120 



375. Intracardiac Inhibitory Center in the Frog. 

 Expose the heart of a frog, divide the frenum and tilt the 

 heart upward to expose the whitish V-shaped crescent between 

 the sinus venosus and right auricle. Stimulate the crescent, 

 using fine electrodes, with an interrupted current ; if the cur- 

 rent is sufficiently strong, the auricles and ventricle, after a 

 brief delay, will cease to beat for a time, but they begin beat- 

 ing again even in spite of continued stimulation. Stimulate 

 the auricles ; there is no inhibition. 



Connect the apex of the ventricle with the heart lever. 

 Use a signal magnet marking seconds, in the primary circuit. 

 Its lever will vibrate when the circuit is closed. Arrange so 

 that its writing point will write immediately under and in 

 the same vertical plane as the writing point of the heart lever. 

 Get a tracing of the normal beat, then stimulate the crescent 

 for one or two seconds as before. Inhibition results. After a 

 pause the beat begins again, the contraction passing as a 

 wave from the sinus, through the auricles to the ventricle. 



Stimulate the auricles. Note any effect upon the tracing. 

 (During inhibition the sinus beats, but the auricles and 

 ventricle do not, because the excitability is so lowered that 

 they do not propagate the excitatory process. 



Stimulate the ventricle mechanically, the heart beats in 

 the reverse order from ventricle through auricles to sinus. 



Apply a few drops of atropine solution to the heart and 

 again stimulate the crescent. There is no inhibitory effect 

 as the atropine paralyzes the inhibitory fibers. 



376. Form and Volume of a Contracting Muscle. 

 Dissect out the gastrocnemius muscle of a frog. Connect the 

 hooked electrodes at each end of the volume tube with the 

 muscle. The tube is to be filled with saline solution which 

 has been boiled and allowed to cool down to the temperature 

 of the room. Replace the stopper in the tube in such a way 

 that all air bubbles shall be excluded. The height of the 

 water in the capillary tube may be adjusted to the proper 

 level by moving the glass rod in the stopper in or out. Con- 

 nect the electrodes of the volume tube with the secondary 

 coil and, using a single induction current, send a maximal 

 break shock into the muscle. Note very carefully the level of 

 the water in the capillary tube before, during and after the 

 contraction of the limb. Does the level of the water in the 

 capillary tube change ? 



