186 THE HUMAN BODY. 



body; a man of average size contains about twelve pounds 

 of bloodc 



The Lymph. The blood lies every where in closed tubes, 

 and consequently does not come into direct contact with 

 any of the cells which make up the body, except those which 

 float in it and those which line the interior of the blood- 

 vessels. At two parts of its course, however, the vessels 

 through which it passes have extremely thin walls, and 

 through the walls of these capillaries liquid transudes 

 and bathes the various tissues. The transuded 

 liquid is called lymph; thejblood makes lymph, 

 and the lymph directly nourishes all the tis- 

 sues except those mentioned above, with which 

 the blood itself comes in contact. 



Dialysis. When two specimens of water 



FIG.OO. A dia- 



containing different matters in solution are 

 separated from one another by a moist animal 

 membrane, an interchange of material will % m5st a animai 

 take place under certain conditions. If A be membrane - 

 a vessel (Fig. 56), completely divided vertically by such a 

 membrane, and a solution of common salt in water be 

 placed on the side b, and a solution of sugar in water on 

 the side <?, it will be found after a time that some salt has 

 got into c and some sugar into b, although there are no 

 visible pores in the partition. Such an interchange is said 

 to be due to dialysis or osmosis, and if the process were 



How much blood is there in an average sized man? 



Why does the blood not directly bathe most of the tissues? What 

 cells come in contact with it? What are the capillaries? What is 

 lymph? What is the nutritive function of lymph? 



What happens when watery solutions of different substances 

 are separated by a moist animal membrane? Illustrate. What is 

 such an interchange called? What would be the result at the end of 

 some hours? 



