238 THE HUMAN BODY. 



But the more the difference of level in the two vessels in- 

 creases, the greater is the force tending to drive water back 

 through I to A, and more will flow back, under the greater 

 difference of pressure, in a given time, until at last, when the 

 water in B has reached a certain level, d', 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 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 driving 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 dia- 

 gram), such as is afforded by internal friction in the capil- 

 laries, 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 sup- 

 posed 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 b will be discontinuous. 

 But if resistance be interposed, then the elastic bag will be- 

 come 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 min- 

 utes 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 occurring 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 continuously through the small arte- 

 ries and capillaries. The steady flow in the latter depends 



