66 ELEMENTARY PHYSIOLOGY. XL 



greatly with the condition of the frog; a slowing and 

 weakening of the heart-beats may be caused instead of 

 cessation. After the stimulation, the beat may become 

 stronger than before. If no effect is obtained with a 

 current distinctly felt on the tongue, place the electrodes 

 on the undissected part of the vagus near the heart. 

 Dissect out and stimulate the other vagus, the effect of 

 one nerve is often more marked than that of the other. 



4. Turn the ventricle forwards and stimulate the line 

 of junction of the sinus and right auricle ; the heart will 

 stop beating; there may be a preliminary period of 

 quickened beats caused by the direct stimulation of the 

 cardiac muscle. 



5. Ligature of sino-auricular junction. Reversed 

 beat. Cut through the tissue on either side of the aortae. 

 Pass a thread underneath them, draw it backwards, tilt- 

 ing the ventricle forwards, tie it firmly round the junc- 

 tion of the sinus with the auricle. The auricles and 

 ventricle will either cease to beat or beat with a slower 

 rhythm than the sinus. If the heart has stopped touch 

 the tip of the ventricle with a seeker ; the ventricle will 

 beat before the auricle (this is best seen somewhat later, 

 when the conductivity of the heart has decreased). If 

 the heart continues to beat, touch the ventricle several 

 times at a rate rather faster than the beat; each touch 

 will cause a reversed beat from ventricle to auricle. 



Stimulate the ventricle with weak faradic (tetanising) 

 currents; the stimulation does not cause tetanus, but 

 either a beat at its beginning and end, or a series of 

 beats. 



