8i6 ELECTRO-PHYSIOLOGY 



Clean the unpolarizable electrodes, and be sure to lower the reservoir 

 of the electrometer ; otherwise the mercury may reach the point of the 

 capillary tube and run out. 



In 4 a galvanometer (p. 699) may be used with advantage by students, 

 if one is available, instead of the electrometer, the unpolarizable elec- 

 trodes being connected to it through a short-circuiting key. The spot 

 of light is brought to the middle of the scale by moving the control- 

 magnet;- or if a telescope-reading (Fig. 222, p. 699) is being used, the 

 zero of the scale is brought by the same means to coincide with the 

 vertical hair-line of the telescope. The short-circuiting key is then 

 opened. 



5. Action Current of Heart. Pith a frog (brain and cord). Excise 

 the heart, and lay the base on one unpolarizable electrode, and the 

 apex on the other, having a sufficiently large pad of clay on the tips of 

 the electrodes to insure contact during the movements of the heart, or 

 having little cups hollowed in the clay and filled with physiological salt 

 solution, into which the organ dips. " Connect the electrodes with the 

 capillary electrometer and open its key. At each beat of the heart the 

 mercury will move (p. 806). 



6. Electrotonus. Set up two pairs of unpolarizable electrodes in the 

 moist chamber. Connect two of them with a capillary electrometer 

 (or galvanometer), and two with a battery of three or four small Daniell 

 cells, as in Fig. 295. Lay a frog's nerve on the electrodes. When the 

 key in the battery circuit is closed, the mercury (or the needle of the 

 galvanometer) moves in such a direction as to indicate that in the extra- 

 polar regions parts of the nerve nearer to the 

 anode are relatively positive to parts more re- 

 mote, and parts nearer to the kathode are rela- 

 tively negative to parts more remote. The 

 direction of movement of the mercury (or gal- 

 vanometer needle) must be made out first for one 

 direction of the polarizing current. Then the 

 latter must be reversed, and the movement of 

 the mercury (or needle) on closing it again noted 

 (p. 803). 



7. Paradoxical Contraction. Pith a frog (brain 

 and cord). Dissect out the sciatic nerve down to 

 the point where it splits into two divisions, one 

 for the gastrocnemius b, and the other for the 

 peroneal muscles a. Divide the peroneal branch 

 as low down as possible, and make a muscle-nerve 

 preparation in the usual way. Lay the central 

 313- Paradoxical end of the peroneal nerve on electrodes con- 

 Contraction, nected through a simple key with a battery of two 

 Daniell cells. When the peroneal nerve is stimu- 

 lated, the gastrocnemius muscle contracts. This result is not due to the 

 current of action, for it is not obtained with mechanical stimulation 

 of the nerve. But it is not the result of an escape of current, for if the 

 peroneal nerve be ligatured between the point of stimulation and the 

 bifurcation, no contraction is obtained. The contraction is really due 

 to a part of the electrotonic current set up in the peroneal nerve passing 

 through the fibres for the gastrocnemius, where they lie side by side 

 in the trunk of the sciatic. 



8. Alterations in Excitability (and Conductivity) produced in Nerve 

 by the Passage of a Voltaic Current through it. Set up two pairs of 

 unpolarizable electrodes in the moist chamber. Connect a battery of 

 two or three Daniell cells, arranged in series through a simple key 



