PHYSIOLOGIC APPARATUS. 



707 



The upper surface of the cell is sealed to prevent evaporation. The electric- 

 ity is generated at the surface of the zinc cup by the chemic action of the 

 chlorin which arises from the dissociation of the ammonium chlorid. When 

 the plates are united by a conjunctive wire the current within the cell flows 

 from the zinc (the positive element) to the carbon (the negative element), 

 and without the cell from the carbon (the positive pole) to the zinc (the 

 negative pole). 



Leads. By means of insulated wires attached to the poles of a cell, the 

 electricity may be conducted from the cell and used for exciting or stimulat- 

 ing purpose. As the wires thus become practically prolongations of the 

 plates their ends become the corresponding poles. In experimental work 

 the ends of the wires are provided with special devices, termed 



Non-polarizable electrodes. The necessity for the employment of 

 such electrodes arises from the fact that when the ends of the wires from a 

 cell are placed in direct contact with the tissues chemic changes are produced 

 in a short time, which lead to their polarization. As a result, a current 

 opposite in direction to that of the cell is 

 developed, which tends to weaken or neu- 

 tralize it. This polarization current vitiates 

 the result of many experiments made with 

 highly irritable tissue such as nerve-tissue. 

 Whether for stimulating purposes or for the 

 purpose of detecting the existence of electric 

 currents in living tissues, it is essential that 

 the electrodes used shall be non-polarizable. 

 The earliest electrodes of this character were 

 made by du Bois-Reymond and were based 

 on the fact discovered by Regnault that a 

 strip of chemically pure zinc or amalga- 

 mated zinc (Matteucci) immersed in a satu- 

 rated solution of zinc sulphate would not 

 polarize. One form made by du Bois-Rey- 

 mond is shown in Fig. 357. It consists of 

 a flattened glass tube attached to a universal 

 joint and supported by an insulated brass 

 stand. The lower end of the tube 

 closed with kaolin or China clay made into 

 a paste with a 0.6 per cent, solution of sodium chlorid. It can be moulded 

 into any desired shape. The interior of the tube is partially filled with a 

 saturated solution of sulphate of zinc in which is immersed the strip of amal- 

 gamated zinc. To the upper end of the zinc the conducting wire is attached. 



The v. Fleischl brush electrode is similar to the preceding except that 

 the end of the tube is closed by the brush of a camePs-hair pencil. 



The d'Arsonval electrode consists of a glass tube containing a silver rod 

 coated with fused silver chlorid. The interior of the tube is filled with 

 normal salt solution 0.6 per cent, and the end closed with a thread or plug 

 of asbestos which is made to project beyond the tube for a short distance. 



Any one of these three electrodes is suitable for physiologic experimenta- 

 tion, as their free ends neither corrode the tissues nor develop electric currents. 



G - 357 NON-POLARIZABLE ELEO 

 1S TRODES. i. Du Bois-Reymond 's. 2. 

 Von FleischPs. 3. d' Arson vaPs. 



