296 EXPERIMENTAL INVESTIGATION OF THE ACTION OF MEDICINES. 



substantiate our conclusion by further experiment. We divide 

 the vagus and galvanise its peiiplieral extremity. If we do this 

 to a normal vagus, the heart will beat more slowly or stand still 

 altogether ; but if either the fibres or ends of tlie nerve have 

 been paralysed, no change will be produced in the heart's rhythm 

 by the application of galvanism to its trunk, and this we find to 

 be the case after the administration of atropia. But this ex- 

 periment does not enable us to decide which part of the nerve 

 is paralysed — the fibres or the ends, for in either case the effect 

 would be the same. We may do this, however, by obser^'ing 

 the effect which irritation of the vagus-trunk produces on such 

 of its fibres as do not go to the heart. If it were the fibres wdiich 

 were paralysed, we should expect that those which go to the 

 heart would not be the only ones affected, but that those going 

 to other parts would be paralysed likewise. 



I have hitherto spoken of the vagus as if it were a simple 

 nerve containing only inhibitory fibres for the heart, but it is 

 really a most complicated bundle, containing centripetal fibres 

 having probably no fewer than eleven different functions, and 

 centrifugal ones having nine or ten ; so it is little wonder that 

 it has long been a puzzle to physiologists, and even yet its 

 functions are not completely investigated. Among these fibres 

 are some which produce contraction of the o^.sophagus and 

 miiscles of the larynx ; and if we find that irritation of the 

 vagus continues to produce contractions in these parts after it 

 has ceased to render the heart's action slow, as is the case after 

 injection of atropia, we conclude that its fibres are not paralysed. 

 Dr. Eutherford has shown that the best mode of observing 

 the effects of irritation of the vagus on the muscles of the 

 larynx, is to open it in front and place the animal in such a 

 position that the light may be reflected from the inner surface 

 of the arytenoid cartilage, as the slightest movements can then 

 be readily detected. In this w^ay it is found that atropia pro- 

 duces complete paralysis of the cardiac branches of the vagus, 

 while the motor fibres supplying the muscles of the larynx 

 remain unaffected ; and we are therefore forced to conclude that 

 it acts not on the fibres but on the ends in the heart. A more 

 direct method is to apply the drug dissolved in an indifferent 



