SOME ECONOMICS OF NATURE. 673 



so treated, discharge themselves in turn onward, and at a rate which 

 corresponds to that with which the force-pump action of the heart 

 charges them from behind. And so, tracing the hydraulics of the cir- 

 culation through its phases we see, firstly, the heart over-distending the 

 elastic arteries. We witness the arteries emptying themselves into 

 their minute continuations, the capillaries, and through these latter into 

 the veins or return-vessels. The economy is witnessed here in the easy 

 means adapted for converting without complications a spasmodic flow 

 of blood into a continuous stream ; insuring also that the amount of 

 of blood which flows from the arteries to the veins during the heart's 

 stroke and pause exactly equals that which enters the circulation at 

 each contraction of the ventricle. In other words, the tremendously 

 high pressure of the arteries of our bodies saves at once the multipli- 

 cation of bodily pumping-engines and conserves the force of the heart 

 itself. 



There are other points connected with the circulation, more or less 

 intimately, to which a passing allusion may be made. The low-press- 

 ure flow of blood in the veins upward to the heart from the lower 

 parts of the body is thus favored by the high pressure of the arterial 

 system, and natural economy of energy is thus again exemplified. The 

 arteries seem to be intent on the work of getting rid of their contents 

 through the capillaries into the veins. There is no resistance, in fact, 

 to the venous flow which is carried on at low pressure. Again, the 

 ordinary muscular movements of the body are utilized in the economy 

 of life, to favor the return of the venous blood. For the veins are 

 compressed in the muscular movements, and, as they are provided with 

 valves which prevent back-flow, the compression can act in one way 

 only namely, to aid the upward or backward return of blood to the 

 heart's right side. 



The overplus of the blood is known as lymph, and is gathered from 

 the tissues by vessels known as absorbents or lymphatics. These 

 return the lymph to the blood-current for future use. Nature " gath- 

 ers up the fragments " here as elsewhere, and sees that the lymph or 

 excess of the blood-supply is once more garnered into the vital stream 

 of the circulation. If we ask how this lymph-flow is maintained from 

 all parts of the body toward the great vein in the neck where the 

 lymph joins the blood, we again light upon the question of high press- 

 ure in one side of matters and low pressure in the other side. All the 

 ordinary movements of our bodies are economically pressed by Nature 

 into the service of the lymph-flow. As in the veins, the valves of the 

 lymphatics prevent backward movement, and as in the veins the muscles 

 compress the vessels, and common movement thus assists a special end. 

 Even the motions of breathing favor the return of the lymph. For, 

 when we inspire, the pressure in the great veins becomes negative in 

 character, and lymph is thus capable of being sucked into the circula- 

 tion from the main tube or duct of the lymph-system. When we 

 vol. xxix. 43 



