100 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



to touch the cut end (x) and the longitudinal surface (g) . When a is 

 stimulated, either directly or through its nerve, as represented in the 

 diagram, the negative charges that pass along the muscle fibers of 

 a with each stimulus cause action currents that will be led off 

 through the nerve of b from x to g. If the nerve is in a sensitive con- 

 dition it will be stimulated by the action currents and thus a series of 

 excitations will be sent into b corresponding exactly in rate with 

 the artificial stimuli given to the nerve of a. The rheoscopic 

 preparation may be used very beautifully to demonstrate the 

 action current in the contracting heart muscle. If the nerve of 

 b is laid upon the exposed beating heart of an animal, the muscle 

 of b will give a single twitch for each beat of the ventricle. An- 

 other interesting method of detecting the action currents, particu- 

 larly in nerves, is by means of the telephone. Wedenski has made 

 especial use of this method, the telephone being connected with 



Fig. 44. Schema to show the arrangement of a rheoscopic muscle-nerve preparation: 

 b. The rheoscopic muscle-nerve preparation, the nerve being arranged to touch the cut sur- 

 face and the longitudinal surface of the muscle, a, whose action currents are to be detected. 

 When the nerve of a is stimulated each contraction of this muscle is followed by a contrac- 

 tion of b, since each contraction of a is accompanied by an action current which passes 

 through the nerve of 6 and stimulates it. 



the nerve in place of the galvanometer. The method has obvious 

 advantages in the fact that it may be used with a nerve to which 

 the muscle is also attached, so that the excitation processes in 

 the nerve and their effect upon the muscle may be studied simul- 

 taneously. In matters of rate the telephone method, appealing 

 to the ear, as it does, is more delicate than the galvanometer or 

 electrometer. 



Relation of the Action Current to the Contraction Wave 

 in Muscle and to the Excitation Wave (Nerve Impulse) in 

 Nerve. The action current, or, to be more accurate, the moving 

 negative charge which gives rise to an action current when two 

 points of the muscle are led off to a galvanometer, has been shown 

 by Bernstein to precede the wave of contraction in muscle. That 

 is, in a stimulated muscle fiber the electrical change at any point 

 precedes the mechanical process of shortening. The electrical change 

 passes over the fiber from the point stimulated in the form of a 



