CH. XV.] LAW OF CONTRACTION IN MAN 177 



In the normal state, nerves can be stimulated through the 

 moistened skin either by induction shocks, or by the make and break 

 of a constant current. In the case of the motor nerves this is shown 

 by the contraction of the muscles they supply ; and in the case of the 

 sensory nerves by the sensations that are produced. In the case of 

 the sensory nerves, the sensation produced by the constant current 

 is most intense at the instant of make and break, or when the 

 strength of the current is changed in the direction either of diminution 

 or increase; but there is a slight sensation due doubtless to the 

 electrotonic alterations in excitability which we have been studying, 

 during the whole time that the current is passing. 



When the nutrition of the nerves is impaired, much stronger 

 currents of both the induced and constant kinds are necessary to 

 evoke muscular contractions than in the normal state. When the 

 nerves are completely degenerated (as, for instance, when they are cut 

 off from the spinal cord, or when the cells in the cord from which 

 they originate are themselves degenerated, as in infantile paralysis) 

 no muscular contraction can be obtained on stimulating the nerves 

 even with the strongest currents. 



The changes in the excitability of the muscles are less simple, 

 because in them there are two excitable structures, the terminations 

 of the nerves, and the muscular fibres themselves. Of these, the 

 nerve-fibres are the more sensitive to induction currents, and the 

 faradic stimulation of a muscle under normal circumstances is by 

 means of these motor nerve-endings. Thus we find that its excita- 

 bility corresponds in degree to that of the motor nerve supplying it. 

 The muscular fibres are, even in the normal state, less sensitive to 

 faradism (that is, a succession of induction shocks) than the nerve, 

 because they are incapable of ready response to stimuli so very short 

 in duration as are the shocks of which a faradic current consists. 

 The proof of this consists in the fact that under the influence of 

 curare, which renders the muscle practically nerveless, the muscle 

 requires a much stronger faradic current to stimulate it than in the 

 normal state. When the nerve is degenerated, the make or break 

 of the constant current stimulates the muscle as readily as in the 

 normal state; but the contraction is propagated more slowly than 

 that which occurs when the nerve-fibres are intact, and is due to the 

 stimulation of the muscular fibres themselves. The fact that, under 

 normal circumstances, the contraction which is caused by the constant 

 current is as quick as that produced by an induction shock, is ground 

 for believing that in health the constant, like the induced current, 

 causes the muscle to contract chiefly by exciting the motor nerves 

 within it. 



When the motor nerve is degenerated, and will not respond to 

 any form of electrical stimulation, the muscle also loses all its power of 



M 



