CHAP, iv.] THE VASCULAR MECHANISM. 251 



pressure curve, the effect of complete cardiac inhibition on blood 

 pressure being most striking. If, while a tracing of arterial 

 pressure is being taken, the beat of the heart be suddenly 

 arrested by vagus stimulation, some such curve as that represented 

 in Fig. 70 will be obtained. It will be observed that two beats 

 follow the application of the current marked by the point a, 

 which corresponds to the signal x on the line below. Then for a 

 space of time no beats at all are seen, the next beat b taking 

 place almost immediately after the shutting off the current at y. 

 Immediately after the last beat following a, there is a sudden fall 

 of the blood pressure. At the pulse due to the last systole, the 

 arterial system is at its maximum of distention; 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 disproportionately large, but it must be re- 

 membered 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 of the ventricle pass at each 

 stroke into the veins, and the mean arterial pressure is established. 

 To this it may be added, that, as we have seen, the force of the 

 individual beats may be somewhat greater after than before inhi- 

 bition. Besides, when the mercury manometer is used, the inertia 

 of the mercury tends to magnify the effects of the initial beats. 



The above is an example of complete inhibition, of a total stand- 

 still for a while of the whole heart, such as may be obtained by 

 powerful stimulation of the vagus ; both auricles and ventricles 

 remain for a period free from all contractions ; and as the 

 previously existing arterial pressure drives the blood onward from 

 the arteries through the capillaries and veins towards the heart, 

 the cavities of the heart become distended with blood, especially 

 on the right side. 



A weaker stimulation of the vagus produces an incomplete 

 inhibition, the heart continues to beat but with a different 

 rhythm and stroke, and by careful observation many interesting 

 features may be observed. If a record be obtained, by one or 

 other of the methods mentioned in 113 or elsewhere, of the 

 behaviour of the auricles and ventricles respectively, it will be 

 observed that the inhibition tells much more on the auricles than 

 on the ventricles. The extent of the auricular contractions is 



