FUNCTIONS OF THE VASCULAR SYSTEM. 193 



and more and more divided the further one follows them 

 from that point, until, in the various houses, they end in 

 numerous but very much smaller tubes. The course of the 

 blood differs, however, essentially from that of the water 

 supplied to a city, for the water does not return to the 

 reservoir, whereas the blood is carried back to the heart: 

 instead Of having a large supply of liquid stored up as in 

 a reservoir, there is at any one time only quite a small 

 amount in the heart, but this is steadily replaced by the in- 

 flow through the veins as fast as it is carried o2 by outflow 

 through the arteries. 



General Functions of the Different Parts of the Vascular 

 System. The blood vascular system is quite closed except at 

 two points, one on each side of the neck, where lymph vessels 

 pour the excess of lymph back into the veins. Valves at 

 these two points let lymph flow into the blood-vessels, but 

 will not allow blood to pass the other way. Accordingly 

 everything which leaves the blood 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 at the points above mentioned: This inter- 

 change through the walls of the vessels takes place only in 

 the capillaries. In India where rain only falls at certain 

 seasons it is stored up in huge tanks, and during the subse- 

 quent dry months distributed over the fields by a set of 

 small ditches and channels, cut through them in all direc- 

 tions, and from which the liquid soaks through the sur- 



How does the blood flow differ from that of water in the mains 

 of a city? How is the supply of blood in the heart kept up? 



Where is the blood vascular system not closed? Wliat occurs at its 

 openings? What is the function of the valves at these openings? How 

 must substances leave the blood ? How enter it? In what vessels does 

 interchange through the walls of the blood-vessels occur? Illustrate the 

 function of the capillaries by comparison with an irrigation system. 



