THE CIRCULATION OF THE BLOOD 



317 



to have an accelerator or an augmentor influence on the heart-beat; with 

 the cessation of the stimulation, and very frequently before, the heart returns 

 to its normal condition. 



The Physiologic Action of the Vagus Nerve in the Frog. Stimulation 

 of the intra-cranial roots of the vagus with very weak induced electric cur- 

 rents is followed by a gradual diminution in the rate and a diminution in the 

 force of the heart-beat. If the induced currents are moderate in strength, 

 the heart will at once come to a standstill in diastole. (Fig. 142.) If the 

 stimulus be applied to the trunk or the peripheral portion of the vagus, for 

 example close to the sinu-auricular junction, an inhibition occurs similar in 



FIG. 142. TRACING SHOWING THE EFFECT ON THE HEART-BEAT OF THE TOAD OF LONG 

 STIMULATION OF THE INTRA-CRANIAL ROOTS OF THE VAGUS WITH MODERATELY STRONG ELECTRIC 

 CURRENTS. (Gaskell.) 



all respects to that which follows stimulation of the intra-cranial roots, and 

 judging from what is known regarding the action of nerve-cells, the inference 

 may be drawn that if the stimulus could be applied directly to the group of 

 nerve-cells from which the efferent fibers arise, the same inhibition would 

 follow; for this reason this collection of nerve-cells is known as the cardio- 

 inhibitor center. Since stimulation of the nerve, either at its center, in its 

 course, or at its periphery, which in all probability exaggerates its normal 

 function, is followed by a period of rest or inactivity, the vagus is said to 

 have a retarding or an inhibitor influence on the beat of the heart. 



During the continuance of the inhibition, the heart-muscle is relaxed, 



FIG. I43--TRACING SHOWING THE DlM IN UTION IN E RATE OF THE HEART-BEAT 



FOLLOWING WEAK TETANIZATION OF THE VAGUS TRUNK. 



its cavities dilated and filled with blood. The dilatation usually exceeds 

 that observed prior to the vagus stimulation, from which it is mferre 

 that some fibers of the vagus at least diminish the tomcity of the hear! 



U After cessation of the stimulation, the heart resumes its activity. At 

 first the beat usually is slow and feeble, but with each succeeding beat both t 

 rate and force increase, until they attain or exceed that observed prior 

 stimulation. In some cases, however, the heart begins to beat with as much 

 and even more vigor than it did prior to the stimulation^ The durati< 

 the inhibitor effect varies with the duration of the stimulation. Thus during 



