INHIBITION OF THE HEAET. 57 



Reflex Inhibition of the Heart. Like most of the direct operations of 

 nerves that can be imitated by electric stimulation, the inhibitory action of 

 the pneumogastrics can be produced by reflex action. The action of the heart 

 may be arrested in the frog by sharply tapping the exposed intestines (Goltz). 

 The same effect has been produced by stimulation of the splanchnic nerves or 

 the cervical sympathetic. In some animals, if one pneumogastric be divided 

 in the neck, the other being left intact, stimulation of the central end of the 

 divided nerve will produce inhibition of the heart, by an action induced in 

 the undivided nerve. In all of these instances, the inhibition is reflex. 

 The stimulation is carried by the afferent fibres of the nerves stimulated, to 

 the inhibitory centre in the medulla oblongata, and is reflected to the heart 

 through the efferent fibres of the pneumogastric. 



While moderate stimulation of ordinary sensory nerves is sometimes fol- 

 lowed by inhibition of the heart, very powerful stimulation arrests the cardio- 

 inhibitory action of the pneumogastrics, as well as certain other reflexes. 



The inhibitory fibres of the pneumogastrics undoubtedly have an impor- 

 tant office in connection with the regulation of the rapidity and force of the 

 cardiac pulsations. It is important, of course, that the heart should act at 

 all times with nearly the same force and frequency. It has been seen that 

 the inherent properties of its fibres and the action, probably, of the cardiac 

 ganglia are competent to make it contract, and the necessary intermittent 

 dilatation of its cavities makes these contractions assume a certain regularity ; 

 but the quantity and density of the blood are subject to very considerable 

 variations within the limits of health, which, without some regulating influ- 

 ence, would undoubtedly cause variations in the heart's action, so considerable 

 as to be injurious. This is shown by the palpitating and irregular action of 

 the heart when the pneumogastrics have been divided. These nerves convey 

 to the heart a constant influence, which may be compared to the insensible 

 tonicity imparted to voluntary muscles by the general motor system. When 

 a set of muscles on one side is paralyzed, as in facial palsy, their tonicity is 

 lost, they become flaccid, and the muscles on the other side, without any effort 

 of the will, distort the features. An exaggeration of this force may be imitated 

 by a feeble Faradic current, which renders the pulsations of the heart less 

 frequent and more powerful, or it may be still farther exaggerated by a 

 more powerful current, which arrests the action of the heart. Phemonena 

 are not wanting in the human subject to verify these views. Causes which 

 operate through the nervous system frequently produce palpitation and 

 irregular action of the heart. Cases are not uncommon in which palpitation 

 habitually occurs after a full meal. There are instances on record of death 

 from arrest of the heart's action as a consequence of fright, anger, grief or 

 other severe mental emotions. Syncope from these causes is by no means 

 uncommon. In the latter instance, when the heart resumes its contractions, 

 the nervous shock carried along the pneumogastrics is only sufficient to arrest 

 its action temporarily. When death takes place, the shock is so great that 

 the heart never recovers from its effects. 



