268 THE HUMAN BODY. 



It is seen that the heart does not stop at once but gives a beat 

 or two and then stops as indicated by the sudden fall of arte- 

 rial pressure and the absence of all pulse-waves from the 

 tracing. ( If the stimulation be stopped at the instant indi- 

 cated by p, the heart does not begin immediately to beat, but 

 when it does, the beats are powerful and soon bring the arte- 

 rial pressure back to its former level, or in many cases to a 

 point above it for some time before the previous pressure and 

 pulse-rate are regained. Such a tracing shows among other 

 things that a certain "latent period" elapses before the 

 stimulation of the inhibitory fibres influences the heart- 

 beat^ and that the influence of the stimulus once established 

 continues a short time after the stimulation is stopped; and 

 that the first beats after cessation of the inhibition are slow 

 and powerful. Of course without any manometer one can 

 detect the effect of cardio-inhibitory stimulus by a finger 

 placed over the -pulse of an animal or by listening to the 

 heart-sounds, but the graphic method above described allows 

 of much more accurate study. 



It has been stated in a previous paragraph that stimulation 

 of the cardiac nerve usually stops or slows the heart-beat of a 

 frog. The reason for the qualifying term is that sometimes 

 the stimulation quickens the beat. This is due to the fact 

 that the nerve (see Fig. 99) is a mixed one and that the 

 fibres it receives from the sympathetic are directly antago- 

 nistic in action to those derived from the vagus. In most 

 cases when the whole trunk is stimulated the vagus fibres get 

 tlie upper hand, but to be sure of pure cardio-inhibitory results 

 the vagus must be stimulated before the sympathetic branch 

 joins it. Then the action is always inhibitory; and certain 

 other important phenomena may be observed, showing that 

 the vagus contains fibres which tend to throw the heart into a 

 better working state. When an exposed frog's heart is dying 

 and has ceased to beat, or when the ventricle has come to rest 

 though the sinus and auricles still work, it not unfrequently 

 happens that a period of vagus stimulation is followed by a 

 set of beats : or similarly that when the whole heart is beating 

 feebly stimulation of the vagus is after a time followed 

 by more forcible contractions. Hence it has been suggested 

 that the nerve contains fibres which tend to promote the 

 nutrition of the cardiac muscle, fibres which are anabolic 

 and favor constructive chemical processes. Whether these fibres 



