194 



INHIBITION AND BLOOD-PRESSURE. [BOOK i. 



The Effects on the Circulation of Changes in the Heart's Beat. 



Any variation in the heart's beat directly affects the blood- 

 pressure unless some compensating influence be at work. The 

 most extreme case is that of complete inhibition. Thus if, while a 

 tracing of arterial pressure is being taken, the beat of the heart be 

 suddenly arrested, some such curve as that represented in Fig. 43 

 will be obtained. It will be observed that immediately after 

 the last beat, there is a sudden rapid fall of the blood pressure. 



Fio. 43. TBACINCJ, SHEWING THE INFLUENCE or CARDIAC INHIBITION ON BLOOD-PBES- 

 BUBE. FBOM A BABBIT. 



The current was thrown into the vagus at a and shut off at b. It will be 

 observed that one beat is recorded after the commencement of the stimulation. 

 Then follows a very rapid fall, continuing after the cessation of the stimulus. 

 With the returning beats, the mercury rises by leaps until the normal pressure 

 is regained. 



At the pulse due to the last systole, the arterial system is at its 

 maximum of distension; forthwith the elastic reaction of the 

 arterial walls propels the blood forward into the veins, and 

 there being no fresh fluid injected from the heart, the fall of 

 the mercury is unbroken, being rapid at first, but slower afterwards, 

 as the elastic force of the arterial walls is more and more used up. 

 With the returning beats, the pressure correspondingly rises in 

 successive leaps until the normal mean pressure is regained. The 

 size of these returning leaps of the mercury may seem dispro- 

 portionately large, but it must be remembered that by far the 

 greater part of the force of the first few strokes of the heart is 

 expended in distending the arterial system, a small portion only 

 of the blood which is ejected into the arteries passing on into the 

 veins. As the arterial pressure rises, more and more blood passes 

 at each beat through the capillaries, and the rise of the pressure at 

 each beat becomes less and less, until at last the whole contents 



