PRACTICAL EXERCISES 



193 



A counterpoise is adjusted on the short arm of the lever in the form 

 of a small leaden weight. Cover a drum with glazed paper and smoke 

 it. The paper must be put on so tightly that it will not slip. To 

 smoke the drum, hold it by the spindle in both hands over a fish-tail 

 burner, depress the drum in the 

 flame, and rotate rapidly. Avoid 

 putting on a heavy coating of 

 smoke, as a more delicate tracing 

 is obtained when the paper is 

 lightly smoked. The speed of the 

 drum can be varied by putting 

 in or taking out a small vane. 

 Arrange an electro-magnetic time- 

 marker for writing seconds (Fig. 

 88). Pith a frog (brain only), 

 expose the heart, and put under 

 it a cover-slip to give it support. 



V 



Pig 87. Arrangement for obtaining a 

 Heart Tracing from a Frog. 



Pin the frog on the myograph- 

 plate, and adjust the foot of the 

 lever so that it rests on the ven- 

 tricle or the auriculo-ventricular 

 j unction . Bring the writing-point 

 of the lever and that of the time- 

 marker vertically under each other on the surface of the drum. Set off 

 the drum at the slow speed (say, a centimetre a second). When the 

 lever rests on the auriculo-ventricular junction, the part of the tracing 

 corresponding to the contraction of the heart will be broken into two 



portions, representing 

 the systole of the auri- 

 cles and ventricle re- 

 spectively. Cut the 

 paper off the drum 

 with a knife (keeping 

 the back of the knife 

 to the drum to avoid 

 scoring it) and carry 

 it to the varnishing- 

 trough, holding the 

 tracing by the ends 

 with both hands, 

 smoked side up. Im- 

 merse the middle of it 

 in the varnish, draw 

 first one end and then 

 the other through the 

 varnish, let it drip 

 for a minute into the 

 trough, and fasten it 

 up with a pin to dry. 



(2) Heart Tracing, 

 with Simultaneous Re- 

 cord of Auricular and Ventricular Contractions. (a) For this purpose two 

 levers may be arranged, one resting on the auricle, the other on the ven- 

 tricle, the writing-points being placed in the same vertical straight line 

 on the drum. A convenient form of apparatus is shown in Fig. 89. 



(b) Gaskell's Method (a modification of). Attach a silk ligature to 

 the very apex of the ventricle. Divide the fraenum, cut the aorta 



Fig. 88. Electro - Magnetic Time - Marker connected 

 with Metronome. The pendulum of the metro- 

 nome carries a wire which closes the circuit when 

 it dips into either of the mercury cups, Hg. 



