114 



PRACTICAL PHYSIOLOGY 



chemical and electrical change be suddenly made or broken, a shock 

 will be produced, for the wires of the electrodes surrounded by the 

 electrolysed fluid will form a minute battery. This can be demon- 

 strated by the following experiment : A pair of electrodes, con- 

 nected with a Du Bois key, is placed under the sciatic nerve, which 

 has been exposed in the thigh of a pithed frog. Making or breaking 

 the circuit causes no contraction. The two wires of a Daniell 

 battery are connected with each side of the Du Bois key, and the 

 current is allowed to pass through the nerve for several seconds. 

 Then these two wires are rapidly disconnected from the battery 

 and key ; the key is closed and opened, and each time a contraction 

 of the muscles of the leg is caused. This make and break can be 

 repeated several times with a similar result, until the polarisation 

 has disappeared. 



This experiment shows the necessity of unpolarisable electrodes in 



experiments upon the effects 

 produced in nerve and muscle 

 by the passage of a constant 

 electric current, and also the 

 necessity of using a Du Bois 

 key as a bridge to short-circuit 

 the electrodes. 



Unpolarisable Electrodes. 



The preceding experiment has 

 shown that the electrolysis 

 occurring around the ordinary 

 metal electrodes may easily act 

 as an exciting electric current, 

 and thus cause errors in experi- 

 ments. In order to avoid this 

 unpolarisable electrodes are used. The electric current from the 

 battery is conducted through media which are not liable to 

 polarisation. 



The structure of Burdon- Sanderson's electrodes is shown in the 

 following diagram (Fig. 112). A smooth amalgamated zinc rod dips 

 into a saturated solution of zinc sulphate, which in turn conducts the 

 current by means of a plug of kaolin or china clay, made into a thick 

 paste with normal saline solution (-75 per cent, sodium chloride). 

 The plug rests upon a small glass tube with a flange ; this delays the 

 spread of the zinc sulphate into the kaolin. The nerve or muscle can 

 be placed in contact with the plug of kaolin, or may be connected 

 thereto by threads saturated with normal saline solution and kaolin. 

 The plug must be kept moist with normal saline solution, for the 

 electrodes have a high resistance. 



The electrodes must be set up with clean hands and material, 

 otherwise polarisation will occur. The solution of zinc sulphate 

 must not be allowed to touch the tissue, for chemical excitation 

 would occur. Kaolin and normal saline solution do not stimulate 

 muscle and nerve. 



FIG. 112. Unpolarisable electrode. 

 Burden-Sanderson's pattern. 



