CAUSE OF STEADY CAPILLARY BLOOD-FLOW. 231 



the greater difference of pressure, in a given time, until at 

 last, when the water in B has reached a certain level, d' y 

 and that in A has correspondingly fallen to d" ', the current 

 through b will carry back in one minute just so much water 

 as the pump sends the other way, and this back- flow will 

 be nearly constant; it will not depend directly upon the 

 strokes of the pump but upon the head of water accumu- 

 lated 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 driving force; and 

 its influence will be inappreciable. We thus gain the idea 

 that an incomplete impediment to the flow from the ar- 

 teries to the veins (from Bio A in the diagram), such as is 

 afforded by internal friction in the capillaries, may bring 

 about conditions which will lead to a steady flow through 

 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 con- 

 sider if the "head of water" can be replaced by some other- 

 form of driving force. It is in fact replaced by the elas- 

 ticity of the large arteries. Suppose an elastic bag in- 

 stead of the vessel B connected with the pump, "a." If 

 there be no resistance to the back-flow the current through 

 1) 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 pow- 

 erful that in a minute or two or five minutes it sends 

 back into A as much as it receives. Thenceforth the 

 back-flow through I 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 occurring in the blood-vessels. 

 The highly elastic large arteries are kept stretched with^ 

 blood by the heart; and the reaction of their elasfic wallsT 



