THE FROG S HEART 



26l 



3. The Rate and Sequence of the Contractions of the Frog's Heart. 

 Destroy the brain of the frog and open the thorax, but do not destroy 

 the pericardium. Count the rate of the heart per minute, then remove the 

 pericardium and make a second determination after the heart is exposed 

 to the air. The different parts of the heart are easily identified and 

 the contractions in definite sequence can be determined without difficulty. 

 Make this determination for the ventricle, auricle, and sinus venosus by 

 direct observation. 



Prepare a cardiac lever as shown in figure 212 or 216, taking special 

 care to arrange the foot so that it will not bind when in motion. Adjust 

 the foot of the lever on the exposed ventricle and bring its point to write 



FIG. 212. Heart Lever for Frog or Turtle Hearts. This lever rests directly on 

 the surface of the heart, the foot consisting of a tiny piece of dry cork bark. The 

 lever can also be used as in figure 216, in this case by attaching the tip of the apex 

 of the ventricle by a tiny hook and thread to the short axis of the lever. Either de- 

 vice can be used for taking records of cardiac sequence. 



lightly on the smoked paper of a recording cylinder. This cylinder should 

 travel at the rate of about 2 cm. per second and its speed be marked by the 

 writing point of an electric magnet. Take care to adjust the time magnet 

 in a vertical line with the writing point of the heart lever, placing the 

 heart lever about i cm. above. The tracing of the ventricle, the cardio- 

 gram, will show rhythmic contraction, relaxation, and pause of the ven- 

 tricle. It will also enable one to measure the exact proportion of the total 

 time of the cardiac cycle consumed by the systole and diastole, and also 

 that portion of the diastole in which the ventricle is wholly at rest. A 

 drum rate of 2 mm. per second gives a more satisfactory record of varia- 

 tions when amplitude and rate alone are studied. 



