PRACTICAL EXERCISES 



183 



circuit, which should also include a simple key. Insert a short- 

 circuiting key in the secondary circuit. Attach the electrodes to the 

 short-circuiting key, push the secondary coil up towards the primary 

 until the shocks are distinctly felt on the tongue when the Neef's 

 hammer is set going and the short-circuiting key opened. Pith the 

 brain of a frog, expose the heart, dissect out the vagus on one side, 

 ligature it as high up as possible, and divide above the ligature. 

 Fasten the electrodes on the cork plate by means of an indiarubber 

 band, and lay the vagus on them. Set the drum off (at slow speed). 

 After a dozen heart-beats have been recorded, stimulate the vagus 

 for two or three seconds by opening the short-circuiting key. If the 

 nerve is active, the heart 

 will be slowed, weakened, 

 or stopped. In the last 

 case the lever will trace an 

 unbroken straight line ; 

 but even if the stimulation 

 is continued the beats will 

 again begin. 



8. Stimulation of the 

 Junction of the Sinus and 

 Auricles. After a sufficient 

 number of the observations 

 described in 7 have been 

 taken with varying time 

 and strength of stimula- 

 tion, take the writing-, 

 points off the drum, apply 

 the electrodes directly to 

 the crescent at the junction 

 of the sinus venosus with 

 the right auricle, and 

 stimulate. The heart will 

 be affected very much in 

 the same way as by stimu- 

 lation of the vagus, except 

 that during the actual 

 stimulation its beats may 

 be quickened and the in- 

 hibition may only begin 

 after the electrodes have 

 been removed (Fig. 59, 

 p. 144). 



9. Effect of Muscarine (or 

 Pilocarpine) and Atropine. 

 Paint on the sinus veno- 

 sus with a small camel's-hair brush a very dilute solution of muscarine 

 (or of pilocarpine) . The heart will soon be seen to beat more 

 slowly, and will ultimately stop in diastole. Now apply a dilute 

 solution of sulphate of atropine to the sinus. The heart will again 

 begin to beat. Stimulation of the vagus will now cause no in- 

 hibition of the heart, because its endings have been paralyzed by 

 atropine. (Muscarine or pilocarpine has also been applied to the 

 heart, but it could be shown by a separate experiment that atropine 

 by itself has the same effect on the vagus endings p. 150). 



10. Stannius' Experiment. Pith a frog. Expose the heart in the 



FIG. 80. RELATION OF THE SYMPATHETIC 

 TO THE VAGUS IN THE FROG. 



i, 2, 3, 4 are spinal uerves. 



