SOME ECONOMICS OF NATURE. 27 



which arises from the heart, than an economical principle of an im- 

 portant kind comes into play. This principle is represented by the 

 elasticity of the arteries which bear the blood to the body. They 

 possess a circular coating of muscle which diminishes in thickness as 

 the vessels grow smaller and smaller, and are therefore removed from 

 the influence of the pumping-engine of the circulation. The arterial 

 coating is itself elastic, and the whole system of these vessels is thus 

 endowed with a high amount of resiliency. Their internal coats are 

 smooth and shining, as also is the lining of the heart's cavities ; 

 friction being thus reduced to its minimum. The united sectional 

 area of the branches of the dividing artery is larger than the same 

 area of its stem, so that the collective capacity of the vessels increases 

 markedly as we pass from the heart outwards to the minuter channels 

 of the circulation. 



The blood is thus driven through an elastic set of tubes presenting 

 the least possible resistance to the flow of fluid through them, and 

 economy of power is thus again witnessed in the details of the human 

 estate. Nor is this all. That there exists resistance to the flow of 

 blood is, of course, a necessary condition in any system wherein 

 large tubes or arteries branch out into small tubes (the capillaries), 

 and these, again, unite to form larger or return vessels the veins. 

 The problem of living Nature would here appear to resolve itself 

 into the inquiry, how the apparently intermittent, or spasmodic, 

 work of the heart may be converted into a constant and continuous 

 action. 



If we suppose that a pump drives water through a rigid pipe, we 

 see, in such a case, just as much fluid to issue from the pipe's end as 

 entered it at the stroke of the pump. Practically, also, the escape 

 of the water from the pipe takes place almost simultaneously with its 

 entrance therein. If we place some obstacle or resistance to the free 

 flow through the pipe, while the pump acts as before, the quantity of 

 water expelled will be less, because less fluid enters the pipe. Just 

 as much water will leave the tube as enters it under the two condi- 

 tions of no resistance and of the presence of such obstacle to the 

 flow. If now we substitute for our rigid pipe an elastic one, the 

 resistance to the water-flow is diminished no doubt, but the fluid 

 will, as before, issue in jets ; that is, in an intermittent and not 

 continuous fashion. There is " easy come and easy go " in the elastic 

 tube, as in the rigid one where no resistance exists. The elasticity, 

 in other words, is not called upon to act in modifying the flow 

 because the course of the fluid is clear and open. Suppose now, 

 that some obstacle or resistance is introduced into the elastic tube. 

 The fluid cannot escape as readily as before, and it tends, as a matter 

 of course, to accumulate on the near or pump side of the obstacle. 

 The tube gives, so to speak, and accommodates the water which is 



