SOME ECONOMICS OF NATURE. 29 



exactly equals that which enters the circulation at each contrac- 

 tion of the ventricle. In other words, the tremendously high pres- 

 sure of the arteries of our bodies, saves at once the multiplication 

 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- 

 pressure flow of blood in the veins upwards to the heart from the 

 lower parts of the body is thus favoured by the high pressure of the 

 arterial system, and natural economy of energy is thus again exem- 

 plified. 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 

 utilised in the economy of life, to favour 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 "gathers 

 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 main- 

 tained from all parts of the body towards the great vein in the neck 

 where the lymph joins the blood, we again light upon the question 

 of high pressure 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 move- 

 ment thus assists a special end. Even the motions of breathing 

 favour 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 circulation from the main tube or 

 duct of the lymph-system. When we "breathe out" the pressure in 

 the large veins increases it is true, but a valve guards the entrance, 

 which in inspiration is free, and untoward consequences are thus 

 prevented. It is a notable fact that in many animals organs known 

 as lymph-hearts are developed. As in the frog, these contractile 

 organs assist the lymph in its return to the circulation. It therefore 

 becomes of interest to note how in the higher walks of existence, 

 the mechanical contrivances and actions of the body undergo an 

 evolution which not only avoids multiplication of parts and organs, 



