308 TEXT-BOOK OF PHYSIOLOGY. 



In addition to the foregoing changes in the heart-beat there is an altera- 

 tion in the sequence of the beat. The natural delay in the conduction of the 

 excitation process from the auricles to the ventricle is increased, in conse- 

 quence of which the auricle completely relaxes before the ventricular con- 

 traction begins. Moreover, the auricular contraction again occurs before 

 the ventricle has completely relaxed. After the effect of the stimulation 

 passes away, the acceleration diminishes, the augmentation declines and a 

 reverse change in the sequence occurs. The lower tracing shows a similar 

 series of effects. If the stimulus be applied to the pre-ganglionic sympathetic 

 nerves, an acceleration or augmentation of the heart follows, similar in all 

 respects to that which follows stimulation of the post-ganglionic or sympa- 

 thetic fibers proper; and the inference may be drawn that if the stimulus 

 could be applied directly to the nerve-cells in the medulla oblongata from 

 which the fibers take their origin, the same acceleration or augmentation 

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

 cardio-accelerator or augmentor center. Since stimulation of the nerve in 

 any part of its course, which in all probability exaggerates its normal function, 

 is followed by an acceleration or an augmentation, the sympathetic is said 

 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 alt 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, 

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

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



