790 SUMMARY OF CURRENT RESEARCHES RELATING TO 



current is in all probability only caused by excitation of inhibitory 

 nerve-fibres scattered through the muscles of the heart. 



Where such nerves are absent, acceleration is the only result of 

 stimulation with the interrupted current. 



The only action of single induction shocks on the muscle is to 

 excite the contraction. When of sufficient strength they act thus 

 during both systole and diastole ; but when just below that strength 

 they are ineffective during systole. 



This is due to the excitability of the heart-muscle undergoing 

 periodic changes, being least during systole and greatest in complete 

 diastole. 



When a natural contraction of the heart has been artificially in- 

 creased or prolonged, the succeeding diastole is also prolonged ; there 

 is a compensatory rest. 



A rapid succession of strong induction shocks may cause a tetanic 

 contraction of the heart. 



The strong constant current may call forth a rhythm during its 

 passage through a resting ventricle, the beats proceeding from the 

 cathode. 



When the current is weak, a making beat occurs at the cathode 

 and a breaking beat at the anode. 



Inhibition of the beating ventricle may be caused by the constant 

 current in two ways : — 



(1) By depression of muscular activity at the anode. 



This is most effective when the anode is on that part of the 

 ventricle whence contractions normally proceed, viz. the auricular 

 end. 



(2) By stimulation of an inhibitory nerve at the cathode. 



In the case of the snail, the current has been found to act also in 

 this way when applied high up on the trunk of the visceral nerve. 



Atropin and muscarin have no visible effect on the inhibitory 

 nerves of Mollusca, and appear to be exclusively muscle-poisons. 



Curari destroys the power of the nerves, but in large doses seems 

 to have a further exciting effect on the muscle. 



The mode of action of the inhibitory nerve of Octopus and Helix, 

 if the view here formed of it at all approaches the truth, is in the 

 present state of our knowledge unique in the animal kingdom. 



Not only in Mammalia and Eeptilia, but even in Amphibia, the 

 heart has been shown to possess two sets of nerve-fibres — accelerators 

 and inhibitors. Whether this differentiation is primitive, or whether 

 it was preceded by a state in which all the fibres were alike in func- 

 tion, we do not know. Nor can we solve this question, nor the 

 further one as to how fibres primitively alike may have become thus 

 differentiated, until we find not only animals possessing the simpler 

 arrangement but also those within the borderland between the two. 

 The apparent uniformity of function in Cephalopoda and Pulmonata 

 undoubtedly points to the single nerve-supply as being, more an- 

 cestral ; but the question is probably not one which can be settled by 

 considering the heart alone. 



A complete solution will entail a comparative study through the 



