454 MANUAL OF PHYSIOLOGY. 



current acts as a stimulus, but a current of exactly even intensity 

 may be made to pass through a muscle without exciting any 

 contraction. The common method employed is that of opening 

 or closing a circuit of which the muscle forms a part, so as to 

 make or break the current ; and thus a variation of intensity, 

 equal to the entire strength of the current used, takes place in 

 the muscle, and acts as a stimulus. 



The irritability of muscle substance is not so great as that of 

 the motor nerves, that is to say, a slight stimulus will make the 

 muscle contract when applied to its nerve, while the same stimulus 

 will have no effect if applied to the muscle directly. In experi- 

 menting on the contraction of muscle, as already stated, the 

 intervention of the nerve is commonly used, the stimulus, by 

 means of an electric current applied to the nerve, being more 

 conveniently and completely distributed to the muscle than when 

 applied directly. 



The current of a battery may be used to stimulate a muscle, 

 but an induced current is more commonly employed on account 

 of the greater efficacy of its action. The instrument in ordinary 

 use in physiological laboratories is Du Bois-Reymond's inducto- 

 rium, in which the strength of the stimulus can be reduced by 

 removal of the secondary coil, and which is supplied with a mag- 

 netic interrupter, by means of which repeated stimuli may be 

 given. (See Fig. 183.) 



CHANGES OCCURRING IN MUSCLE ON ITS ENTERING THE 

 ACTIVE STATE. 



Changes in Structure. The examination of muscle with the 

 microscope during its contraction is attended with considerable 

 difficulty, and in the higher animals has not led to satisfactory 

 results. In the muscles of insects, where the differentiation of 

 the contractile substance is more complicated, certain changes 

 can be observed. The fibres, and even the fibrillse within them, 

 can easily enough be seen to undergo changes in form correspond- 

 ing to those of the entire muscle, namely, increase in thickness 

 and diminution in length. A change in the position and relative 

 size of the singly and doubly refracting portions of the muscle 



