226 



THE CIRCULATION OF THE BLOOD 



auriculoventricular junction. The function of the ganglia is to serve as 

 cell stations on the course of the vagus nerves. (Fig. 68.) 



Nicotine is a drug which in certain concentrations, if applied locally 

 to sympathetic ganglia, specifically paralyzes the synapses between the 

 ends of the preganglionic fibers and the cells from which the post- 

 ganglionic fibers arise. If this drug is applied in a 1 per, cent solution 

 to the heart, stimulation of the vagus trunk no longer produces inhibi- 

 tion, but if the stimulus is applied to a portion of the heart known as 



Fig. 69. Frog heart tracing showing the action of nicotine. The vagus trunk was stimulated 

 as indicated. In the normal (lower) tracing inhibition occurs but after nicotine (second tracing) 

 no inhibition follows. Stimulation of the crescent in the next two lines still is followed by inhibi- 

 tion. The final effects of the drug are' shown in the last two (upper) tracings. (From Jackson.) 



the white crescentic line, inhibition still occurs, because at this point the 

 postganglionic nerve fibers come near to the surface and therefore are 

 stimulated (Fig. 69). On the other hand, atropine is a drug which 

 paralyzes the postganglionic fibers, so that after its application to the 

 heart inhibition can not be produced by stimulating either the vagus 

 trunk or the white crescentic line. Pilocarpine and muscarine are drugs 

 which have an action exactly opposite or antagonistic to that of atro- 



