THE CIRCULATION OF THE BLOOD 511 



the ventricle which opens into the arteries. 

 When the ventricle, having by its contraction 

 raised the pressure in the arteries, begins to 

 relax, the pressure within its cavity becomes less 

 than that in the arteries. The outflow valve 

 then shuts. Otherwise the arteries would be 

 placed iii direct communication with an area of 

 low pressure and the relaxation of the ventricle 

 would undo in part the work of the contraction, 

 the purpose of which was the creation of a pres- 

 sure in the arteries great enough to force the 

 blood through all the blood vessels. 



It is obvious from these general considerations 

 that the problems of the circulation are in the 

 first instance those presented by any system of 

 closed tubes through which liquid is driven by a 

 pump. 



The Circulation Scheme. 1 The artificial scheme 

 (Fig. 68) to illustrate the mechanics of the circulation 

 in the highest vertebrates consists of a pump, a sys- 

 tem of elastic tubes, and a peripheral resistance. The 

 inlet and the outlet tubes of the pump are furnished 

 with valves that permit a flow in one direction only. 

 The peripheral resistance is the friction which the 

 liquid undergoes in flowing through the minute chan- 

 nels of a piece of bamboo. To this must be added 



i Science, 1905, xxi, pp. 752-754. 



