THE CARDIAC NERVES. 



513 



This inhibitory influence upon the heart beat was first described 

 in 1845 by the two brothers, Edward Weber and E. H. Weber. 

 It was a physiological discovery of the first importance, not only 

 as regards the physiology of the heart, but from the standpoint 

 of general physiology, since it gave the first clear instance of the 

 possibility of inhibitory action through nerve fibers. 



If the heart is examined during its complete inhibition it will 

 be seen that it stops in diastole, 

 and indeed the diastole is more 

 complete than normal, the heart 

 dilates to a very large extent, and 

 becomes swollen with blood. This 

 latter fact is taken usually as proof 

 that the action of the inhibitory 

 fibers not only prevents the usual 

 systole, but also removes the to- 

 nicity of the musculature. Some 

 observers believe that the unusual 

 dilatation is due simply to the effect 

 of the increased venous pressure 

 (Roy and Adami). Examination of 

 the heart shows also that the inhi- 

 bition affects the whole heart, both 

 auricles and ventricles are slowed or 

 stopped, as the case may be. That 

 the vagus nerve in man also con- 

 tains inhibitory fibers to the heart 

 is made highly probable by every- 

 thing known concerning the condi- 

 tions under which the heart is 

 slowed or stopped temporarily, and 

 has, moreover, been demonstrated 

 directly in several instances upon 

 living men.* These inhibitory fi- 

 bers have been shown to exist in all 

 classes of vertebrates and in a num- 

 ber of the invertebrates, a fact 

 which in itself would indicate the 

 great importance of their influence 



upon the effective activity of the heart. In the mammals gener- 

 ally employed in laboratory experiments the inhibitory fibers 

 occur in both vagi; in some of the lower vertebrates, however, 



Fig. 212. To show the inhibition 

 of the terrapin's heart due to stimula- 

 tion of the vagus nerve. The upper 



tracing (/) records the contractions of 



the left auricle ; the lower 



tractions of the ventricle, me vagus 



auricle ; the lower (II) the con- 

 The 

 was stimulated three times, each 

 chamber coming to a complete stop. 

 On removing the stimulus it will be 

 noted that the auricular contractions 

 increase gradually to their normal, 

 while the ventricular contractions 

 start off at full strength. 



* See especially Thanhoffer, " Centralblatt f. d. med. Wiss.," 1875, who 

 gives an account of an experiment in which the vagi were compressed in the 

 neck, with a resulting stoppage of the heart and loss of consciousness. 

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