THE HEARTS BEAT 149 



shell-fish. Perhaps if oysters, too, had red blood, there 

 would be a prejudice against eating them in the uncooked 

 condition. 



The heart is essentially an enlargement of the great 

 stem or main blood-vessel which, like the trunk of a 

 tree, has branching roots at one end of it and ordinary 

 branches at the other. The trunk branches, and roots of 

 the "heart-tree" are, of course, hollow blood-holding 

 tubes, not solid fibrous structures, as are the woody 

 branches and trunk of a vegetable tree. Further, the 

 finest rootlets and the finest terminal branches in the 

 case of the heart-tree are connected to one another by 

 the network of very fine branches or by great blood- 

 holding cavities, which occupy all parts of the body of an 

 animal. The enlarged part of the trunk of the tree-like 

 system of blood-vessels the heart has powerful muscles 

 forming its walls, the fibres disposed so as to surround 

 the contained chamber. When these muscular fibres 

 contract, they squeeze the walls of the chamber together 

 and drive the blood out of it into the forward branches, 

 called " arteries." It is prevented from going backwards 

 into the hinder branches called " veins " (which we com- 

 pare to the roots of a tree) by flaps which are so set on 

 the inside of the great vessel at the entrance to those 

 branches that the flaps are made to move out across the 

 space by the backward current, and thus prevent any 

 backward flow, whilst a forward current merely presses 

 them flat against the wall of the vessel, and thus no 

 obstruction to a forward flow is presented. These 

 flaps are called the valves of the heart. The con- 

 sequence of this arrangement is that whilst blood 

 flows freely into the heart from the veins or hinder 

 (root-like) set of vessels, it is driven by the muscular 

 contraction of the heart only in one direction namely, 

 forwards into the arteries. This movement in one 



