EXCITABILITY OF THE VAGUS. 



509 



This experiment shows that when both vagi are divided the 

 injection of a small dose of casca no longer slows the heart; 

 and therefore the slowing usually observed after its administra- 

 tion is due to stimulation of the vagus-roots and not to stimu- 

 lation of the inhibitory apparatus in the heart itself. 



It also shows that a large dose completely paralyses the ends 

 of the vagus in the heart, so that a strong interrupted current 

 applied to the trunk of the nerve produces no slowing of the 

 cardiac pulsations. 



Action of a Small Dose on the Excitability of the Vagus. 



As it has been stated that digitalis in small doses increases 

 the excitability of the ends of the vagus in the heart, so that a 

 slight irritation applied to the trunk of the nerve will cause 

 slowing or stoppage of the heart after the administration of the 

 drug, although previously it had no effect, it seemed advisable 

 to ascertain whether or not a similar action was possessed by 

 casca. The following experiment was therefore tried : — 



Minimum Irritation of Vagus {peripheral end). 

 Experiment XXXVI.— April 6. 



A cat, weight 6 lbs., was chloroformed, and kept under chloro- 

 form the whole time of the experiment. 



A cannula was inserted into the right carotid artery and 

 into the right femoral vein. 



Both vagi were then cut, and the peripheral end of the right 

 vagus attached to Von Basch's electrodes. 



Operation lasted half an hour; the cat at first very feeble, 

 afterwards recovered. 



