84 LABORATORY MANUAL OF GENERAL PHYSIOLOGY 



cardiac muscle, against the corresponding time intervals. Can you 

 distinguish : 



The Absolute Refractory Period? 

 The Relative Refractory Period? 

 The Period of Hyperexcitability? 



Compare with Adrians experiment (J. Physiol., vol. 54, p. 1, 1920). 

 Explain the normal rhythmicity of the heart (cf. Bayliss, Fig. 139, p. 455). 



8. The Effect of a "Tetanizing" Current. — Record on a slowly turning 

 drum the effect of stimulating the ventricle for a short interval (5 seconds) 

 with the weakest effective tetanizing current. Can cardiac muscle of 

 vertebrates exhibit true complete tetanus? The apparent tetanus you 

 obtain is probably contracture. 



9. Gaskell's Clamp and Heart Block. — Expose the beating heart of a 

 frog with brain pithed. Draw the ventricle between the edges of the 

 Gaskell clamp. Screw the edges carefully against the auriculo-ventricular 

 junction. 



Continue to increase the pressure very cautiously. A pressure will be 

 found at which the ventricle does not respond to every beat of the auricle, 

 i.e., the contraction wave does not always pass from auricle to ventricle 

 through the partial block. The clamp depresses the rate at which a hypo- 

 thetical substance recovers its power of transmission. 



What numerical ratios are noted between the contractions of the 

 auricles and the contractions of the ventricle, as the pressure is slowly 

 increased, and again as the pressure is lessened. Cf. Bayliss, p. 681; 

 Gilson, Am. J. Physiol., 110: 376, 1935- Text p. 432. 



10. Engelmarm's Incisions. — With small scissors cut about three- 

 fourths across the ventricle alternately on the two sides, beginning just 

 below the auriculoventricular junction. Thus the ventricle is transformed 

 into a zig-zag strip. If the contraction wave is still conducted into the 

 ventricle, note whether the wave passes over the strip. If no part of the 

 ventricle is active, stimulate the base with a minimal induction shock. 

 Next apply the same stimulus to the apical end of the strip. The waves 

 of contraction should travel equally well in either direction (cf. Engel- 

 mann, Pfliiger's Arch. 61: 275, 1895; Schmitt, Am. J. Physiol., 85: 332, 

 1928). Text p. 433. 



11. Inhibition of the Heart by Vagus Stimulation. — Remove the lower 

 jaw of a frog. Carefully pick away muscles in the two depressions on 

 each side of the occipital region. Expose the vagus as it leaves the skull. 

 (Fig. 8.) Explore with electrodes while dissecting. Arrange the heart 

 to record its contractions clearly on a slowly revolving drum. Set in the 

 primary circuit of an inductorium a simple key and an electromagnetic 

 signal. Using a weak interrupted current, stimulate the vagus nerve; first 



