LTII.] 



THE frog's heart-beat. 



263 



still within its pericardium, and arrange a heart-lever so that it rests 

 lightly on the pericardium over the beating heart. Adjust the 

 lever to write on a revolving cylinder, moving at a suitable rate 

 (5-6 cm. per second). Take a tracing of the beating of the heart. 



(b.) Before commencing the experiment, make a suitable heart-lever with 

 a straw about 12 inches long, or a thin strip of wood about the same len^'th. 

 Thrust a needle transversely either through the straw or through a piece of 

 cork slipped over the straw about 2 inches from one end of the lever. Tlie 

 needle forms the fulcrum of the lever, and works in bearings, whose height 

 can be adjusted. To the end of the lever nearest this is attached, at right 

 angles, a needle with a small piece of cork on its free end. The lever is so 

 adjusted that the cork on the needle rests on the heart. The long arm of the 

 lever is provided with a writing-style of copper- foil, or a writing point made 

 of parchment paper, fixed to it with sealing-wax. By using a long lever a 

 sufficient excursion is obtained. Another form of heart-lever is shown in fig. 

 182. It consists of a thin glass rod, fixed as shown in the figure. The frog is 

 laid on its back on a frog-plate covered with cork. The heart-lever is fixed 

 into the cork by means of the two pins (6), while C is so adjusted as to rest on 

 the heart. 



the 



Fio. 182.— Simple Frog's Heart-Lever, a. Fulcrum ; L. Glass lever with knob to act as 

 counterpoise ; b. To fix the apparatus Into the cork of a frog-plate ; C. Cork to rest on 

 the heart. 



(c.) Open the pericardium, expose the heart, and adjust the 

 cork on the lever. To obtain a good tracing, it is well to put a 

 resistant body behind the heart. Raise the ventricle, ligature the 

 fraenum, and divide the latter dorsal to the ligature ; behind 

 heart place a pad of blotting- 

 paper moistened with normal 

 saline, or a thin glass-cover 

 slip. Adjust the cork pad of 

 the lever on the junction of 

 the auricles and ventricle, to 

 write on the drum, moving at 

 a slow rate (5-6 cm. per 

 second), and take a tracing. 

 Fix the tracing (fig. 183). 



(d.) In the tracing note a 

 first ascent, due to the auricular contraction, and succeeding this a 

 second ascent, due to the contraction of the ventricle, followed by 

 a slow subsidence, due to the continuation of the ventricular 

 systole, and then a sudden descent, due to the diastoHc relaxation 

 of the heart. 



Fig. 183.— Tracing taken with a Frog's Heart- 

 Lever resting on the Auriculo-ventiiciJlar 

 Groove. A. Heart tracing ; T. Time ; each 

 interval represents one second. 



