148 



ADVANCED PHYSIOLOGY 



The chief arteries and the organs to which they go are diagram- 



matically shown in Figure 78. 



The Capillaries. 1 Each artery divides and sub-divides into 



smaller and smaller branches, and the smallest twigs are 

 distributed to the tissues in every part 

 of the body. If we follow a single one 

 of these branches, we find that each 

 ultimate twig finally breaks up into a 

 profusion of extremely minute vessels, 

 the capillaries, too small to be seen with 

 the naked eye; Fig. 79. They branch 

 abundantly and unite in the form of a 

 network, so that the blood which flows 

 into them has no definite course but 

 may go through the network in an 

 direction. These capillaries (Fig. 80. 

 are of great importance, for it is throug 

 them that the blood gives up its nu 

 triment to the tissues and takes i 

 turn the waste materials which ma 

 have collected in them. 



The Veins. After passing throug 

 the capillaries the blood collects in ve 

 sels, called veins. The smallest of these 

 join others from other sets of capill 



FIG. 80. SHOWING THE 

 DISTRIBUTION OP CAP- 

 ILLARIES IN MUSCLES 

 The black irregular lines 

 are the capillaries. Their 

 minuteness and abundance 

 may be inferred from the 

 fact that the muscle fibres 

 themselves are only about 

 one five-hundredth of an 

 inch in diameter. 



a- 

 ries until they soon become vessels of 



good size. Every artery ends in a set 

 of capillaries, and each set of capillaries 

 empties into minute veins which unite 

 with others to form main trunks, carry- 

 ing the blood back toward the heart. 

 The blood from the head returns in two large veins on 

 each side of the neck, known as the jugular veins (Fig. 

 81), and these join the large veins coming from each 

 ^ee Demonstration, Appendix, Section 14. 



