The Body-Machine and Its Work 333 



The most learned physicians had the weirdest ideas about the 

 function of the heart and the flow of the blood. Nowadays the 

 essential facts are familiar. The heart, which one feels beating 

 about the lower part of the breast-bone (though drawn a little 

 to the left), is the central pumping-station. From it goes a 

 great tube, or artery, which branches out much as the trunk of 

 a tree divides into branches, and finally into twigs until its finest 

 ramifications have carried the blood into the remotest tissues of the 

 body, even into the teeth and bones. There the little twigs turn 

 back, as it were, and become veins, and the veins from all parts 

 join each other and at last bring the blood back to the heart. 



In a sense it is as if the fresh-water circulation and the 

 sewage circulation of a great city were managed from the same 

 pumping-station. One set of pipes would convey water to every 

 tap; another set would bring back the foul water to the pump. 

 The difference is that in the animal body the two sets of pipes 

 join on to each other and form a continuous system. But, obvi- 

 ously, fresh and foul blood must not mix, and this has been 

 secured by the evolution of a heart with the two halves com- 

 pletely separated from each other. We can trace the evolution 

 of the heart by studying it in various types of lower vertebrates. 

 In most reptiles the two halves are still imperfectly separated, 

 and "mixed" blood (pure and impure, fresh and foul) goes to 

 the greater part of the body. In the mammals and birds the 

 separation is complete. 



The heart is a thick muscular pouch with walls about half 

 an inch at their thickest part in man which has to drive the 

 blood to the tissues on the one hand, and to the lungs for purifi- 

 cation on the other. That is why it has separate halves. Each 

 half, moreover, has a little chamber for receiving the blood (an 

 auricle) and a larger chamber for pumping it (a ventricle), and 

 valves are cunningly contrived at each opening so that the blood 

 can flow only in one direction when the pump works. 



So remarkable is the mechanism of the heart, that we do 



