THE CARDIAC NERVES. 539 



That is, it is not due to automatic processes generated within the 

 nerve cells by their own metabolism or by changes in their liquid 

 environment, but to stimulations received through sensory nerves. 

 The continuous though varying inflow of impulses into the central 

 nervous system through different nerve paths keeps the center in 

 that state of permanent gentle activity which we designate as 

 "tone." It is possible, of bourse, that certain afferent paths may 

 be in specially close functional relationship to the center. One may 

 suppose, from the anatomical relations and from physiological 

 experiments, that the afferent paths from the abdominal viscera 

 play such a role. 



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 the pilocarpin or muscarin may have a deeper 

 effect in that it acts directly on the heart muscle itself, and that 

 the antagonistic atropin affects the muscular tissue also as well 

 as the endings of the fibers. A final statement can not 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 



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

 Philadelphia, 1903. 



