CHAPTER XXXV 

 THE CIRCULATION OF THE BLOOD 



215. The vessels. In all backboned animals the blood is 

 entirely inclosed within a set of tubes, so that everything enters 

 or leaves the blood only by passing through the walls of these 

 tubes (see p. 180). 



The tubes or vessels reach all parts of the body, so that if we could 

 somehow remove all of a person except the blood vessels, we should 

 have a pretty good imitation of the whole body. 



There is a special muscular enlargement of the blood vessels 

 known as the heart. Through the contractions of this organ 

 the fluid is kept in constant and continuous motion not in 

 a back-and-forth motion, as the ancients believed. 



The vessels in which the blood flows from the heart are 

 called arteries ; those in which the blood returns to the heart 

 are called veins. 



The main arteries and veins run in parallel courses and 

 branch and subdivide until the smallest tubes are too small to 

 be seen without a microscope. The smallest branches of the 

 arteries connect with the smallest branches of the veins, so 

 that the blood that has been moving into smaller and smaller 

 vessels presently begins to flow into larger and larger ones. 

 These smallest tubes, connecting the arteries with the veins, 

 are called capillaries. 



The walls of the capillaries are so thin that diffusion is constantly 

 taking place through them ; and the white corpuscles work their way 

 through them, passing between the cells (Fig. 68). All the changes 

 in the composition of the blood take place while the blood is in capil- 

 laries in various parts of the body. 



185 



