222 HUMAN BIOLOGY 



preventing a back-flow; the pressure on the contents rises 

 until it opens the outlet valves, whereupon the blood is 

 driven forth through these valves into the outleading 

 vessels, from the right chamber into vessels distributing 

 to the lungs, from the left chamber into the great main 

 vascular trunk of the body, the aorta. The heart then 

 relaxes, and when the pressure within it becomes less than 

 that in the vessels, the outlet valves close. Thus the heart 

 is emptied, and made ready for being recharged. 



The vessels leading away from the heart are hke the 

 elaborate branchings of a thickly growing tree. The major 

 trunk is the aorta. Large minor trunks reach out to the arms 

 and legs, to the head and to the organs of the abdomen, e.g. 

 the stomach, the hver and the intestines. In each of these 

 regions the minor trunks ramify again and again into smaller 

 and smaller twigs until every part of the body is supphed. 

 The vessels leading away from the heart are the arteries, 

 and this intricate branching system is sometimes called 

 the "arterial tree." The arteries have relatively thick 

 elastic walls, and because provided with muscle their 

 capacity can be varied. When the heart discharges its 

 load into the arteries it starts a distending wave along their 

 contents, which can be felt in any superficial branch, e.g. 

 in the wrist at the base of the thumb, as a "pulse." 



We must remember always that the virtue of the cir- 

 culating blood is to serve the cells which are far removed 

 from the sources of supply and from the conveniencies for 

 voiding their rubbish. It is clear that this service must be 

 performed through the walls of the vessels. The arterial 

 walls are too thick to permit the passage of material to and 

 fro. The process of exchange occurs through the walls of 

 capillaries, extremely minute tubules with walls so exceed- 

 ingly tenuous that gases, such as oxygen and carbon dioxide, 

 and sugar and salts in solution, pass readily through them. 

 The capillaries, about 1/4000 of an inch in diameter, form a 

 rich and intricate network, intimately insinuated between 

 the layers and masses of the cells everywhere in the body. 

 Into this network the fine twigs of the arterial tree, the 

 arterioles, pour the blood; and from it the blood is gathered 

 into the fine twigs of another tree, the tree of veins. From 



