THE STAIRCASE. 



221 



resumes its rhythmical beating, and continues to do so as 

 long as the constant current passes through the living pre- 

 paration. 



3. The Staircase. 



(a.) To a glass slide used for microscopic purposes (3x1) 

 fix with sealing-wax two copper wires in the long axis 

 of the slide, and let their two free ends be about 3 milli- 

 metres apart. They will act as electrodes. Connect the 

 other ends of the wires to a du Bois key introduced into 

 the secondary circuit of an induction machine. Arrange 

 the primary coil for single induction shocks, introducing 

 a Morse key in the circuit. 



(b.) Expose the heart of a frog, make an " apex prepara- 

 tion," and place it on the electrodes on the glass slide. 

 Rest on the heart a heart-lever properly balanced and 

 arranged to record its movements on a slow-moving drum 

 (5 mm. per second). The preparation does not contract 

 spontaneously, but responds to mechanical or electrical 

 stimulation. 



(c.) Stimulate the apex preparation with single break 

 induction-shocks at intervals of about ten seconds. To do 

 this un-short-circuit the secondary circuit, depress the 

 Morse key, short-circuit the secondary circuit, and close the 

 Morse key again. Repeat this several times every ten 

 seconds, and note that the amplitude of the second contrac- 

 tion is greater than the first, that of the third than the 

 second, the third than the fourth, and 

 then the successive beats have the same 

 amplitude (Fig. 102). Allow the heart- 

 apex to rest for a few minutes, and 

 repeat the stimulation. Always the 

 same result is obtained. From the 

 graduated rise of the first three or four 

 beats after a period of rest, the pheno- _^_ ... 



menon is known as the " staircase." The . , no c , . 



, . -i i riff. WZ. otaircase 



increment is not equal in each successive character of Heart 



beat, but diminishes from the beginning 

 to the end of the series. 



Beat. 



(d.) If, while the apex is relaxing, it be stimulated by a 



