128 The Structure and Rhythm of the Heart in Fishes. [Jan. 29, 



Poisoning with curare (a minimal dose) prevents the occurrence of 

 local inhibition in the auricle as a result of the passage of a weak 

 interrupted current. And when an isolated piece of auricle exhibiting 

 an independent rhythm is subjected to the influence of a weak inter- 

 rupted current, its behaviour differs markedly from that of the 

 same piece of tissue whilst it formed part of an intact auricle, for in 

 the case of an automatically contracting isolated piece of auricle the 

 application of a weak interrupted current produces no inhibitory 

 effect. When the current is strengthened a depression of the contrac- 

 tion force is induced, but the rhythm remains unchecked, unless, 

 perhaps, by very strong currents. The rhythm may even be accelerated, 

 and when very strong currents cause an apparent standstill, it is 

 doubtful whether this standstill is due to a true inhibition of rhythm. 



(3.) Effects of Vagus Stimulation on the Sinus. 

 A. On the Interjugular Part. 



During the inhibitory standstill of the heart the interjugular part 

 is directly affected by nerve influence ; it does not merely stop action 

 because the leading parts have stopped. During vagus stimulation 

 there is a marked depression of the inherent rhythmic tendency. The 

 interjugular part can, by vagus stimulation, be kept quiescent for a 

 very much longer time than would be required for the manifestation 

 of the independent rhythm of the part ; there is evidently an inhibition 

 of the inherent rhythmic property. This depression of rhythmic power 

 seems to pass off from the interjugular part earlier than it does from 

 the neighbouring ostial parts. 



As a result of vagal stimulation the excitability of the interjugular 

 part (as well as its inherent rhythmic power) is depressed. During 

 moderate excitation of the vagus nerve the interjugular part responds 

 less readily to a direct stimulus, and during powerful inhibition the 

 interjugular part fails to contract at all on the application of a direct 

 excitation. Moreover, when recommencing action begins after a pro- 

 longed inhibitory standstill, the contraction is often seen to begin at 

 the dorsal aspect of the interjugular part, and to be for a time limited 

 to a small area of the tissue ; at each succeeding beat the contraction 

 extends, and soon involves the whole interjugular part. Such facts 

 indicate the occurrence during inhibition of a marked depression of 

 the excitability of the part. 



As a secondary effect of vagus stimulation, there is sometimes a 

 distinct increase in the rhythmic power, and probably also in the 

 excitability of the interjugular part. 



The passage of an interrupted current through the interjugular 

 part very readily brings about an arrest of the whole heart. A very 

 weak current is sufficient for this purpose ; frequently a current too 



