4io 



THE PROPERTIES OF STRIPED MUSCLE. 



is represented in Fig. 224. A glass U-tube u, containing saturated solution of 

 zinc sulphate, is supported by a brass block b and holder which slides on a 

 horizontal vulcanite rod r. The block also supports an L-shaped zinc wire 

 L, of which the arm dips into the solution in the U-tube. It is also provided 

 with a terminal, for attaching the wire by which the electrode is con- 

 nected with the circuit. The arm of each zinc rod is flattened by hammer- 

 ing, at the end which dips into the zinc solution, so as to take the form of the 

 blade of an oar, and its surface is carefully smoothed, and then amalgamated. 

 The straight tube T (too narrow in the figure) is of such width that it fits 

 loosely into the other end of the U-tube ; it is prevented from slipping in by 

 the flange /. It is charged with paste, made by kneading kaolin with the zinc 

 solution. To prepare the electrodes for use, a lump of kaolin paste k, made 

 with 0'6 per cent, solution of sodium chloride, is stuck on the summit 

 of the straight tube, a groove being made across the top of this lump with an 

 ivory knife, in which the end of a "cable" of cotton wick thoroughly soaked 

 in physiological salt solution is placed. By this means the clay is brought 

 into connection with the surface of the muscle to be examined. 



Before establishing this connection, it is necessary to test the electrodes by 

 making contact between the two lumps. If the galvanometer is sufficiently 

 sensitive, some current will generally be indicated, the electromotive force 



and direction of which can be 

 readily ascertained with the aid 

 of the compensator. If it is con- 

 siderable (electromotive force ex- 

 ceeding Yoijoo v °lt)> f ne electrodes 

 must be remade. To avoid mis- 

 takes of direction, it is desirable 

 to label the electrodes N and S 

 respectively — it being understood 

 that the wire of the N electrode 

 runs to the north terminal of the 

 galvanometer, and accordingly that 

 a current flowing therefrom is 

 indicated by a movement of the 

 spot of light southwards. 



In exploring a muscle in the 



Fig. 224. — Non-polarisableelectrode. 



resting state galvanometrically, the differences of potential between different 

 parts of its surface are measured in hundredths of the divided metre of the 

 compensator, i.e. in millivolts. In such observations the " muscle current " is 

 balanced by a current of opposite direction from the compensator, by working 

 the slider until the galvanometer is at zero. 



When the capillary electrometer is substituted for the galvanometer, the 

 arrangement of the circuit is the same. But, with the aid of a scale by which 

 the excursion of the mercurial meniscus from its normal or zero position is 

 measured, the observer is enabled to determine approximately the difference of 

 potential between the two contacts, provided that the value of the graduation 

 in decimals of a volt is known. 



In the gastrocnemius, as has been indicated above, however carefully 

 it is prepared, a considerable difference of potential usually presents 

 itself between the tendon end and the most positive part (Fig. 224,/) 

 of the natural surface, notwithstanding that Hermann found that 

 under conditions which absolutely exclude sources of injury this differ- 

 ence is inappreciable. The difference, when present, diminishes as the 

 electrode on the tendon is brought step by step nearer to the other. 

 Tn uninjured parallel-fibred muscles, on the other hand, the differences 



