ACTION OF THE INHIBITOR V NERVES. 



213 



when the nerve is stimulated. The curve so obtained is given in Fig. 118, 

 and shows clearly the fact of the positive electrical variation in consequence 

 of vagus stimulation, and also that such electrical change takes place so slowly 

 that its course can easily be registered by an observer who is noting the 

 position of the galvanometer needle only every rive seconds. Such a rate of 

 change is remarkably different from the rapid movement in the opposite direc- 

 tion, which occurs when a contraction takes place. This action of the vagus 

 is abolished by atropine, or by ligaturing the coronary nerve tightly, so as to 

 destroy its physiological continuity with the vagus in the neck. ■ 



Such, then, are the primary effects of the stimulation of the vagus 

 nerve : depression of rate, depression of excitability, depression of con- 



?x 



Z3- 



*4- 



Sir 



?; 



sa 



^ 



RSymp 



RVcuj 



Fig 118. — Curve of demarcation current caused by heating apex of auricle. Divisions of 

 ordinate represent divisions of the galvanometer scale in centimetres. Fine divisions 

 of abscissre represent intervals of five seconds. As is seen, the demarcation current 

 diminishes at first rapidly, then more slowly, during the course of the experiment. At 

 intervals, as represented by the arrows, the right vagus or right augmentor nerves 

 were stimulated. 



traction force, depression of conductivity, depression of tonicity, and 

 increased positivity of the muscular substance. 



AVe may therefore conclude that the nerve acts in the same way on 

 all the attributes of auricular muscle, that its terminal fibrils in the 

 muscle act in the same way as its main fibres in the neck, and that 

 atropine paralyses these terminal fibrils ; and that, therefore, as far at all 

 events as the auricle is concerned, any explanation of inhibition must 

 be consistent with a direct action of an inhibitory nerve on muscle. 



The evidence shows, further, that in all cold-blooded animals, without 

 exception, the vagus acts powerfully on the auricle. When, however, 

 we consider its action upon the ventricle, we come immediately face to 

 face with one of the most puzzling and most important facts in the 

 innervation of the heart of cold-blooded vertebrates, namely, that as far as 

 is at present known the vagus has no effect whatever upon the ventricle 

 of any cold-blooded vertebrate except the Amphibia. In my experiments 1 



1 Jour a. Physiol, Cambridge and London, 1S87, vol. viii. PI. XII. Figs. 1 and 2. a. 



