270. Inhibition of ventricle by constant current. X<> record 

 need be taken. Place an indifferent electrode in contact with the 

 muscles of the frog's throat or other indifferent region ; the other 

 electrode is placed in contact with the tip of the ventricle by means 

 of a thin strip of cotton soaked in 7^ NaCl. The two electrodes are 

 connected through a key and pole-changer with the battery. With 

 the anode in contact with the ventricle, make the current. Note 

 the change in appearance of the ventricle ; explain. Then reverse 

 the current and break just before the beginning of systole. The 

 cathode is now in contact with the ventricle. Any result ? At what 

 poles does the inhibition appear (a) at make and <b) at break of 

 constant current? Compare with the conditions for stimulation. 



271. Stimulation by constant current. Bring the ventricle to 

 rest by a ligature at the auriculo-ventricular junction. Using the 

 same arrangement as before, determine at which pole stimulation 

 appears (a) at make, and (b) at break of constant current. 



III. INHIBITION OF HEART. 



272: Rcfle.v inhibition. Etherize a frog lightly by placing under a 

 glass jar with a piece of ether soaked cotton. Fasten the animal 

 in the holder ventral surface upward. Apply ether at intervals if 

 needed. Expose the heart, preventing loss of blood. Now tap 

 the abdomen rapidly with the handle of a scalpel, noting any change 

 in the number of beats per minute. The normal rate of the heart 

 should be determined before tapping begins. Now expose the intes- 

 tine and try the effect of direct stimulation of the intestine, both 

 mechanical and electrical. What is the effect on the rate of heart 

 beat? If the above method of reflex inhibition fails, expose the 

 sciatic nerve, ligature, and stimulate the central end. Effect on the 

 heart? 



273. Situation of cardio-inhibitory mechanism in central ner- 

 vous system. (Cardio-inhibitory center.) Remove carefully the 

 cerebrum and optic lobes. The medulla is letft intact. Now stimu- 

 late the intestine as before, after allowing time for shock to pass 

 off. Result? Stimulate the medulla directly by platinum electrodes. 

 Effect on the heart beat? Now destroy the medulla and repeat 

 reflex stimulation. Result? 



274. Iiitra-cardiac inhibitorv mechanism. Stimulate the heart 

 directly at the "white crescent" marking sinn-auricular junction. 

 Result? After inhibition note carefully the manner in which the 

 beat is resumed. 



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