MECHANICAL STIMULI. ELECTRICAL STIMULI. 119 



soon becomes arrested, and continues at rest if kept at this temperature. Up to 

 38 C. Landois has seen it recover if removed quickly. The inner surface of the 

 heart reacts much more readily to all degrees of temperature than the external 

 surface. If the heart, after having been arrested, is removed from the warm 

 bath, it begins to beat rapidly after a pause, which may be interrupted by one or 

 two beats, the frequency gradually diminishing until the normal rate is attained. 

 If the ventricle alone is heated, the frequency of pulsation is not increased. 

 The volume and the extent of the cardiac contractions increase up to a tem- 

 perature of about 20 C., and beyond that point they begin to diminish again. 

 The functional power increases between 8 and 33 C.; but the frequency increases 

 more than the efficiency of the pulsations. The duration of the contraction at 

 20 C. is only about one -tenth of what it is at 5 C. The heated heart reacts to 

 rapidly intermittent stimuli, the cold heart only when the intervals are of consid- 

 erable length. The mammalian heart ceases to beat at from 44.5 to 45 C. 



As the heat of the blood diminishes, the heart pulsates more slowly. 

 When a frog's heart is placed on ice between two watch-glasses, its rate diminishes 

 considerably; between 4 C. and o C. the pulsations of the frog's heart cease. 

 When a frog's heart is suddenly removed from warm water and placed on ice, the 

 beat is accelerated; conversely, when it is transferred from ice to warm water, 

 the beat is at first slowed and only after a time accelerated. 



(b) Mechanical Stimuli. Pressure applied to the outside of the heart causes 

 an acceleration of the cardiac action. In man also light pressure applied to the 

 auriculo-ventricular junction of an exposed heart gave rise to a secondary shorter 

 contraction of both ventricles following each heart-beat. Heavy pressure causes 

 an irregular, undulatory contraction of the muscle, such as may be produced by 

 compressing the excised heart of a warm-blooded animal between the fingers. 

 Increase of the blood-pressure in the interior of the heart effects a similar accelera- 

 tion, and decrease of the pressure a corresponding diminution in the number of 

 heart-beats. When the intracardiac pressure is excessive, the overstimulation 

 results in irregularity or even slowing of the heart-beat. A resting heart that is 

 still irritable will react by a single contraction to a mechanical impulse (prick). 



(c) Electrical Stimuli. A moderately strong constant current passing continu- 

 ously through the heart produces an increase in its rate. Ziemssen succeeded in 

 accelerating the beat of an exposed heart two-fold or three-fold by passing a 

 strong galvanic current uninterruptedly through the ventricles. Exceedingly 

 strong constant currents, as well as tetanizing faradic currents, produce tetanic 

 undulatory contractions of the heart-muscle, with lowering of the blood-pressure. 



If the ventricle of the frog's heart has been permanently relaxed by being 

 clamped at the auriculo-ventricular junction, and one electrode of a constant 

 current is applied to the ventricular wall, and the other to any portion of the 

 trunk, systolic contraction of the ventricle takes place when the 'current is closed 

 only if the kathode is placed in contact with the ventricle; conversely when the 

 current is opened only if the anode is in contact with the heart-wall. The feeblest 

 faradic currents accelerate the heart-beat; stronger currents produce irregularities, 

 which may go on to fibrillation. 



A single induction-impulse applied to the ventricle in systolic contraction has 

 no effect either in the frog or in the mammal. When, however, it is applied to 

 the ventricle in diastolic relaxation, the succeeding systole takes place earlier. The 

 auricles and the apex of the heart, which is devoid of ganglia, but may be excited 

 to activity by suitable stimulation, react in the same way. During their systole 

 an induction-impulse is ineffective, but in diastolic rest the impulse gives rise to a 

 contraction, which is followed by a ventricular contraction. Even strong tetan- 

 izing induction-currents applied to the heart are unable to produce tetanus of the 

 entire musculature. There develop between the electrodes localized, white, cylin- 

 drical elevations, as in the muscles of the intestines, which may persist for several 

 minutes. After severe and continued tetanization the undulatory contractions 

 outlast the stimulus. Also the isolated apex of warm-blooded animals may exhibit 

 this undulatory contraction only so long as the stimulus lasts. The heart of a 

 previously warmed frog, as well as the isolated apex, reacts to electric stimuli by 

 flickering. The fibrillating or flickering rabbit's heart often returns spontaneously 

 to its normal contractions, the dog's heart with greater difficulty. After the 

 contractions of the frog's heart have become weak and irregular, they can be 

 made regular and isochronous with the rhythm of the stimulus by means of elec- 

 tric stimuli applied in rhythmical succession. The feeblest stimuli that are at all 

 efficient act as well in this connection as the strongest; even with the weakest 



