294 



TEXT-BOOK OF PHYSIOLOGY. 



FIG. 131. TRACING SHOWING DIMINISHED 

 AMPLITUDE AND SLOWING OF THE PUL- 

 SATIONS OF THE AURICLE AND VENTRI- 

 CLE WITHOUT COMPLETE STOPPAGE 

 DURING IRRITATION OF THE VAGUS. 

 (From Brunton, after Gaskell.') 



diminution in the rate of the beat (Fig. 129). Stimulation with 



strong tetanizing currents is followed by complete inhibition, and 



for a short time the heart re- 

 mains quiescent; but notwith- 

 standing the continued stimu- 

 lation, the heart commences 

 again to beat .(Fig. 130). 

 Though at first the beat is 

 feeble, it soon regains and far 

 exceeds its former rate and 

 force. This may be attributed 

 to a fatigue of the terminals 

 of the inhibitory fibers or to 

 an overpowering action of the 

 augmentor fibers arising after 

 a rather long latent period. 

 The foregoing facts are also 



illustrated in Figs. 131 and 132, as published by Gaskell. In these 



experiments the heart was suspended and clamped in the auriculo- 



ventricular groove, thus permitting both auricle and ventricle to be 



attached to recording levers. 

 In addition to the changes 



in the rate and force of the 



heart caused by stimulation 



of the inhibitor and the 



augmentor nerves, it is stated 



by Gaskell that there is also 



during inhibition a decrease 



in the conductivity of the 



heart at both the sinu-auric- 



ular and auriculo- ventricular 



junctions, and an increase 



in the conductivity during 



acceleration. The decrease 



in conductivity may be so 



pronounced that only every 



second or third contraction 



of the auricle will be followed 



by a contraction of the ventri- 



FIG. 132. TRACING SHOWING THE ACTIONS 



OF THE VAGUS ON THE HEART. Aur., 



Auricular; Vent., ventricular tracing. 

 The part between perpendicular lines 

 indicates period of vagus stimulation. 

 C.8 indicates that the secondary coil 

 was 8 cm. from the primary. The part 

 of tracing to the left shows the regular 

 contractions of moderate height before 

 stimulation. During stimulation, and 

 for some time after, the beats of auricle 

 and ventricle are arrested. After they 

 commence again they are single at first, 

 but soon acquire a much greater ampli- 

 tude than before the application of the 

 stimulus. (From Brunton, aiter Gas- 



cle. In other instances both 

 auricles and ventricles remain 

 at rest while the sinus main- 

 tains its usual rate. 



The increase in conductivity is shown by first artificially blocking 

 the contraction wave at the auriculo- ventricular junction with the 



