100 ACTION OF MECHANICAL AND ELECTRICAL STIMULI. 



appearing to remain contracted. The ventricles usually cease to beat before the 

 auricles (Schelske). The size and extent of the contractions increase up to about 

 20C., but above this point they diminish (Fig. 35). The time occupied by any 

 single contraction at 20C. is only about -^ of the time occupied by a contraction 

 occurring at 5"C. 



A heart which has been warmed is capable of reacting pretty rapidly to inter- 

 mittent stimuli, while a heart at a low temperature reacts only to stimuli occurring 

 at a considerable interval. If a frog be kept in a cold place its heart beats slowly 

 and does little work, but if the heart be supplied with the extract of a frog which 

 has been kept warm, it is rendered more capable of doing work (Gaule). 



Cold. When the temperature of the blood is diminished, the heart beats slower 

 (Kielmeyer, 1793). A frog's heart, placed between two watch-glasses and laid on 

 ice, beats very much slower (Ludwig, 1861). The pulsations of a frog's heart stop 

 when the heart is exposed to a temperature of 4C. to (E. Cyon). 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 in 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 demonstration 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 con- 

 tractions are readily made visible at a distance.] 



[Gaskell fixes the heart by means of a clamp placed round the auriculo-ven- 

 tricular groove, while levers are placed horizontally above and below the heart. 

 These levers are fixed to part of the auricles and to the apex by means of threads. 

 Each part of the heart attached to a lever, as it contracts, pulls upon its own 

 lever, so that the extent and duration of each contraction may be registered. This 

 method is applicable for studying the effect of the vagus and other nerves upon the 

 heart (Roy).] 



(&.) Mechanical Stimuli. Pressure applied externally to the heart 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 pressure 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. 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 aud slower 

 (Heidenhain). A heart which has ceased to beat may, under certain circum- 

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



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

 increases the number of heart-beats, v. Ziemssen found in the case of Frail 

 Serafin (p. 74, 3), that the number of beats was doubled, when a constant uninter- 

 rupted 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 (Ludwig and 

 Hoffa), and of course this results in a fall of the blood- pressure (Sigm. Mayer). 



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 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 



