STRUCTURE AND FUNCTIONS OF BLOOD AND LYMPH 293 



flu* epidermis are protected from rapid evaporation. All these 

 bloodless parts together form a group of non-vascular tissues; 

 they alone excepted, a wound of any part of the Body will cause 

 bleeding. 



In many of the lower animals there is no need that the liquid 

 representing their blood should be renewed very rapidly in dif- 

 ferent parts. Their cells live slowly, and so require but little food 

 and produce but little waste. In a sea-anemone, 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 

 warm-blooded ones, 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 medium in their neighborhood very rapidly. Besides, 

 we have seen that in complex animals certain cells are set apart 

 to get food for the whole organism and certain others to 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 sys- 

 tem of tubes and forces it out again into the other. Sent by this 

 pump, the heart, 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 

 excreting cells; and so the round goes on. 



The Lymph. 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, however, the vessels through which it passes 

 have extremely thin coats, and through the walls of these capil- 

 laries 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. 



The Renewal of the Lymph. Osmotic phenomena (p. 18) play 



