252 



PHYSIOLOGY 



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-polarisable. A more convenient form is that employed -by Burdon 

 Sanderson, in which the glass tube is bent into a U (Fig. 77). 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 current through the 

 nerve or muscle to the metallic part of the circuit may be represented 

 as follows : 



Zn 



Zn Na 



SO* CL 



Na Zn 

 CL SO* 



f 



Zn 



> 



FIG. 78. 



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

 two non-polarisable electrodes connected with a delicate galvano- 

 meter 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 the ends to the middle, and in the 

 external circuit from the middle (or equator) to the ends. It was 

 formerly thought that this current was always present in all normal 

 muscles, and it was spoken of as the " natural muscle current " ; 

 the muscle was said to be made up of a series of electromotive mole- 

 cules, the equator of each molecule being positive to the two poles 

 (du Bois Raymond). It has been conclusively shown, however (by 

 Hermann and others), that this current of resting muscle is not a 

 natural current at all, but is due to the effects of injury in making 

 the preparation. The less the preparation is injured, the smaller 

 is the current to be obtained from it, and in some contractile tissues, 

 such as the heart, there may be absolutely no current during 

 quiescence. 



Hermann describes the fact of the existence of currents of rest 

 thus : "In partially injured muscles every point of the injured part 

 is negative towards the points of the uninjured surface." Fig. 79 

 shows the direction of the current in a muscle with two cut 

 ends. When the whole muscle is quite dead, this current of rest, 



