60 



HUMAN PHYSIOLOGY 



the microscope. These capillaries are the finest 

 bloodvessels of the body, and convey the blood to 

 the most minute cells and tissues. The blood itself 

 passes through the fine walls of the minute blood- 

 vessels, and thus comes directly in contact with the 

 parts it is intended to nourish (Fig. 16). This fact 

 has already been noted in the section dealing with 

 the work of " absorption." The circulation is, how- 

 ever, continued beyond these finest bloodvessels 

 onwards into another and different 

 set of vessels known by the name 

 of veins. Thus arteries end in 

 capillaries, and veins begin in 

 them. As the veins leave the 

 further parts of the body they 

 tend to become larger and larger, 

 receiving branches in their course, 

 and at last ending in two large 

 vessels which return blood from 

 the lower and upper parts of the 

 body respectively, these large 

 veins entering the right auricle 

 of the heart. Another large vein, 

 the portal vein, as we have seen, 

 returns blood from the digestive 

 organs and carries it to the liver 

 for purposes already mentioned. 

 We have thus three sets of blood- 

 vessels concerned in the distribu- 

 tion of blood through the body, namely arteries, 

 capillaries, and veins. The blood is naturally 

 pumped by the force of the left side of the heart 

 through the system of arteries and capillaries, the 

 flow of blood slowing down in the capillaries just as 

 the force of a river current is much lessened when 



Fig. 16. CAPIL- 

 LARIES AND BODY 

 CELLS 



The blood fluid 

 passes through the 

 capillary walls and 

 nourishes the cells. 



