2O2 



THE CIRCULATION OF THE BLOOD 



The effect of the stimulus of the vagus is twofold to slow the rate, or 

 even to bring the heart to a complete standstill, and to produce a decrease 

 in the amplitude. The slowing does not take place until after the lapse of 

 a short latent period during which one or more contractions may occur. 

 The stoppage may be due either to prolongation of the diastole or to diminu- 

 tion of the systole. Vagus stimulation inhibits the spontaneous beats of 

 the heart only, it does not entirely suppress the irritability of the heart 

 muscle, since mechanical stimulation may bring out a beat during the pause 

 caused by vagus stimulation. The inhibition of the beats varies in duration 

 according to the strength of the stimulus and the animal stimulated. The 

 heart of the terrapin can be completely inhibited for hours with a strong 

 stimulus. The heart of a dog escapes from inhibition in a few seconds. 

 When the beats reappear, the few first are usually feeble, and may be auric- 

 ular only; after a time the contractions become more and more strong, and 



FIG. 18 1. Arterial Blood Pressure of the Dog, Showing the Effect on the Heart Rate 

 of Cutting both Vagus Nerves as marked. The scale to the left shows the pressure in 

 millimeters of mercury. Time in seconds. The momentary inhibition just before the 

 nerves were cut is probably due to mechanical stimulation of the nerves. (New figure by 

 Hill and Chilton.) 



very soon exceed both in amplitude and frequency those which occurred 

 before the application of the stimulus. This phenomenon is shown in 

 figure 179, which illustrates the action of the vagus on the terrapin's heart. 



The inhibitory fibers have their origin in nerve cells in the motor nucleus 

 of the vagus, and of the glosso-pharyngeal location in the floor of the fourth 

 ventricle. These cells have not been exactly identified, but the center is 

 called the cardio-inhibitory center. The center is a bilateral one and the 

 fibers from it pass into the great vagus trunk to be distributed to the heart 

 through superior and inferior cardiac branches which help to form the cardiac 

 plexus. Within the heart the inhibitory fibers form synapses with cells 

 whose axones reach the cardiac muscle cells. The cardiac-inhibitory center 

 is in constant tonic activity, and the tonic influence is eliminated when both 

 nerves are cut, figure 181. 



