a solution of com- 

 mon salt in water be placed on the side #, 

 Suit | Sugal and a solution of sugar in water on the side- 

 ^ c, it will be found after a time that some 



FIG 9 A dia- sa ^ nas S^ i n ^ c au ^ some sugar into #, al- 

 tnou gh tnere are no visible pores in the parti- 



containing: two tion. Such an interchange is said to be due 



liquids, b and 



c, separated by to cUali/sis or osmosis, and if the process were 



a moist animal ,. . A ., 



membrane. allowed to go on for some hours the same 

 proportions of salt and sugar would be found in the solu- 

 tions on each side of the dividing membrane. 



The Renewal of the Lymph. Osmotic phenomena play a 

 great part in the nutritive processes of the Body. The 

 lymph present in any organ gives up things to the cells there 

 and gets things from them; and so, although it may have- 

 originally been tolerably like the liquid part of the blood, it 

 soon acquires a different chemical composition. Diffusion. 

 or dialysis then commences between the lymph outside and 

 the blood inside the capillaries, and the latter gives up to- 

 the lymph new materials in place of those which it has lost 

 and takes from it the waste products it has received from 

 the tissues. "When this blood, altered by exchanges with 

 the lymph, gets again to the neighborhood of the re- 

 ceptive cells, having lost some food materials it is poorer 

 in these than the richly supplied lymph around those cells, 

 and takes up a supply by dialysis from it. When it reaches 

 the excretory organs it has previously picked up a quantity 

 of waste matters and loses these by dialysis to the lymph 

 there present, which is specially poor in such matters^ 



