170 



A MANUAL OF PHYSIOLOGY 



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 corre- 

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



FIG. 59. ARRANGEMENT FOR OBTAINING A HEART TRACING FROM A FROG. 



portions, representing the systole of the auricles and ventricle re- 

 spectively. Cut the paper off the drum \\ith a knife and carry it 

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



FIG. 60. ELECTRO-MAGNETIC TIME-MARKER CONNECTED WITH METRONOME. 



The pendulum of the metronome carries a wire which closes the circuit when it dips 

 into either of the mercury cups, Hg. 



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. 



(2) Heart Tracing, with Simultaneous Record of Auricular and 



