CHAPTER XVII 

 THE ACTION OF THE HEART 



Having studied the methods for measurement and the main factors con- 

 cerned in the maintenance of the arterial blood pressure, we may now pro- 

 ceed to study in greater detail the two most important of these ; namely, the 

 action of the heart, and the peripheral resistance. 



The heart action has to be studied from two viewpoints, the physical 

 and the physiological. From the physical viewpoint we have to study 

 the heart as the pump of the circulation. We must see how it acts so as 

 to raise the pressure of the blood within it, and how the valves operate 

 so as to direct the bloodflow always in one direction. We must also ex- 

 plain the causes of certain secondary physical phenomena, such as the 

 heart sounds which accompany the heart action, and of certain secondary 

 changes in pressure produced in the other thoracic viscera by each heart- 

 beat. From the physiological viewpoint we must investigate the conditions 

 responsible for the constant rhythmic activity of the heart and the con- 

 trol to which this is subjected through the nervous system. 



THE PUMPING ACTION OF THE HEART 



When the heart is viewed in the opened thorax of an animal kept alive 

 by artificial respiration and lying in the prone position, it can be noted 

 that with each contraction the ventricles become smaller and harder, that 

 the apex tends to rise up a little, so that if the thorax were intact it 

 would press more firmly against the walls, and that it rotates slightly 

 from left to right, but does not move nearer the base of the heart. If 

 the auriculoventricular groove is carefully observed, it will often be 

 noted that it moves slightly toward the apex with each systole, whereas 

 the base of the heart itself, where it is attached to the large vessels, re- 

 mains fixed. The auricles can often be seen to contract and relax before 

 the ventricles. 



The most noteworthy results of this inspection are that during sys- 

 tole the apex of the heart does not move toward the base, but that 

 the auriculoventricular groove moves slightly toward the apex. That 

 these same movements occur in the intact animal can be shown by the 

 very simple experiment of pushing two long steel knitting needles 



145 



