84 ACTION OF MECHANICAL AND ELECTRICAL STIMULI. 



contraction at 20 C. is only about T \> of the time occupied by a contraction occurring at 5 C. 

 A heart which has been warmed is capable of reacting pretty rapidly to intermittent stimuli, 

 while a heart at a low temperature reacts only to stimuli occurring at a considerable interval 

 (Oaule). 



Cold. When the temperature of the blood is diminished, the heart beats more slowly. A 

 frog's heart, placed between two watch-glasses and laid on ice, beats very much more slowly. 

 The pulsations of a frog's heart stop when the heart is exposed to a temperature of 4 C. to 0. 

 If a frog's heart be taken out of warm water, and suddenly placed upon ice, it beats more 

 rapidly, and conversely, if it be taken from ice and placed over warm water, it beats more 

 slowly at first and more rapidly afterwards (Aristow). 



[Methods. The effect of heat on a heart may be studied by the aid of the frog-manometer, the 

 fluid in which the heart is placed being raised to any temperature required. For demonstra- 

 tion purposes, the heart of a pithed frog is excised and placed on a glass slide under a light 

 lever, such as a straw. The slide is warmed by means of a spirit-lamp. In this way the 

 frequency and amplitude of the contractions are readily made visible at a distance.] 



(6) Mechanical Stimuli. Pressure applied to the heart from without accelerates its action. 

 In the case of Frau Serafin, v. Ziemssen found that slight pressure on the auriculo-ventricular 

 groove caused a second short contraction of both ventricles after the heart-beat. Strong pres- 

 sure causes a very irregular action of the cardiac muscle. This may readily be produced by 

 compressing the freshly excised heart of a dog between the fingers. The intra-cardiac pressure 

 also affects the heart-beat (p. 83). If the pressure within the heart be increased, the heart- 

 beats are gradually increased, if it be diminished the number of beats diminishes (Ludwig and 

 Thiry). If the intra-cardiac pressure be very greatly increased, the heart's action becomes very 

 irregular and slower. A heart which has ceased to beat may, under certain circumstances, be 

 caused to execute a single contraction if it be stimulated mechanically. 



(c) Electrical Stimuli. A constant electrical current t of moderate strength increases the 

 number of heart-beats. V. Ziemssen found, in the case of Frau Serafin ( 47, 3), that the 

 number of beats was doubled when a constant uninterrupted strong current was passed through 

 the ventricles. If the constant current be very strong, or if tetanising induction currents be 

 used, the cardiac muscle assumes a condition resembling, but not identical with, tetanus 

 (Ludung and Hoffa), and of course this results in a fall of the blood-pressure. If the auriculo- 

 ventricular groove be compressed so as to cause the ventricle of a frog's heart to cease to beat, 

 on placing one electrode of a constant current on the ventricular wall and the other electrode 

 on an indifferent part of the body, we obtain on making the current, a systolic contraction of 

 the ventricle only when the cathode touches the ventricle ; and conversely on breaking, only 

 when the anode is on the heart (Biedermann). 



When a single induction shock is applied to the ventricle of a frog's heart during systole, it 

 has no apparent effect ; but if it is applied during diastole, the succeeding contraction takes 

 place sooner. The auricles and also the apex behave in a similar manner. Whilst they are 

 contracted, an induction shock has no effect; if, however, the stimulus is applied during 

 diastole, it causes a contraction, which is followed by systole of the ventricle. Even when 

 strong tetanising induction shocks are applied to the heart, they do not produce tetanus of the 

 entire cardiac musculature, or as it is said, "the heart knows no tetanus" (Kronecker and 

 Stirling). Small white local weal-like elevations such as occur when the intestinal muscula- 

 ture is stimulated appear between the electrodes. They may last several minutes. A frog's 

 heart, which yields weak and irregular contractions, may be made to execute regular rhythmical 

 contractions synchronous with the stimuli, if electrical stimuli are used (Bowditch). 



[Break induction shocks, if of sufficient strength, cause the heart to contract, while weak 

 stimuli have no effect ; on the other hand, moderate stimuli, when they do cause the heart to 

 contract, always cause a maximal contraction, so that a minimal stimulus acts at the same time 

 like a maximal stimulus. The heart either contracts or it does not contract, and when it con- 

 tracts the result is always a " maximal " contraction (Kronecker and Stirling). Bowditch found 

 that the excitability of the heart was increased by its own movements, so that after a heart had 

 once contracted, the strength of the stimulus required to excite the next contraction may be 

 greatly diminished, and yet the stimulus be effectual. Usually the amplitude of the first beat 

 so produced is not so great as the second beat, and the second is less than the third, so that a 

 ''staircase " (" Treppe") of beats of successively greater extent was produced (fig. 59). Under 

 certain circumstances, however, a skeletal muscle gives contractions of a " staircase " character. 

 This staircase arrangement occurs even when the strength of the stimulus is kept constant, so that 

 the production of one contraction facilitates the occurrence of the succeeding one. A staircase 

 arrangement of the pulsations is also seen in Luciani's groups (p. 81). The question, whether 

 a stimulus will cause a contraction, depends upon what particular phase the heart is in when 

 the shock is applied. Even comparatively weak stimuli will cause a heart to contract, provided 

 the stimuli are applied at the proper moment and in the proper tempo, i.e. to say, they become 

 what are called " infallible." If stimuli are applied to the heart, at intervals which are longer 

 than the time the heart takes to execute its contraction, they are effectual or "adequate," but 

 if they are applied before the period of pulsation comes to an end, then they are ineffectual 



