108 



EXPERIMENTAL PHYSIOLOGY 



Upon a heart brought to a standstill by the Stannius ligature we 

 are further able to study the effect of two successive stimuli, which 

 enables us to demonstrate one of the great physiological characteristics 

 of heart muscle, namely, its long refractory period. 



Fig. 89.- -A Single Contraction of the Frog's Ventricle. Time, 30 per sec. 



Fig. 90. — A Piepetition of the Tracing of Fig. 89 with a Slower 

 Movement of the Recording Surface. Time, Tracing, 8 per sec. 



Experiment 6. — Arrange the drurn as for recording the effect of two 

 successive stimuli on muscle (fig. 58). The drum should be so geared that it 

 rotates at the rate of 2 to 3 cm. per second. Pith a frog, expose its heart, 

 and apply the Stannius ligature. Pass a bent hook through the apex of 

 the ventricle and attach to the recording lever, fig. 83. First remove one 

 of the contacts and record a single beat as in the previous experiment. 

 Mark the point of stimulation and draw an abscissa line. Now arrange the 

 second contact at such a distance behind the first that the second point ot 

 stimulation will fall near the end of the period of relaxation and record the 

 result of the double stimulation. Mark the points of stimulation on the 

 curve. Move the two contacts nearer and repeat the stimulation. Carry out 

 the process until a distance between the points of stimulation is reached, 

 when only a single contraction results. 



In this way tracings such as those reproduced in fig. 91 are 

 obtained. 



Curve I. gives a record of the single beat. The time from the 

 point of stimulation to the highest point of the curve is § £ths sec. 

 The total duration is ^hs sec - 



Curve II. This is the result of two successive stimuli, the second 

 following the first after an interval of f£ths sec. It therefore fell near 



