MUSCLE-NERVE PHYSIOLOQY. 



503 



in one direction or another, the strength and direction of the currents in the 

 piece of muscle can be estimated. It is necessary to use non-polarizable and 

 not metallic electrodes in this experiment, as otherwise there is no certainty 

 that the whole of the current observed is communicated from the muscle itself, 

 and is not derived from the metallic electrodes arising in consequence of the 

 action of the saline juices of the tissues upon them. The form of the non- 

 polarizable electrodes is a modification of Du Bois Reymond's apparatus (fig. 

 318) , which consists of a somewhat flattened glass cylinder, a, drawn abruptly 



Fig. 317. Reflecting galvanometer. (Thomson.) A. The galvanometer, which consists of 

 two systems of small astatic needles suspended by a fine hair from a support, so that each set of 

 needles is within a coil of fine insulated copper wire, that forming the lower coil is wound in an 

 opposite direction to the upper. Attached to the upper set of needles is a small mirror about 

 14 inch in diameter ; the light from the lamp at B is thrown upon this little mirror, and is re- 

 flected upon the scale on tfie other side of B, not shown in figure. The coils I I are arranged 

 upon brass uprights, and their ends are carried to the binding screws. The whole apparatus is 

 placed upon a vulcanite plate capable of being levelled by the screw supports, and is covered 

 by a brass-bound glass shade, L, the cover of which is also of brass, and supports a brass rod, 

 6, on which moves a weak curved magnet, m. C is the shunt by means of which the amount of 

 the current sent into the galvanometer may be regulated. When in use the scale is placed 

 about three feet from the galvanometer, which is arranged east and west, the lamp is lighted, 

 the mirror is made to swing, and the light from the lamp is adjusted to fall upon it, and it is 

 then regulated until the reflected spot of light from it falls upon the zero of the scale. The 

 wires from the non-polarizable electrodes touching the muscle are attached to the outer binding 

 screws of the galvanometer, a key intervening for short-circuiting, or if a portion only of the 

 current is to pass into the galvanometer, the shunt should intervene as well with the appropriate 

 plug in. When a current passes into the galvanometer the needles and, with them, the mirror, 

 are turned to the right or left according to the direction of the current. The amount of the de- 

 flection of the needle is marked on the scale by the spot of light travelling along it. 



to a point, and fitted to a socket capable of movement, and attached to a stand, 

 A, so that it can be raised or lowered as required. The lower portion of the 

 cylinder is filled with china clay moistened with saline solution, part of which 

 projects through its drawn-out point ; the rest of the cylinder is fitted with a 



