262 



PHYSIOLOGY. 



cardiac trouble ensued. The permanent regulation of the work of the 

 heart is then conditioned on the central nervous system and the sym- 

 pathetic ganglia. If the vagus is divided and the peripheral end 

 irritated, as the Weber brothers did in 1845, then we have an arrest 

 of the heart in diastole. This inhibitory action of the vagus is seen in 

 all animals. Czermak, who had an exostosis on the cervical vertebra, 

 by pressing his finger on the right vagus, mechanically irritating it, 

 could at any time slow the heart and even bring it to a complete arrest 

 for a moment. The right vagus often has a more powerful inhibitory 

 effect than the left. In the slowing of the heart by irritation of the 

 vagus the diastole is lengthened, both auricles and ventricles are 

 stopped, and the heart dilates and is swollen with blood. This 



mwi 



Fig. 79. Tracing by Lever Attached to Frog's Heart on Stimulation 

 of the Pneumogastric Nerve. (FOSTER.) 



a-& shows time of stimulation by electricity. As the tracing shows, the 

 heart's movements were arrested for some time. 



inhibitory effect on the heart lasts some time after the irritation of 

 the vagus stops. During this inhibitory action by irritation of the 

 vagus the heart's excitability is diminished, its contents are not so 

 great and the blood-pressure falls. The slowing of the ventricle is 

 supposed to be due to an inhibition of the auricle, which slows or does 

 not permit the usual exciting impulses to travel from it to "shoot off" 

 the ventricle. But it has been shown by Tigerstedt that dividing the 

 connection between the auricle and the ventricle does not bring on a 

 pause, as it should if the ventricular contractions were due to impulses 

 coming from the auricle. It is probable then that inhibition is exerted 

 mainly on the auricles and in part upon the ventricles. Erlanger 

 with a clamp on the auriculo-ventricular bundle of His found that 

 stimulation of the vagus only affects the auricle in that case. 



It makes no difference whether one irritates the center of the 

 pneumogastrics, their trunk, or peripheral ends within the heart, the 



