PRACTICAL EXERCISES 



169 



of the ligature, and taking care to cut wide of the sinus venosus. 

 Place the heart in a small porcelain capsule on a little blotting-paper 

 moistened with normal saline. Observe that it goes on beating. 

 Put a little ice or snow in contact with the heart, and count the 

 number of beats in a minute. The rate is greatly diminished. Now 

 remove the ice and blotting-paper, cover the heart with normal 

 saline, and heat, noting the temperature with a thermometer. Observe 

 that the heart beats faster and faster as the temperature rises. At 

 40 C. to 43 C. it stops beating in diastole (heat standstill). Now 

 at once pour off the heated liquid, and run in some cold normal 

 saline. The heart will begin to beat again. 



4. Cut off the apex of the ventricle a little below the auriculo- 

 ventricular groove. The auricles, with the attached portions of the 

 ventricle, goon beating. The apex does not .contract spontaneously, 

 but can be made to 



beat by stimulating 

 it mechanically (by 

 pricking it with a 

 needle) or electrically. 

 Divide the still con- 

 tracting portion of 

 the heart by a longi- 

 tudinal incision. The 

 two halves go on 

 beating. 



5. Heart -tracings. 

 (i) Fasten a myo- 

 graph-plate (Fig. 59) 

 on a stand. Take a 

 long light lever con- 

 sisting of a straw or 

 a piece of thin chip, 

 armed at one end 



with a writing-point of parchment-paper, supported near the other 

 end by a horizontal axis, and pierced not far from the axis by 

 a needle carrying on its point a small piece of cork or a ball of 

 sealing-wax. 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. 60). 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 



FIG. 58. FROG'S HEART WITH STANNIUS LIGA- 

 TURES IN POSITION (CYON). 



Anterior surface of heart shown on the left, posterior 

 surface on the right, a, right auricle ; b, left auricle ; c, 

 ventricle ; d, bulbus arteriosus ; e, f, aortoe ; g, sinus 

 venosus. 



