VAGUS-FIBRES — VAGUS-ENDS. 30} 



rendered slow, taking care to keep up artificial respiration alT 

 the time. If the drug cause convulsive movements whicli 

 interfere with the proper performance of artificial respiration,, 

 curare should be given so as to prevent their occurrence, and 

 the experiment sliould be again repeated. 



Are the Vagus-fibres Irritated? — To ascertain this we apply 

 the drug dissolved in an indifferent fluid, such as a solution of 

 half per cent, of chloride of sodium, or serum, to the nerve„ 

 and notice whether any change occur in the heart-beats. Care 

 must be taken that the solution of the drug be not applied in 

 too concentrated a solution, as false conclusions might thus be 

 arrived at, for it might then have an irritant action, which it 

 could not have if it reached the part through the circulation 

 and became diluted by the blood before reaching the nerve. 



Is their Conducting Power Increased, so that the Boots can act 

 through them on the Heart more readily and ijowerfidly ? — If 

 their conducting power be increased, other stimuli as well as- 

 those from the roots will act more powerfully on the heart. We 

 therefore divide the vagi and apply a stimulus to the peripheral 

 end of one or both by irritating them with one of Du Bois- 

 Eeymond's induction-coils, and note at what distance from the 

 primary coil the secondary one must stand in order to produce 

 stoppage or slowness of the heart ; we then apply the drug to- 

 the nerve below it and again irritate. If the excitability of the 

 nerve be increased, stoppage or slowness should be produced 

 when the distance between the primary and secondary coils is- 

 greater — that is, when the current is weaker than before. It is- 

 generally assumed that the fibres are not likely to be affected, 

 and these experiments are rarely performed. 



Are the Vagus-ends Excited ? — We may test this in the same 

 way as the action on the roots, by injecting the drug at one time 

 into the jugular and at another into the carotid. If it increase 

 the excitability of the ends without affecting the roots, we should 

 find it produce, when injected into the jugular vein, an imme- 

 diate slowing of the pulse, which does not become greater in a 

 quarter of a minute afterwards, when the drug has reached the 

 roots. When injected into the carotid, no slowness should occur 

 till sufficient time has elapsed for it to pass round to the heart. 



