62 THE HEART 



the base of the ventricles. S is due to contraction at 

 the apex. Then follows a prolonged iso-electric period 

 which is succeeded by the slow base-negative wave T. 

 As to the nature of this last wave, there is much uncertainty. 

 It may be due to the ventricular contraction ending at the 

 base, at the opening of the aorta. 



THE WORK OF THE HEART 



The aorta and large arteries may be said to form a 

 reservoir at high pressure from which blood is suppUed 

 to the various tissues. The needs of the tissues for blood 

 are constantly fluctuating according to physiological 

 activity. We shall see how in the different tissues the 

 supply is made to meet the demands. It is only necessary 

 to say here that the fluctuation is greatest in the abdomen 

 and hmbs, least in the brain. It follows that if there were 

 no compensating mechanism, the arterial blood-pressure 

 would vary as the flood-gates into the tissues^ — for instance, 

 the muscles — were open or shut, and the brain would be 

 exposed indirectly to a diminution in its blood-supplv at 

 the very time when this organ would need blood most for 

 the increased cerebral activity which accompanies physical 

 exertion. But in the intact animal when the arterial 

 reservoir is being drained abnormally rapidly the pressure 

 within it, so far from falhng, actually rises. There exists, 

 therefore, a mechanism which seems to have for its object 

 the proper nourishment of the brain under all circumstances. 



In whatever this mechanism may be found to consist, 

 it must involve ultimately a variation in the output of the 

 heart, since it is only by an alteration in the amount of 

 blood which enters the arterial reservoir that the pressure 

 here can be maintained constant in the face of alterations 

 in the rate at which blood leaves the reservoir. 



The work done by the heart may therefore be said to 

 consist in the maintenance of a constant or nearly constant 

 arterial pressure. From a mechanical point of view, this 



