ANATOMY OF THE HEART AND BLOOD-VESSELS 289 



lymph is poured in and mixed directly with the blood. Accord- 

 ingly everything which leaves the bloo$J must do so by oozing 

 through the walls of the blood-vessels, and everything which enters 

 it must do the same, except matters conveyed in by the lymph 



lu 



del 



mvs 



mvp 



Fio. 101. The arteries of the hand, showing the communications or anasto- 

 moses of different arteries and the fine terminal twigs given off from the larger 

 trunks; these twigs end in the capillaries which would only become visible if 

 magnified. R, the radial artery on which the pulse is usually felt at the wrist; 

 U, the ulnar artery. 



at the points above mentioned. This interchange through the 

 walls of the vessels takes place only in the capillaries, which form 

 a sort of irrigation system all through the Body. The heart, 

 arteries, and veins are all merely arrangements for keeping the 

 capillaries full and renewing the blood within them. It is in the 

 capillaries alone that the blood does its physiological work. 



The Position of the Heart. The heart (h, Fig. 1) lies in the 



