CH. xvi.] ELECTRICAL RESPONSES IN MAN. 185 



because here it forms a most valuable method of diagnosis in 

 cases of disease. The following account of this is chiefly an 

 abstract from Sir William Gowers' Manual of Diseases of the 

 Nervous System. 



In the normal state, nerves can be stimulated 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 

 arc 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 termina- 

 tions 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 q, muscle under normal circumstances 

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

 excitability 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 

 "f the muscular fibres themselves. The fact that, under normal 

 circumstances, the contraction which is caused by the constant 



