150 SCIENCE FROM AN EASY CHAIR 



direction is helped in some elongated hearts by the 

 contraction of the wall of the heart beginning behind 

 and spreading quickly forward like a wave. The heart 

 of the common earth-worm and of small transparent 

 worms with red blood like it, which are common in the 

 mud of ponds and rivers and can be easily watched with 

 the microscope so that one can see through their glass- 

 like skin what is going on inside them, shows very 

 beautifully this wave of contraction. The heart in these 

 worms is a long contractile vessel which runs the whole 

 length of the body along the back. You can watch the 

 red blood flowing into it through the veins in each ring 

 or segment of the worm's body slowly swelling it out 

 so that it looks like a long red cord. Then, sud- 

 denly, there is a movement like a flash in its rapidity, 

 passing from behind forwards ! The walls of the red 

 cord-like heart contract so as to drive the blood forward 

 into the arteries, which also are present in every ring of 

 the worm's body. At the same time you can see the 

 .valves, which hang at the entrance of the veins to the 

 heart, swing with a sudden " chuck " and close those 

 vessels against the driven blood. The red cord becomes 

 colourless progressively from behind forwards, owing to 

 the squeezing out of the blood, and by the time the 

 movement has reached the head of the worm, the hinder 

 part of the cord-like heart is beginning slowly to dilate 

 again with the influx of red blood from the veins. 



What causes the muscles of the heart to contract at 

 regular intervals ? There is no doubt that the " stimulus " 

 which excites the heart muscles to contraction is in these 

 simpler animals merely the tension or strain produced by 

 the presence of a sufficient quantity of blood which has 

 flowed into the heart from the veins. The heart muscle, 

 after its rapid contraction, rests ; it has no other rest, no 

 sleep, as have all the other parts of the body. It must 



