THE INTERNAL MEDIUM. 41 



of the eye (see Chap. XXXI.), but these interior parts are 

 moistened with liquid of some kind, and unlike the epi- 

 dermis are protected from rapid evaporation. All these 

 bloodless parts together form a group of non-vascular tis- 

 sues; they alone excepted, wounding any part of the Body 

 will be followed by bleeding. 



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

 liquid representing their blood should be renewed very 

 rapidly in different 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 those with an 

 elevated temperature, such an arrangement, or rather ab- 

 sence of arrangement, as this would not suffice. In them 

 the constituent cells live very fast, making much waste and 

 using much food, and so alter the blood in their neigh- 

 borhood very rapidly. Besides, we have seen that in com- 

 plex 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 this is ac- 

 complished 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 hearty 

 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 



