550 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



hand, if the current is very strong, the heart may be completely arrested 

 and yet start again during the passage of the current. With weak cur- 

 rents no actual arrest of the heart takes place, but the pauses between 

 the beats are prolonged during the earlier part of the application of the 

 current, and the pulse is thus rendered slow. If the pneumogastric be 

 stimulated in a dog during a blood-pressure experiment some such curve 

 as represented in the diagram will be produced (Fig. 232). 



The blood pressure is seen to undergo a rapid fall shortly after the 

 application of the current, from the fact that on the cessation of the 

 pulsation of the heart the arteries, through their contractility, empty 

 themselves into the venous system. As the heart again commences to 

 pulsate it throws its contents into the arterial system, which has been 

 already largely depleted of blood, and, as a consequence, the walls of the 

 arteries are rapidly stretched, and we have a correspondingly rapid 



Fig. 232.— Manometer Tracing from the Carotid op a Rabbit on Stim- 

 ulation of the Pneumogastric Nerve. (Foster.) 



The current entered the nerve at a and was shut off at b. 



increase in pressure, and therefore a rapid rise in the mercury in the 

 manometer. When the heart is only slowed by stimulation of the 

 pneumogastric, and not completely arrested, the mercury in the manom- 

 eter undergoes extensive oscillations, partly due to inertia, which 

 exaggerates these movements, and partly due to the same cause already 

 mentioned. In other words, between the pulsations of the heart, which 

 succeed each other slowly, the arterial system has time to partially empty 

 itself into the veins. 



Inhibition of the heart may not only be produced by direct stimu- 

 lation of the pneumogastric, but also may be produced reflexly. If one 

 pneumogastric nerve be divided, and the central end, in connection with 

 the brain, be stimulated with the induction current, the heart will he 

 arrested or slowed as before. If the abdomen of a frog be opened, and 

 the intestine struck sharply, as with the handle of a scalpel, the heart 



