144 



ADVANCED PHYSIOLOGY 



drain 



' Vaqus. 

 \Sifmpathetic 



CAUSE AND REGULATION OF HEART BEAT 



What makes the heart beat? No one as yet understands 

 life sufficiently to give a satisfactory answer to this question, 

 but we do know that most activities are brought about at the 

 command of the brain; most of the muscles will not contract 

 at all unless it orders them to do so. How is it with the heart? 

 Does it need orders from the brain or can it direct its' own 

 beating? That it can act independently of the brain has 

 been shown in the instances of many animals, where the 

 heart has been entirely removed from the body and has yet 

 continued to beat for a considerable length of time, even so 



long as two days. Evidently then, 

 the heart contains in itself some 

 agency that causes it to beat. 



The heart is thus automatic 

 and would of itself continue to 

 beat regularly through life; but 

 such regularity would be very 

 unsatisfactory, for the amount 

 of blood which the working or- 

 gans need varies at different 

 times. When the muscles are 

 active they need much blood, 

 when they are quiet they need 

 little. When one is asleep, the 

 organs need less blood than when 

 he is awake, and all through life 

 occasions are constantly occur- 

 ring where the body demands a 

 more or less rapid circulation 

 of blood than usual. To meet these varying demands, the 

 brain has the power of regulating the heart beat. 



Two sets of nerves pass to the heart; one set arises in 

 the medulla of the brain, the other in the sympathetic 



-Hearth 



FIG. 77. DIAGRAM 



Showing the nerves controlling heart 

 beat. 



