THE CIRCULATION OF THE BLOOD. 



131 



rhythm occurs without nervous stimulation, nor indeed is this at all 

 likely. 



Inhibition of the Heart's Action. Although, under ordinary con- 

 ditions, the apparatus of ganglia and nerve-fibres in the substance of the 

 heart forms the medium through which its action is excited and rhythmi- 

 cally maintained, yet they, and through them, the heart's contractions, 

 are regulated by nerves which pass to them from the higher nerve-cen- 

 tres. These nerves are branches from the pneumogastric or vagus and 

 the sympathetic. 



The influence of the vagi nerves over the heart beat may be shown 

 by stimulating one (especially the right), or both of the nerves, when a 

 record is being taken of the beats of the frog's heart. If a single induc- 

 tion shock be sent into the nerve, the heart, as a rule after a short inter- 

 val, ceases beating, but after the suppression of several beats resumes its 

 action. As already mentioned, 

 the effect of the stimulus is not 

 immediately seen, and one 

 beat may occur before the 

 heart stops after the applica- 

 tion of the electric current. 

 The stoppage of the heart may 

 occur apparently in one of two 

 ways, either by diminishing 

 the strength of the, systole or by 

 increasing the length of the 

 diastole (Figs. 121, 122) 



The stoppage of the heart 

 may be brought about by the 

 application of the electrodes to 

 any part of the vagus, but 



FIG. 121. Tracing showing the actions of the va- 

 gus on the heart. Aur., auricular; Vent., ventricu- 

 lar tracing. The part between perpendicular lines 

 indicates period of vagus stimulation. C.8 indicates 

 that the secondary coil was 8 c.ra. from the primary. 

 The part of tracing to the left shows the regular con- 

 tractions of moderate height before stimulation. Dur- 

 ing stimulation and for some time after the beats of 

 auricle and ventricle are arrested. After they com- 

 mence again they are single at first, but soon acquire 

 a much greater amplitude than before the application 

 of the stimulus. (.From Brunton, after Gaskell.) 



most " effectually if lihey are 

 applied near the position of Kemak's ganglia. It is supposed that the 

 fibres of the vagi, therefore, terminate there in the ganglia in the heart - 

 walls, and that the inhibition of the heart's beats by means of the vagus 

 is not a direct action, but that it is brought about indirectly by stimu- 

 lating these centres in the heart itself. If this idea be correct, it may 

 be supposed that the inhibitory centres are paralyzed by injection of 

 atropine, as after this has been done no amount of stimulation of the 

 vagus, or of the heart itself, will produce any effect upon the cardiac 

 beats. Also that urari in large doses paralyzes the vagus fibres, but as 

 the inhibitory action can be produced by direct stimulation of the heart, 

 it is inferred that this drug does not paralyze the ganglia themselves. 

 Muscarin and pilocarpine appear to produce effects similar to those ob- 

 tained by stimulating the vagus fibres. They stimulate the inhibitory 

 ganglia. " 



The remarkable effects of ligaturing the heart at various parts (Stan- 



