276 MANUAL OF PHYSIOLOGY. 



EXTRINSIC CARDIAC NERVES. 



The intrinsic nerve mechanism of the heart just described is 

 under the immediate control of the great nervous centres through 

 the medium of fibres passing along the vagus and sympathetic 

 nerves from the medulla oblongata. 



Some of these fibres check the action of the intrinsic ganglia, 

 and cause the heart to beat more slowly ; hence they are called 

 inhibitory. Others cause it to beat more quickly, and are called 

 accelerator^/. 



INHIBITORY NERVES OF THE HEART. 



It was observed by Weber that electric stimulation of the vagus 

 nerve caused a slowing of the heart's rhythm, and if increased 



FIG. 123. 



Tracing showing the effect of Stimulation of Vagus Nerve. Stimulus 

 applied between vertical lines. (Kecording surface moved from left to 

 right.) 



gave rise to a standstill of the heart in diastole ; the heart beat 

 gradually recommencing some time after the stimulus had been 

 removed. On the other hand, the section of both vagi produced 

 an increase in the rapidity of the heart beat varying according 

 to the kind of animal experimented upon. 



Section of only one vagus, however, has not this effect. From 

 these experiments it would appear 1. That some fibres of the 



