3 i8 TEXT-BOOK OF PHYSIOLOGY 



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. 139.) 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. 139. 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. 140. TRACING SHOWING THE DIMINUTION IN THE 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 inferred 

 that some fibers of the vagus at least diminish the tonicity of the heart- 

 muscle. 



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 the 

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

 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 duration of 

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

 and after a stimulation of thirty-eight seconds the heart of the toad remained 

 at rest for 292 seconds (Gaskell) ; the heart of a snake for from one-half to 



