318 PHYSIOLOGY CHAP. 



He found that the apex-beats called out at regular intervals 

 by induction shocks did not increase in intensity with increased 

 strength of stimulus, as is the case with normal muscle. When 

 an induction current, no matter of what intensity, is strong enough 

 to provoke a contraction, this is invariably maximal, i.e. as strong 

 as can be obtained from the heart at the given moment (" all or 

 nothing," Law of Bowditch). This fact, which was fully confirmed 

 by the observations of Luciani, Kronecker, and Stirling, proves that 

 the contraction of cardiac muscle depends essentially upon its 

 inner conditions, and to a much less degree upon the external 

 stimulus, as if the effect of the latter was limited to enabling the 

 muscle to serve up a spontaneous contraction, of which it would 

 not have been capable without such a stimulus. 



This view is justified by other phenomena elucidated by 

 Bowditch. When cardiac muscle is stimulated with weak in- 

 duction shocks, the reaction sometimes occurs and sometimes fails 

 (stimolazioni fallaci ). If the strength of stimulus is increased, or 

 the interval between each is diminished, the number of effective 



FIG. 141. Bowditch's ascending staircase from frog's heart ligatured at the auricles, with a series 

 of induction shocks thrown in at intervals of 4". , (Luciani.) The tracing shows a gradual in- 

 crease of both systolic and diastolic excursions. 



shocks increases also. If the current is still further strengthened, 

 there will be a response to every stimulus (stimolazioni infallibili). 

 After a long series of regular contractions a weaker current is seen 

 to produce the same effect. Hence a strength of current which 

 is at first uncertain (fallace) becomes certain (infallibile) after 

 a sequence of shocks. These facts prove that the excitability 

 of cardiac muscle to external stimuli is very variable, and oscillates 

 from one moment to another, and that the contractions are capable 

 of determining the said oscillations of excitability. 



On experimenting with frogs' hearts, ligatured at the auricles, 

 we obtained the same results with Bowditch's method of electrical 

 stimulation as he discovered for the apex of the heart. 



Another phenomenon that can be observed on experimenting 

 either with the apex or the auricular ligature, is the so-called 

 Bowditcli staircase. After complete rest for 5 to 10 minutes, 

 rhythmical excitation with induction shocks of uniform strength, 

 thrown in at intervals of 4 to 6 seconds, produces a series of 

 contractions, which steadily increase in height up to a certain 

 maximum. We were able to demonstrate that Bowditch's stair- 

 case expresses not merely an increment in systole, but an increment 

 in diastole also. This means that the prolonged rest produces a 



