THE INHIBITORY NERVES 211 



both vagi produces slowing of the rhythm of the heart. It has since been 

 shown, in all of the higher vertebrate animals experimented with, that 

 this is the normal reaction to vagus stimulation. Moreover, a section 

 of one vagus, or at any rate of both vagi, produces acceleration of the 



FIG. 1 80. Tracing Showing Action of the Vagus on the Heart of the Terrapin. A ur, 

 Auricular; vent, ventricular tracing. The part between words "on" and "off" indicates 

 a period of vagus stimulation. The part of tracing to the left shows the regular contrac- 

 tions before stimulation. During stimulation, and for some time after, the beats of the 

 auricle and ventricle are arrested. After they commence again the auricle contracts 

 weakly at first, but soon acquires a much greater amplitude. The ventricular contrac- 

 tions that follow the first weak auricular contractions are maximal in the terrapin, but 

 not so in the frog. See next figure. Time in seconds. (Carr.) 



pulse by breaking the pathway from the vagus center to the heart; 

 stimulation of the distal or peripheral end of the divided nerve normally 

 produces slowing or stopping of the heart beats, showing that the fibers 

 are efferent and thus carry the nerve impulses toward the heart. 



FIG. 181 Tracing Showing Diminished Amplitude and Slowing of the Pulsations of 

 the Auricle and Ventricle of the frog without Complete Stoppage during Stimulation of 

 the Vagus. (Gaskell.) 



It appears that any kind of stimulus, either chemical, mechanical, elec- 

 trical, or thermal, produces the same effect, but that of these the most 

 potent is a rapidly interrupted induction current. A certain amount of 

 confusion has arisen as to the effects of vagus stimulation in consequence 

 of the fact that fibers of the sympathetic nerve run within the trunk of the 



