198 



THE CIRCULATION OF THE BLOOD AND LYMPH 



crystals to keep it saturated. After using the Daniell, it must always 

 be taken down. The outer pot is left with the copper plate and the 

 sulphate solution in it. The zinc is washed and brushed bright. The 

 sulphuric acid is poured into the stock bottle, and the porous pot put 

 into a large jar of water to soak. 



The Bichromate Cell contains only one liquid a mixture of i part 

 of sulphuric acid with 4 parts of a 10 per cent, solution of potassium 

 bichromate. In this is placed one, or in some forms two, carbon 

 plates and a plate of amalgamated zinc. After using the battery, take 

 the zinc out of the liquid. 



The Leclanche battery consists of a porous pot filled with a mixture 

 of manganese dioxide and carbon packed around a carbon plate, which 

 forms the positive pole. The pot stands in an outer jar of glass filled 

 with a saturated solution of ammonium chloride, into which dips an 

 amalgamated zinc rod, which constitutes the negative pole. Various 

 forms of dry batteries can be conveniently used for running indue tion- 



coils or time-markers, but are not 



rrvi 



LqryngeaL 

 branch of 

 ' 



adapted for yielding constant cur- 

 rents of long duration. 



7. Stimulation of the Vagus in 

 the Frog. Make the same arrange- 

 ments as in 5 (i) (p. 195), but in 

 addition set up an induction 

 machine arranged for an inter- 

 rupted current (Fig. 93), with a 

 Daniell, a bichromate, a Leclanche, 

 or a dry cell in the primary circuit, 

 which should also include a simple 

 key. Insert a short-circuiting key 

 in the secondary circuit. Attach 

 the electrodes to the short-circuit- 

 ing 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 stimulation, take the writing-points 

 off the drum, apply the electrodes directly to the crescent at the junc- 

 tion of the sinus venosus with the right auricle, and stimulate. The 

 heart will be affected very much in the same way as by stimulation of 

 the vagus, except that during the actual stimulation its beats may be 

 quickened and the inhibition may only begin after the electrodes have 

 been removed (Fig. 70, p. 158)'. 



Fig. 91. The Relations of the Vagus 

 in the Frog. 



