420 THE FUNCTIONS OF CROSS-STRIATED MUSCLES 



Often it is necessary to have the stimuli follow one another very rapidly. 

 The device most commonly employed for this purpose is that known as the 

 Wagner hammer (Fig. 159). The current, starting from the battery K, passes 

 through the post g, the spring h provided with an armature, and the screw f to 

 the primary coil c, and from there through the electro-magnet b back to the 

 battery. If the current is closed at the screw f, & is magnetized and draws the 

 armature of the spring h down ; in this way the current is broken at f , the mag- 



FIG. 158. Induction coil of Du Bois-Reymond, after Porter. The strength of the induced cur- 

 rents is varied by sliding the secondary coil on the horizontal bars and also by revolving it 

 about its axis. 



net consequently is demagnetized, the spring h is released until it again touches 

 f , when the current is once more closed, and so on. The number of interruptions 

 per second can be varied by the position of the screw f. The make and break 

 shocks from such an interrupter are not, however, of equal strength. In order 

 to equalize them a side wire is inserted between g and f and the screw at f is 

 raised until the hammer can no longer touch it. The screw f on the other 

 hand, is raised so that the spring in its downward motion comes in contact 

 with it. Now when the hammer vibrates the primary current is never entirely 

 broken, but varies between two extreme values. Consequently the make and 

 break shocks are weaker but (for reasons which we cannot go into here) they 

 are also more nearly equal in strength. 



2. The General Law of Electrical Stimulation. All the effects of an elec- 

 tric current upon the medium through which it flows depend upon the strength 

 and the density of the current. With the same conductor the density of course 

 is directly proportional to the strength. 



In 1843 Du Bois-Reymond, on the basis of his discoveries concerning the 

 electrical stimulation of motor nerves, laid down the following general law: 

 The electric current does not stimulate by means of its absolute density, but 

 by means of the alterations which it undergoes from one moment to another; 

 hence the impetus toward a movement which results from these alterations 

 is greater the more rapidly they occur, or the more extensive the alteration 

 in a unit of time. The contraction of a muscle produced by an increasing 

 density of the current was called the " closing contraction" that produced by 

 decreasing density, the " opening contraction." 



This law was supported by such facts as the following: a current passing 

 through a nerve may, if increased very gradually, reach a high density without 



