PRACTICAL EXERCISES 



1 79 



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FIG. 



75- ARRANGEMENT FOR OBTAINING A HEART 

 TRACING FROM A FROG. 



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, de- 

 press the drum in the 

 flame, and rotate 

 rapidly. Avoid put- 

 ting on a heavy 

 coating of smoke, as 

 a more delicate trac- 

 ing 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. 76) . Pith a frog 

 (brain only), expose 

 the heart, and put under it a cover-slip to give it support. Pin the frog 

 on the myograph-plate, and adjust the foot of the lever so that it rests 

 on the ventricle or the auriculo- ventricular junction. 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-ven- 

 tricular junction, 

 the part of the 

 tracing correspond- 

 ing to the contrac- 

 tion of the heart 

 will be broken into 

 two portions, repre- 

 senting the systole 

 of the auricles and 

 ventricle respec- 

 tively. Cut the 

 paper off the drum FIG. 76. ELECTRO-MAGNETIC TIME-MARKER CONNECTED 

 with a knife (keep- WITH METRONOME. 



ing the back of the The pendulum of the metronome carries a wire which 

 knife to the drum closes the circuit when it dips into either of the mercury 

 to avoid scoring it) cups, Hg. 

 and carry it to the 



varnishing-trough, holding the tracing by the ends with both hands, 

 smoked side up. Immerse 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. 



12 2 



Time- Marker 



Battery 



Metronome O\ 



