THE CARDIAC NERVES. 



573 



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. Exam- 

 ination of the heart shows also that 

 the inhibition affects the whole 

 heart, — both auricles and ventricles 

 are slowed or stopped, as the case 

 may be. That the vagus nerve in 

 man also contains inhibitory fibers 

 to the heart is made highly probable 

 by everything known concerning 

 the conditions under which the heart 

 is slowed or stopped temporarily, 

 and has, moreover, been demon- 

 strated directly in several instances 

 upon living men.* These inhibitory 

 fibers have been shown to exist in 

 all classes of vertebrates and in a 

 number of the invertebrates, — a 

 fact which in itself would indicate 

 the great importance of their in- 

 fluence upon the effective activity of 

 the heart. In the mammals gener- 

 ally employed in laboratory experi- 

 ments the inhibitory fibers occur in 



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

 the terrapin, the inhibitory fibers may be found exclusively or 

 mainly in the right vagus. 



Fig. 239. — 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 (//) the con- 

 tractions of the ventricle. The vagus 

 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 

 otart 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. 



