THE CARDIAC NERVES. 593 



designate as "tone." It is possible, of course, that certain 

 afferent paths may be in specially close functional relationship 

 to the center, and the fact that at each heart beat its own 

 sensory fibers are stimulated (p. 618, Fig. 280) would suggest 

 that these fibers may have this function. 



The Action of Drugs on the Inhibitory Apparatus. The 

 existence of the inhibitory fibers to the heart furnishes a means 

 of explaining the cardiac action of a number of drugs, atropin, 

 muscarin or pilocarpin, nicotin, curare, digitalis, etc., for the 

 details of which reference must be made to works on pharmacology.* 

 The action of the first three named illustrates especially well the 

 application that has been made of physiology in modern pharma- 

 cology. Atropin administered to those animals, such as the dog 

 or man, in which the inhibitory fibers of the vagus are in constant 

 activity, causes a quickening of the heart rate. Indeed, the heart 

 beats as rapidly as if both vagi were cut. After the use of atropin, 

 moreover, stimulation of the vagus nerve fails to produce inhibition. 

 The action of atropin is satisfactorily explained by assuming that 

 it paralyzes the endings of the (postganglionic) inhibitory fibers 

 in the heart muscle, just as curare paralyzes the terminations of 

 the motor fibers in skeletal muscle. Atropin exercises a similar 

 effect upon the nerve terminations in the intrinsic muscles of the 

 eyeball and in many of the glands. On the contrary, when mus- 

 carin or pilocarpin is administered it causes a slowing and finally 

 a cessation of the heart beat. Since this effect may be removed 

 by the subsequent use of atropin it is assumed that the two former 

 drugs excite or stimulate the endings of the inhibitory fibers in 

 the heart and thus bring the organ to rest in diastole, as happens 

 after electrical stimulation of the vagus nerve. Some authors, 

 however, believe that these drugs do not act upon the terminals 

 of the vagus fibers, but upon the muscular tissue itself or upon a 

 specialized " receptive substance " (Langley) contained in the 

 muscle. A final statement cannot be made upon this point, but 

 the current belief is that the atropin paralyzes while the muscarin 

 or pilocarpin stimulates the endings of the inhibitory fibers in the 

 substance of the heart. 



The Nature of Inhibition. Since the discovery of the inhibi- 

 tory nerves of the heart furnished the first conclusive proof of the 

 existence in the body of definite nerve fibers with apparently the 

 sole function of inhibition, it seems appropriate in this connection 

 to refer to the views regarding the nature of this process. Several 

 general views of the nature of inhibition have been proposed, but 

 the one that is most definite and has met with most favor is that 



* Consult Cushny, "Text-book of Pharmacology and Therapeutics," Phila- 

 delphia. 



