250 



THE CIRCULATION OF THE BLOOD 



mains fairly constant, while the time of the diastole increases or decreases 

 with the rate, a fact to which Hiirthle has drawn attention, figure 157. 



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 

 tLe 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 when exposed are easily iden- 

 tified and the contractions which take place 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, taking special care to ar- 

 range the foot so that it will not bind on the lever when in motion. Adjust 



FIG. 212. Heart Lever for Frog or Turtle Hearts. 



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

 the smoked paper of a recording cylinder. This cylinder should travel at 

 the rate of about i cm. per second and its speed be determined by the writing 

 point of an electric magnet which is connected with the electric- clock circuit 

 marking seconds. 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 magnet lever. The tracing of the ventricle's movement, or cardio- 

 gram, will show alternate contraction, relaxation, and pause of the ventricle. 

 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. 



After one has obtained the ventricular tracings and has learned the diffi- 

 culties of adjusting the apparatus, a second heart lever should be adjusted 



