SOME ECONOMICS OF NATURE. 671 



ing of the hand. While the " flexors " of the fingers placed on the 

 front palm or surface of the limb close the hand, it is the " extensors " 

 of the opposite aspect of the forearm (whose sinews we see in the 

 back of the hand), which open or extend our digits. There may be 

 multiplication of organs here, it is true ; but, given the original power 

 to produce them, there is a clear economy of vital wear and tear exer- 

 cised in the avoidance of too onerous tasks being laid upon any one 

 muscle. 



It is something of this principle which we find reflected also in the 

 circulation of the blood. Here we see the heart's left ventricle (or 

 larger cavity of the left side) driving blood, as does a force-pump, out 

 into the great system of arteries, which everywhere throughout the 

 body carry the nutrient stream. No sooner, however, has the blood- 

 stream, impelled by the contraction of the muscular walls of the 

 heart's ventricle, passed into the great main artery (the aorta) which 

 arises from the heart, than an economical principle of an important 

 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 influ- 

 ence 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 outward 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 



