610 Professor C. S. Sherrington [April 19 



composing the whole complex living animal. The lives of these 

 myriad minute entities all depend on their supply of blood, and there- 

 fore the life of the whole creature depends on the contraction of the 

 heart. At each beat the heart by squeezing the blood out of its 

 arterial end maintains the flow of blood, and this flow resulting from 

 its own contraction refills it, because the blood returns to it by the 

 veins. 



This beating is all which the heart has to do. Whatever happens 

 it must continue to do this, or the creature perishes. Life-long, night 

 and day, winter and summer, it must do this. Whatever act the 

 creature may be accomphshing, sitting, walking, feeding, sleeping, 

 catching its prey, or escaping its enemies, this beating must go on, in 

 the frog about 10 times a minute, in ourselves about 70 times a 

 minute. The task is monotony itself. How admirably is the heart 

 muscle adapted to fulfil it ! 



Self -adjustment to meet the environmental conditions differentiates 

 animate from inanimate nature. As characteristic as this self- 

 adjustment itself is its constant trend toward what has sometimes 

 been termed " purpose." Animate objects are observed to adjust 

 themselves to their own advantage, that is, so as to prolong their 

 individual existence or that of their species. The more we know of 

 them the more complete appears to us this trend in their reactions. 

 The living organism advantageously adapts itself to its surroundings. 

 And every part of a living organism exhibits this power. The heart- 

 muscle reveals it clearly. It must not tire, and under normal circum- 

 stances the healthy heart, unUke other muscles, shows no fatigue. 

 Its beat must always be strong enough to press its contents over into 

 the artery against considerable resistance which opposes it. A heart- 

 beat which did not expel the blood would be useless, worse than 

 useless, wasteful, because it would be energy spent in vain. Its task 

 can be roughly likened to that of a man with a bucket who has to 

 keep lifting water from a tank at his feet to pour it over a wall of 

 certain height before him. If he lift the bucket much above the 

 wall he expends more energy than he need do ; if he lift it less than 

 the wall's height his work fails altogether. If he still, when the 

 bucket is emptied, keep it above the wall's height, his work stops 

 although his effort does not. 



The heart, whether its stimulus be weak or strong, beats always 

 with sufficient power : it thus avoids the useless labour of a beat 

 too weak to fulfil the ofiice of a beat. If the heart were to give too 

 prolonged contraction it would defeat its own purpose : after its 

 beat which empties it of blood, it must relax to refill for the next 

 beat : to keep contracted would be for its purpose as harmful as to 

 cease from beating ; it would stop the blood instead of pumping 

 it onward. In harmony with this, we find a prolonged stimulus to 

 the heart does not keep the heart contracted ; after the heart has 

 replied to the stimulus by a beat it exhibits a refractory phase, 



