296 INHIBITION IN THE MAMMAL. [BOOK i. 



vertebrate animals including mammals, and indeed we meet 

 similar phenomena in the hearts of invertebrate animals. 



If in a mammal the heart be exposed to view by opening the 

 thorax, and the vagus nerve be stimulated in the neck, the heart 

 may be seen to stand still in diastole, with all the parts flaccid 

 and at rest. If the current employed be too weak, the result as 

 in the frog is not an actual arrest but a slowing or weakening of 

 the beats. If a light lever be placed on the heart a graphic record 

 of the ^standstill, or of the slowing, of the complete or incomplete 

 inhibition may be obtained. The result of stimulating the vagus 

 is also well shewn on the blood-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, some such curve as that represented in 

 Fig. 56 will be obtained. It will be observed that two beats follow 



FIG. 56. TRACING, SHEWING THE INFLUENCE OF CARDIAC INHIBITION ON BLOOD- 

 PRESSURE. FROM A BABBIT. 



x the marks on the signal line when the current is thrown into, and y shut off 

 from the_vagus. The time marker below marks seconds, the heart, as is frequently 

 the case in the rabbit, beating very rapidly. 



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 corre- 



