1885.] of the Heart in Fishes, especially that of the Eel. 125 



Ventricular tracing showing an inhibitory standstill — a result of vagus stimulation. 

 The beat marked + was elicited by direct stimulation of the ventricle during 

 the period of inhibition. 



(2.) Effects of Vagal Stimulation on the Auricle. 



The auricle, though possessing an inherent rhythmic power of its 

 own, has this independent rhythm masked and rendered latent (like 

 the lower rhythmic power of the ventricle) by the more rapid rhyth- 

 mical lead of the sinus. The auricle then is, in consequence of the 

 rapid action of the sinus, made to beat at a considerably quicker rate 

 than it would otherwise do in virtue of its own inherent rhythm. 



When the vagus is strongly stimulated, and the whole heart 

 brought to a standstill, the auricle becomes relaxed, and a marked 

 diminution of its ordinary tone is evident. It stands still not merely 

 in the way the ventricle does because of the arrest of the dominant 

 part which normally starts off its contraction, but because of a pro- 

 found influence directly exerted on its muscular tissue. Its inherent 

 rhythmic power is markedly depressed, for vagal stimulation can keep 

 the auricle in a motionless state much longer than would be required 

 for the exhibition of its independent rhythmic power. If the auricle 

 merely stopped in consequence of the stoppage of the sinus, and if 

 the inherent auricular rhythmic power was intact, an independent 

 action would very soon be exhibited by the auricular tissue. It is 

 obvious that this inherent automatic rhythmic property is held in 

 abeyance by vagal stimulation. 



Moreover, there occurs in the auricle a striking depression of the 

 excitability of the muscular tissue as tested by direct stimulation. 

 The auricular excitability is, during strong inhibition, annulled for 

 the time being. Such a loss of excitability does not depend on over- 

 distension of the auricle with blood during the cardiac standstill ; it 

 occurs in the bloodless auricle as well. As the inhibitory phase is 

 passing away direct stimulation of the auricle leads to localised con- 

 traction in the stimulated area. These contractions seem to be 

 localised in consequence of the great depression of excitability which 

 still pervades the auricular tissue, and prevents the contraction from 

 spreading over the whole auricle as it would do in the normal state. 

 Very weak stimulation of the vagus nerve causes a distinct depres- 



