ADVANCED EXPERIMENTAL PHYSIOLOGY 



313 



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 

 experiments. 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. 190). 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 solu- 

 tion ('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 arid 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, other- 

 wise 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. 



The previous experiment on the polarisation of electrodes should be 

 repeated with the unpolarisable electrodes. The result will be negative 

 if the electrodes have been well and truly made. 



FIG. 190. Unpolarisable electrode. Burdon- 

 Sanderson's pattern. 



CHAPTER X. 

 TRANSMISSION OF A NERVOUS IMPULSE IN BOTH DIRECTIONS. 



THE excitatory state produced by stimulation of a nerve can be 

 transmitted in both directions. This can be shown by the following 

 experiments. 



Sartorius Experiment. The sartorius muscle is dissected out and its 



