THE INHIBITORY NERVES 



181 



since mechanical stimulation may bring out a beat during the pause caused 

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

 ing 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 first few are usually feeble, and may be auricular only; 

 after a time the contractions become more and more strong, and very soon 

 exceed both in amplitude and frequency those which occurred before the 

 application of the stimulus. This phenomenon is shown in figure 178, 

 which illustrates the action of the vagus on the frog's heart. 



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



FIG. 1 80. 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.) 



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 180. 



The center is also influenced by afferent impulses which may reach it 

 from the heart itself, by the depressor nerve, or from other parts of the body. 

 These reflex stimulations of the heart through the vagus center are constantly 

 occurring during our daily life and are the most potent factor in the coordi- 

 nations going on between the heart and the rest of the body. 



Rhythmical alterations of the heart rate occur in association with the 

 effects of the mechanical variations of pressure of the thorax on the heart 

 and blood-vessels. Apparently the cardio-inhibitory center is stimulated 



