48 CIRCULATION OF THE BLOOD 



removes the organ from the characteristic and peculiar effects of the 

 poison. 



Feeble stimulation of one or both pneumogastrics, when it produces 

 any effect, almost always slows the action of the heart. In some 

 animals, however, the pneumogastrics contain a few accelerator fibres, 

 and feeble excitation sometimes is followed by a slight increase in the 

 rapidity of the cardiac pulsations, but this is unusual. 



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. The same effect has been produced by stimulation 

 of the splanchnic nerves or the cervical sympathetic. In some animals, 

 if one pneumogastric is divided in the neck, the other being intact, 

 stimulation of the central end of the divided nerve will produce inhibi- 

 tion of the heart by an action induced in the undivided nerve. In all 

 these instances the inhibition is reflex. The stimulation is carried by 

 the afferent fibres of the nerves stimulated to the inhibitory centre in 

 the bulb and is reflected to the heart through the efferent fibres of the 

 pneumogastric. 



While moderate stimulation of ordinary sensory nerves is sometimes 

 followed by inhibition of the heart, powerful stimulation may arrest the 

 cardio-inhibitory action of the pneumogastrics as well as certain other 

 reflexes. 



The inhibitory fibres of the pneumogastrics undoubtedly have an 

 important 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 the con- 

 tractions assume a certain regularity ; but the quantity and density of 

 the blood are subject to considerable variations within the limits of 

 health, which, without some regulating influence, would cause varia- 

 tions in the heart's action so considerable as to be injurious. This 

 is shown by the 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 



