THE HEARTS BEAT 151 



rest and take refreshment after each effort. Whilst it 

 rests the blood quietly flows in and dilates the heart's 

 cavity ; then the rested muscular wall of the heart, gently 

 stretched by the recovery after compression of its elastic 

 components, nourished and oxygenated by the blood, is 

 ready for another " stroke," and again it contracts tightly, 

 emptying its cavity of blood, which is driven into the 

 arteries. So it goes on effort and rest, effort and rest 

 alternating without cease. Whilst it is the stroke of the 

 heart which causes the blood to flow through the arteries 

 into the finest network of hair-like vessels, what is it that 

 causes the blood to flow on through the collecting veins, 

 to reach the heart, and actually to distend that collapsed 

 cavity after its stroke ? It must be remembered that a 

 very low pressure is enough to effect this. In the simplest 

 arrangements of worms and such-like animals, there is 

 probably some pressure transmitted to the blood in the 

 veins by the heart-stroke ; but the elasticity of the heart- 

 wall and its necessary tendency to resume its dilated con- 

 dition after its squeezing by its rings of muscle, is what 

 is chiefly effective in drawing on the blood in the veins 

 into the heart. 



In man and the higher animals the whole mechanism 

 of the heart is greatly complicated by the action of the 

 nervous system upon it and upon the contraction or ex- 

 pansion of the blood vessels. In this way the rate of the 

 beat of the heart is affected and brought into relation with 

 the needs of the blood circulation in remote parts of the 

 body. The beat of the heart in the human species is 

 more rapid in children than in adults, and more rapid in 

 women than in men, and it differs in all individuals under 

 differing conditions. Before birth it is 140 per minute, 

 in the first month after birth 130, and gradually diminishes 

 to 90 at nine years of age, and at twenty-one to 70 in 

 man and to 80 in woman. But these figures only repre- 



