THE NERVOUS REGULATION OF THE HEART 1095 

 latent period of the vagus in the mammal is considerably less than one 

 second, that of the accelerator nerves may amount to ten or even 

 twenty seconds (Fig. 450). Hence if the vago-sympathetic of the frog be 

 stimulated, the first effect is inhibition due to vagus action. The vagus 

 nerve-endings then become fatigued, and the influence of the accelera- 

 tor fibres makes itself apparent, and the heart commences to beat, and 

 the beats become more rapid and forcible than before (Figs. 451, 452). 



FIG. 451. Tracing to show effect of .stimulation of the vago-sympathetic nerve 

 on the frog's heart. The rhythm is unaltered, but the beats of auricle and 

 ventricle are much decreased in size. On ceasing the stimulation the beats 

 become augmented. (GASKELL.) 



FIG. 452. A tracing similar to Fig. 451. In this case, however, the stimulation 

 caused complete stoppage (inhibition) of both auricular and ventricular 

 beats. (GASKELL.) 



Like the vagus, the sympathetic nerve fibres appear to exercise a 

 tonic influence on the heart, so that after extirpation of the stellate 

 ganglion on each side, the pulse frequently becomes permanently 

 slowed. 



THE HEART REFLEXES 



The part of the nervous system chiefly concerned in the central 

 co-ordination of the various afferent impulses which act on the heart is 

 the medulla oblongata. It is in this situation that we find the nerve- 

 cells giving origin to the efferent fibres of the vagus nerves, and also 

 the collection of grey matter in which the afferent fibres of the vagus 

 terminate. Direct stimulation of the vagus centre may cause slowing 

 and stoppage of the heart. The tonic influence of the vagi can be 



