XLVI.] NERVE-CURRENTa 237 



towards zero. This is the "negative variation of the muscle- 

 current." If the gastrocnemius be used, stimulate the sciatic nerve. 

 Care must be taken that the muscle does not shift its position on the 

 electrodes. According to Hermann's theory, it is brought about as 

 follows : — An injured part of a muscle (or nerve) is negative to an 

 uninjured part — 'negativity of injury," and similarly an active 

 part of a muscle is negative to an inactive part — "negativity of 

 activity." The demarcation-current or injury-current passing in 

 the galvanometa.'from the longitudinal + to the transverse - surface 

 is diminislied, because, when the muscle contracts, there is a current 

 set up — action-current — in the opposite direction, which diminishes 

 the total current acting on the galvanometer. 



ADDITIONAL EXERCISEa 



6. Brush Electrodes of V. Fleischl (fig. 160) consist of glass tubes 5 mm. 

 in diameter and 4 cm. long. Into one end is fitted a perfectly clean camel's- 

 hair pencil, and into the other dips a well-amalgamated rod of zinc with a 

 binding screw at its free end. Place some clay in the lower part of the tube, 

 and then fill it with a saturated solution of zinc sulphate. A piece of india- 

 rubber tubing fits as a cap over the upper end of the glass tube. The brushes 

 are moistened with a mixture of kaolin and normal saline. 



7. D'Arsonval's Non-Polarisable Electrodes (fig. 161).— The electrodes 

 consist of a silver wire coated with fused silver chloride. The silver wire is 

 held in a suitable stand, while the silver chloride coated part is placed in a 

 tube tapering to a point below and filled with normal saline. At the lower 

 tapered end there is a small aperture into which is introduced a thick thread. 

 The tube is closed above with a cork (C), through which passes the silvei 

 electrode (A). The tapered points are brought into contact with the tissues. 

 They should be kept in the dark. 



Vertical Electrodes of Fick, — Into a vertical glass tube the amalgamated 

 zinc is introduced from below, the tube is filled with a saturated solution of 

 ZnS04, but the nerve rests on a hammer-shaped piece of baken porcelain, 

 such as is used for porous cells for batteries. It is soaked with salt solution, 

 and has a process which dips into the zinc sulphate. Several of these can be 

 arranged side by side in a suitable holder. 



LESSON XLVL 



NERVE - CURRENTS — ELECTRO - MOTIVE PHENO- 

 MENA OP THE HEART— CAPILLARY ELECTRO- 

 METER. 



1. Demarcation-Current of Nerve. 



(a.) Render the galvanometer as sensitive as possible by adjusting at a suit- 

 able height the north pole of the magnet over the north pole of the uppei 

 needle. 



