70 ELEMENTARY PHYSIOLOGY. XII. 



When the heart begins to beat after vagus stimulation 

 it will for a time probably beat more strongly than be- 

 fore the stimulation, and if the contractions of the ven- 

 tricle have been of unequal strength, they will become 

 equal. By repeated stimulation the beat will (usually) 

 be considerably improved. 



2. Put a piece of thin india-rubber on the tissue under 

 the nerve. Fasten small electrodes to the frog board 

 with two pins (cp. Fig. 1 b) and place the nerve on the 

 electrodes. With the key in the secondary circuit closed, 

 set the vibrator of the induction coil in action, and 

 stimulate the nerve by opening the key in the secondary 

 coil. The nerve must be kept moist with Ringer's fluid. 



3. A signal marker (Fig. 14) may be arranged with its writing 

 point to mark vertically under the point of the heart lever. The 

 bobbins of the marker are connected with a battery and an in- 

 circuit key. This key is closed during the time the key of the second- 

 ary circuit is opened, and so marks on the tracing the beginning 

 and end of the stimulation. 



4. a. Effect of nicotine. Let fall on the heart two 

 drops of 1 p.c. nicotine. There will be temporary slowing 

 of the beat. In a few minutes stimulate the vagus, no 

 inhibition will be produced, but there may be increase 

 in rate and strength of the heart-beats. Stimulate the 

 junction of the sinus and auricle, the heart will stop. 



Nicotine has a brief stimulating effect on the peri- 

 pheral ganglia, it then paralyses the pre-ganglionic fibres, 

 but does not (except in large amount) paralyse the post- 

 ganglionic fibres. 



b. Effect of arecoline (and pilocarpine). Let fall 

 on the heart two drops of 1 p.c. arecoline nitrate. The 



