THE CARDIAC NERVES. 



515 



one may, by using weak stimuli, obtain only a weakening of the 

 auricular beats without any interference with the rate (Fig. 214), 

 while by increasing the stimulus the slowing in rate becomes evident 

 combined with a diminution in force or extent. Although the 

 force of the beat may be influenced without altering the rate, the 

 reverse does not hold. Usually, for the auricle, at least, any stimulus 

 that slows the beat also weakens the individual beat. Whether 

 the vagus fibers exercise a similar double influence directly upon 

 the ventricle is not so clear. Some observers find that when the 

 ventricle is inhibited the beats, although slower, are stronger, while 

 others obtain an opposite result. It seems probable, as stated 

 by Johansson and Tigerstedt, that the result obtained depends 

 largely on the strength of stimulus 

 used. These observers found* that 

 with relatively weak stimuli the 

 contractions of the ventricle, though 

 slower, are stronger, while with 

 stronger stimuli the contractions are 

 diminished in strength as well as 

 rate. The question is complicated 

 by the difficulty of separating the 

 direct effect of the vagus on the 

 ventricle from the indirect effect 

 brought about by the changes in the 

 auricular beat. The inhibitory in- 

 fluence makes itself felt also upon 

 the conductivity of the heart. This 

 fact has been noted by several ob- 

 servers. A striking example is seen 

 in the case of partial heart block. 

 When as the result of some injury 

 or pressure in the auriculo-ventricu- 

 lar region or from some other less 

 evident cause there is a partial block, so that the ventricle con- 

 tracts once to two or three beats of the auricle, vagus stimulation 

 may be followed at once as an after-effect by a return to the 

 normal beat, a re-establishment of a one-to-one rhythm. Under 

 other circumstances the contrary effect of vagus stimulation 

 has been described. From the results cited it seems evident that 

 the vagus nerve may affect the rate and the force of the con- 

 tractions, and also the conductivity or the propagation of the 

 wave of contraction. These separate influences have been referred 

 by some authors to the existence of different kinds of nerve fibers, 

 each exerting its own influence, but it seems preferable to assume, 

 * See Tigerstedt, " Lehrbuch der Physiologic des Kreislaufes," 1893, p. 247. 



Fig. 214. To show the effect of 

 vagus stimulation on the force only of 

 the auricular beat in the terrapin's 

 heart: A, Record of the auricular 

 beats; V, record of the ventricular 

 beats. The vagus was stimulated be- 

 tween x and x. It will be noted that 

 the ventricular beats are not affected, 

 and that the auricular beats diminish 

 in extent without any change in rate. 



