THE CIRCULATION OF THE BLOOD 359 



it will not depend directly upon the strokes of the pump, but 

 upon the head of water accumulated in B; which head of water 

 will, it is true, be slightly increased at each stroke of the pump, 

 but the increase will be very small compared with the whole driv- 

 ing force, and its influence will be inappreciable. We thus gain 

 the idea that an incomplete impediment to the flow from the 

 arteries to the veins (from B to A in the diagram), such as is 

 afforded by friction in the capillaries, may bring about conditions 

 which will lead to a steady flow along the latter vessels. 



But in the arterial system there can be no accumulation of 

 blood at a higher level than that in the veins, such as is supposed in 

 the above apparatus; and we must next consider if the "head of 

 water" can be replaced by some other form of driving force. It 

 is in fact replaced by the elasticity of the large arteries. Suppose 

 an elastic bag instead of the vessel B connected with the pump 

 "a." If there be no resistance to the back-flow the current 

 through 6 will be discontinuous. But if resistance be interposed, 

 then the elastic bag will become distended, since the pump sends 

 in a given time more liquid into it than it passes back through b. 

 But the more it becomes distended the more will the bag squeeze 

 the liquid inside and the faster will it send that back to A, until 

 at last its squeeze is so powerful that each minute or two or five 

 minutes it sends back into A as much as it receives. Thenceforth 

 the back-flow through b will be practically constant, being im- 

 mediately dependent upon the elastic reaction of the bag, and only 

 indirectly upon the action of the pump which keeps it distended. 

 Such a state of things represents very closely the phenomena oc- 

 curring in the blood-vessels. The highly elastic large arteries are 

 kept stretched with blood by the heart ; and the reaction of their 

 elastic walls, steadily squeezing on the blood in them, forces it con- 

 tinuously through the small arteries and capillaries. The steady 

 flow in the latter depends thus on two factors : first, the elasticity 

 of the large arteries; and secondly, the resistance to their empty- 

 ing, dependent upon internal friction in the small arteries and the 

 capillaries, which calls into play the elasticity of the large vessels. 

 Were the capillary resistance or the arterial elasticity absent the 

 blood-flow in the capillaries would be rhythmic. 



Weber's Schema. It is clear from the statements made in the 

 last paragraph that it is the pressure exerted by the elastic arteries 



