42 THE HUMAN BODY. 



for example, there is no special arrangement to keep the 

 blood moving; it is just pushed about from part to part by 

 the general movements of the body of the animal. But in 

 higher animals, especially those with an elevated temperature, 

 such an arrangement, or rather absence of arrangement, as 

 this would not suffice. In them the constituent cells live 

 very fast, making much waste and using much food, and 

 altering the blood in their neighborhood very rapidly. Be- 

 sides, we have seen that in complex animals certain cells are 

 set apart to get food for the whole organism and certain 

 others to finally remove its wastes, and there must be a sure 

 and rapid interchange of material between the feeding and 

 excreting tissues and all the others. This can only be brought 

 about by a rapid movement of the blood in a definite course, 

 and that is accomplished by shutting it up in a closed set of 

 tubes, and placing somewhere a pump, which constantly takes 

 in blood from one end of the system of tubes and forces it 

 out again into the other. Sent by this pump, the hearf, 

 through all parts of the Body and back to the heart again, 

 the blood gets food from the receptive cells, takes it to the 

 working cells, carries off the waste of these latter to the ex- 

 creting cells; and so the round goes on. 



The L'ymph. The blood, however, lies everywhere in 

 closed tubes formed by the vascular system, and does not 

 come into direct contact with any cells of the Body except 

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

 blood-vessels. At one part of its course, how- 

 ever, the vessels through which it passes have 

 extremely thin 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 

 Fro. 12. Adia- the immediate nutrient plasma -of the tissues 

 fug" Apparatus" exce pt the few which the blood moistens di- 



containing two vppHv 

 liquids, 6 and c, r ' U J; 



separated by a Dialysis. When two liquids containing dif- 



moist animal . 



membrane. ferent matters 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. 12) completely divided vertically by such a mem- 

 brane, and a solution of common salt in water be placed on 

 the side Z>, and a solution of sugar in water on the side c, it 



