212 THE CIRCULATION OF THE BLOOD 



vagus nerves of some animals, for example, the frog. Speaking generally, 

 however, excitation of any part of the trunk of the vagus produces inhibi- 

 tion, the stimulus being particularly potent if applied to the points where 

 the nerves enter the substance of the heart at the situation of the sinus 

 ganglia. The stimulus may be applied to either vagus with like effect. 

 There are quantitative differences, however, between the right and left 

 vagi. The right vagus usually has the greater effect on rhythm. 



The effect of the stimulation of the vagus is threefold to slow the rate, 

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

 in the amplitude, and to delay conduction through the bundle system. 

 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 diminution 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 stimu- 

 lated. The heart of the terrapin can be completely inhibited for hours 

 with a strong stimulus. This phenomenon is shown in figure 180, which 

 illustrates the action of the vagus on the terrapin's heart. 



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

 When the beats reappear, the first few are usually feeble, after a time the 

 contractions become more and more strong, and may soon exceed both in 

 amplitude and frequency those which occurred before the application of 

 the stimulus. If the stimulation is prolonged, the inhibition escapes 

 to a slow rate, much under the normal rate. It is held there with some 

 variations until the stimulus ceases. This is due to the fact that, in the 

 dog at least, the stimulation reacts more strongly on rhythm production 

 at the sino-auricular nodal center, holding it in check with a strength that 

 does not inhibit the auriculo-ventricular nodal rhythm. The funda- 

 mental rhythm of the latter center is at a slower rate. The escape is to the 

 auriculo-ventricular nodal rhythm. 



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

 the vagus, and of the glosso-pharyngeal, located 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 more or less constant tonic activity, and the tonic influence is 

 eliminated when both nerves are cut, figure 182. 



