86 LABORATORY MANUAL OF GENERAL PHYSIOLOGY 



for 5 seconds with the weakest current producing any effect, next with a 

 strong current for the same length of time, and finally with the strong 

 current for a longer time. The signal should record directly below the 

 writing point of the heart lever. Cf. Mitchell, 2nd Ed., p. 37. Text 

 p. 437. 



PICK OFF 

 0VERLYIN6 

 MUSCLES 



VA6US NERVI 



Fig. 8. — Stimulation of vagus nerve in the frog. Ventral aspect of skull. Lower jaw removed. 



12. Influence of Temperature on the Frog's Heart. — Record on kymo- 

 graph and count the frequency of the heart beats while Ringer solution 

 of a known temperature is poured gently over the heart. Use several 

 temperatures from 10° to 25°C. Calculate the "Temperature character- 

 istic." Cf. Crozier, J. Gen. Physiol., 9: 531, 1926. If frog's heart could 

 function at 37°C. would its frequency be similar to that of a warm blooded 

 animal like the rabbit? Cf. Clark, "Comp. Physiol, of Heart," p. 40, 

 1927. Text p. 441. 



13. Electrocardiogram of Frog. — Place excised frog heart on non- 

 polarizable boot electrodes (Zn — sat. ZnS04) wired to the capillary 

 electrometer. Observe the movement of the meniscus between the Hg 

 column and the 20% H2SO4. Explain movement of Hg (by change in 

 surface tension). Compare frequency of contraction with the electrical 

 variation. Make a diagram of the instrument. The meniscus may be 

 adjusted by the windlass at right. For description and limitations of 

 capillary electrometer cf. Bayliss, p. 642. Try effect of M/8 KlI. M/12 

 CaCl2 (cf. Clark, "Comp. Physiol, of Heart," p. 129). 



EXPERIMENTS ON THE TORTOISE HEART 



With a hook draw out the head. Smash the skull with screw driver 

 and hammer. Pith brain with wire. Saw off the plastron. Slit the 

 pericardium and expose the heart. Identify the different parts and trace 

 the sequence of the contraction wave from the sinus to the bulbus arterio- 

 sus. Place the animal on its back in a pan. Keep neck extended by a 

 cord passing under the pan and tied to a pelvic girdle. With pipette 



