ELECTRO-MOTIVE PHENOMENA OF NERVE. 



193 



the electrodes are hook-shaped, and one is adjusted over 

 the other. The upper hooked electrode 

 has a groove on its concavity com- 

 municating with the interior of the tube 

 (Fig. 83). Place only one plug in the 

 shunt between A and B. 



(c.) Dissect out a long stretch of the 

 sciatic nerve, make a fresh transverse sec- 

 tion at either end, hang it over the upper 

 N.P. electrode (N), and resting with its 

 two cut ends on the lower electrode (C), 

 thus doubling the strength of the current 

 (Fig. 83). 



(d.) Remove the plug from C in the 

 shunt, and pass the whole of the demar- 

 cation-nerve current through the gal- 

 vanometer, noting the deflection. 



Fig. 83. Nerve N. 

 P. Electrodes. 

 N, Nerve; C, clay 

 of electrodes ;-Zn, 

 Zincs. 



(e.) Instead of adjusting the nerve as 

 in (c.), it may be so placed on the ordinary 

 tube N.P. electrodes, that the cut end rests on one electrode, 

 and the longitudinal surface 011 the other, thus leaving part 

 of the nerve free. Observe the deflection in this way. 



2. Action Current of Nerve. 



(a.) Observe the amount of deflection as in (e.) Stimulate 

 with an interrupted current the free end of the nerve, and 

 observe that the spot of light travels towards zero. This 

 was formerly called the " negative variation " of the nerve 

 current. 



3. Electro -motive Phenomena of the Heart. The arrangement 

 of the apparatus is precisely the same as in Lesson XXXVIII. 



(a.) Make a Stannius preparation, of the heart, using only 

 the first ligature (Lesson XLVI., 1) to arrest the heart's 

 action. Lead off with brush N.P. electrodes from base and 

 apex of the heart ; there is no deflection. 



(b.) Pinch the apex so as to injure it, it becomes negative. 



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