1888.] 



Inhibition of the Mammalian Heart. 



209 



Effects of Vagus Stimulation on the Auricles. 



1. The vagus appears as a rule to influence the auricles more 

 readily and more powerfully than the ventricles. 



2. Vagus stimulation leads to a slowing or an arrest of the rhythmic 

 beat, and a very marked weakening of the contraction force. 



The recommencing auricular beats that occur when the period of 

 inhibition is passing away are very weak ;; and any contractions 

 excited by direct stimulation (e.g., with induction shocks) during the 

 period of standstill are strikingly enfeebled. 



3. Vagus stimulation causes a pronounced depression of the excita- 

 bility of the auricular tissue to direct stimulation. 



During the period of inhibition resulting from vagus stimulation it 

 is much more difficult than usual to excite an auricular beat by direct 

 excitation ; a much stronger stimulus is necessary to elicit any con- 

 traction at all. 



4. The tone of the auricular muscle appears to be markedly dimi- 

 nished. 



5. These results occur when the vagus is stimulated, even when the 

 superior and inferior vense- cava©- have been clamped, so that the 

 cavities of the heart are no longer filled with blood. 



6. The vagus nerve seems to exert a powerful influence of a more 

 or less direct nature on the muscle itself, not merely by inhibiting or 

 weakening the motor impulses which are commonly assumed to pass 

 from nerve centres in the heart to the muscular fibres. For if it were 

 true that the vagus acted simply by depressing the motor centres of 

 the heart, it is very difficult to conceive how the responsiveness of the 

 auricular muscle to direct stimuli should be so greatly diminished, 

 and how the contraction force should be so strikingly reduced when 

 the auricular muscle is made to contract by induction shocks applied 

 to the auricular tissue. 



It would seem that whatever changes the vagus may induce in the 

 nerve-cells and ganglia occurring plentifully in the auricles, it can 

 also exert an important influence on the contractile tissue itself. 



7. Upon the whole, the influence of the vagus nerve upon the mam- 

 malian auricles presents a close parallelism to what holds good in the 

 auricles of many cold-blooded animals. 



Effects of Vagus Stimulation on the Ventricles. 



Besides causing slowness or standstill, the vagus can cause other 

 important changes in the ventricular part of the heart. 



1. The contraction force is markedly diminished. When a period of 

 standstill has ended, the recommencing beats are usually weak ; and 

 beats excited by direct stimulation (e.g., single induction shocks) 

 during the period of standstill are of diminished size. 



