SECTION VII 



ELECTRICAL CHANGES IN MUSCLE 



IF a current from a battery be passed between two plates of platinum immersed in 

 acidulated water or salt solution, electrolysis of the water takes place, bubbles of oxygen 

 appearing on the positive plate (anode), and bubbles of hydrogen on the negative 

 plate (cathode). If now we remove the battery, and connect the two plates (electrodes) 

 by wires with a galvanometer, a current passes through the galvanometer and water 

 in the reverse direction to the previous battery current. 

 This current is called the polarisation current, and is due 

 to the electrolysis of the water that has taken place. 

 The vessel in which the electrodes are immersed has in 

 fact become a galvanic cell, the platinum covered with 

 oxygen bubbles being the positive element, and that covered 

 with hydrogen bubbles the negative element. Exactly the 

 same process of electrolysis or polarisation takes place 

 when we pass currents through the tissues of the body by 

 means of metallic electrodes. 



Hence before we can study accurately the delicate 

 FIG. 75. Diagram of non- electrical changes that may occur normally in living 

 polarisable electrode. tissues, it is necessary to have some form of electrodes in 

 o,coveredwire;&,amal- which this polarisation will not occur. The 'non-polari- 

 gamated zinc rod ; c, sa ^i e electrodes which are most generally used for this 

 7^0 U I t' ' - Sa U l & G f P ur P se are m ade in the following way. A glass tube 

 zinc sulphate clay'; /, plug (Fig. 75) is closed at one end with a plug of kaolin made 

 of normal saline clay. into a paste with a saturated solution of zinc sulphate. 

 The rest of the tube is filled with a similar solution. 



Dipping into the zinc sulphate solution is a rod of pure zinc, amalgamated. Just 

 before use, a plug of china clay made with normal saline solution is put on the 

 end of the tube, so as to effect a connection between the zinc sulphate clay and the 

 nerve or muscle which it is desired to stimulate or lead off. In these electrodes 

 there is no contact of metals with fluids that 

 can produce dissimilar ions (e.g. hydrogen or 

 oxygen) at the surface of contact, and hence 

 they may be regarded as practically non-polari- 

 sable. A more convenient form is that employed 

 by Burdon Sanderson, in which the glass tube is 

 bent into a U (Fig. 76). The mouth of the tube is 

 closed by a smaller glass tube plugged with clay, 

 and bearing a plug of normal saline clay. 



In such electrodes the conduction of the cur- FlG ' 76 ' U ; sb fP^ non-polarisable 



electrodes, 

 rent through the nerve or muscle to the metallic 



part of the circuit may be represented as shown on the opposite page (see Fig. 77). 



If a muscle such as the sartorius be removed from the body, and two non- 

 polarisable electrodes connected with a delicate galvanometer be applied 

 to two points of its surface, there will be a deflection of the mirror attached 

 to the galvanometer, showing the presence of a current in the muscle from 



224 



