COMPLEX FUNCTIONS OF THE VAGUS. 207 



fluid, siicli as half per cent, solution of chloride of sodium, to 

 the vaous itself, and then to irritate the nerve about this point. 

 This may be done by dropping the solution on the nerve after 

 •placing a piece of gutta-percha tissue below it, so as to keep the 

 fluid from reaching the tissue below and being absorbed. If the 

 drug paralyse the fibres, the irritation wliich is applied to the 

 nerve above the part which is moistened by the solution will 

 not be conveyed by the paralysed part of the nerve, and will 

 consequently have no effect on the heart. 



Hoiu does a Drug render the Hearfs Action Slow t — We have 

 now gone over the experiments which are necessary to deter- 

 mine what the structure is through which a drug quickens the 

 heart's action, and we have now to consider those which we 

 require when investigating the action of a drug which renders 

 it slow. It may do so by irritating the vagus-roots, fibres, or 

 ends ; by increasing their excitabilit}^, so that they act more 

 strongly when stimulated ; or by paralysing the sympathetic, 

 the cardiac ganglia, or the muscular substance of the heart 

 itself. 



Does it act hy irritating the Vagus roots ? — In order to answer 

 this question, we divide both vagi and then inject the drug. 

 We thus separate the heart from the vagus-roots and deprive 

 them of any influence over it, so that, if they have been the 

 cause of slowness of the pulse in previous experiments, it will 

 not occur in this ; but if the slowness have been due to other 

 causes it will, with one or perhaps two exceptions, be noticed in 

 this experiment, just as it would had the vagi been intact. 

 These exceptions, which we will consider afterwards, are in- 

 creased excitability of the vagus-fibres and ends. The vagus- 

 roots can only act on the heart through the medium of the fibres 

 and ends ; if the drug itself should affect these structures, its 

 action on the heart may be much altered or even destroyed. If 

 the vagus-ends be paralysed, the roots can exert no more action 

 on the heart than they can after we have cut through the 

 trunks ; and if the excitability of the ends be increased, the 

 power of the roots over the heart will be greatly augmented, so 

 that the heart's action may be made slow without there being 

 any actual irritation either of the roots or ends. In order, then, 



