106 



EXPERIMENTAL PHYSIOLOGY 



effect. We have already found that skeletal muscle gives a similar 

 result (pp. 63 and 69). 



Fig. 86. — Electrical Stimulation of the Ventricle of the Frog's Heart 

 in Standstill by the Stannius Ligature to Show the ' Staircase ' Effect. 

 Interval between the Stimuli 5 secs. 



Experiment 5. — Record a single beat of the heart. Arrange the drum 

 to rotate at the rate of 2 cm. per second. Fit up the apparatus as for record- 

 ing a single muscle twitch (fig. 37). For electrodes in the secondary circuit 

 make a pair with fine silk-covered wire as in fig 87. Cement the two wires 

 together by a touch of sealing-wax or marine glue at a point, c, near to their 



Fig. 87. — A Simple Form of Flexible Electrodes. 



free ends. Scrape off the insulation from their projecting ends, e, which 

 should be cut of equal length. Take a piece of flat cork of the shape of k 

 and pass two pins, f and g, through it. Slit up the cork in two places at 

 its apex and pass the wires through these slits by which they are held 

 firmly ; then wind each wire round one of the pins and attach the two 

 ends to the Du Bois key in the secondary circuit. Apply a Stannius liga- 

 ture to a heart and, passing a bent pin through its apex, attach it to the 

 recording lever, as in fig. 83. Twist the electrodes, e, until the wires lie 

 above one another and pin down the electrodes to the cork base of the 

 myograph, so that the wires touch the base of the ventricle. Arrange a chrono- 

 graph vibrating thirty times per second to record directly under the heart 

 lever. Bring the writing point to the surface and record a contraction in the 

 same way as if recording a simple muscle twitch. Mark the point of stimu- 

 lation and draw vertical lines with the recording lever to mark on the time 

 tracing the point of stimulation and the points (1) when contraction begins, 

 (2) when it reaches its maximum, and (3) when relaxation ends. Draw a zero 

 abscissa line. Bepeat the experiment, varying the rate of the recording surface. 



The tracing obtained is of the form shown in fig. 88, and presents 



