82 EXPERIMENTAL PHYSIOLOGY 



pure zinc, and the solution in which they were immersed was a strong 

 solution of zinc sulphate, that no polarisation occurred ; and the same 

 was found to be the case with less purified zinc if its surfaces were 

 thoroughly amalgamated. 



If instead of a solution a piece of fresh animal tissue connects a. 

 pair of wire electrodes the same polarisation occurs. Chemical changes 

 are set up where the wires touch the tissue which can act in the 

 reverse manner, and set up a small current if the battery be removed 

 and the electrodes connected by a conductor. This acts as a source 

 of fallacy in many experiments, and becomes of great importance 

 where we are dealing with a very excitable tissue, such as nerve. 

 The existence of this polarisation current may be proved, as in the 

 previous example, by sending it through a galvanometer ; but we are- 

 also able to show it by its effect in exciting a nerve, as in the following 

 arrangement : — 



Experiment 10. — Arrange the apparatus as shown in fig. 72, open the kejr 

 K, and close the key k,. The current is thus sent through the nerve by the 

 electrodes e, which it will polarise. Note that there is no contraction during 



Fig. 72. — Method of Arranging the Apparatus to Show- 

 Polarisation of Electrodes. 



the time the current is passing. In about a minute open k ; and then rapidly 

 close and open K 2 , when contractions will occur which are due to the closure 

 and opening of the small current set up by the polarised electrodes. The- 

 contractions rapidly diminish in amount as the nerve becomes depolarised. 



This experiment illustrates the necessity of avoiding this polarisa- 

 tion, if it be possible, when we are experimenting upon nerves or other 

 excitable tissues, and Du Bois-Eeymond utilised Eegnault's discovery 

 for making electrodes which would not show this defect. In his 

 original form (fig. 73, 1) each electrode consisted of a zinc trough on an 

 insulating vulcanite base, the outer surface being thoroughly varnished 

 and the inner well amalgamated. Into this a thick pad of filter 

 paper thoroughly soaked in a saturated solution of zinc sulphate is 

 fitted, and the part of the pad lying in the trough is then covered 

 with the saturated zinc sulphate solution. If, now, the piece of tissue 

 be placed between two such pads the zinc salt rapidly produces 

 corrosive effects, and the tissue is rapidly destroyed. To prevent 

 this, little masses of china clay worked up into a stiff paste with 

 normal saline solution are used, upon which to rest the tissue and 



