264 PHYSIOLOGY 



strate the production of excitation at cathode on make, and at anode on 

 break of a constant current. 



A muscle with parallel fibres, such as the sartorius, is injured at one 

 end, and a constant current passed, first from the injured to the uninjured 

 end, and then in the reverse direction (Fig. 112). It is found in the former 

 case, when the anode is on the injured part (which is therefore less excitable), 



that break of the current is ineffec- 

 tive, and in the latter, when the 

 contraction at make. cathode is on tne injured surface, 



that the make stimulus is ineffec- 

 tive, showing that the part excited 



kath d8 ^ ln J ured ^ corresponds to the cathode at make 



T"= J no contraction at make. and to the anode at break 



* Xv ^|^-v'V With a current of very short 



b duration no excitation is produced 



Fio. 112. Diagram to show the effect of local, f IT EVPTV induction shork 

 injury on the excitability of a muscle. &, at break ' - KjV6r 3 r 



battery ; m. muscle. The arrows indicate can be therefore regarded as a 

 the direction of the current. make stimulus, and when such a 



shock is led through a muscle the contraction in each case will start at 

 the cathode, i.e. the point at which the induction shock leaves the muscle. 

 The results of stimulating nerve fibres are similar to those obtained by 

 stimulating muscle fibres directly. 



Under normal circumstances, if a constant current be passed through the 

 nerve of a nerve-muscle preparation for a short time, the muscle responds 

 only at the make and the break of the current, remaining perfectly quiescent 

 all the time the current is passing. If the nerve be in a very excitable 

 condition, it is possible that the muscle may be thrown into a tetanus or 

 continued contraction during the whole time that the current is passing 

 (' closing tetanus '). On the other hand, if a strong ascending current be 

 passed through the nerve for a considerable time, the muscle when the current 

 is broken may go into continued contraction, which may last some time. 

 Normally the muscle simply responds with a single twitch at the make 

 and break of the current, although, on investigating the condition of the 

 nerve during the passage of the current, we find that it is considerably 

 modified. This modification in the condition of the nerve is spoken of as 

 electrotonus^nd includes change* in its irritability and its electrical condition. 



To investigate these changes the following apparatus is necessary : two constant 

 batteries, induction-coil, a reverser and keys, a pair of non-polarisable electrodes, 

 and a pair of ordinary platinum electrodes. Fig. 113 represents roughly the arrange- 

 ment of the experiment. A constant current from the battery is led through a part 

 of the nerve by means of non-polarisable electrodes, which are about one inch apart. 

 In this circuit we put a reverser, by means of which the direction of the current of 

 the nerve may be changed at will, and a key to make or break the current. This is the 

 polarising circuit. The other battery is arranged in the primary circuit of the coil, a key 

 being interposed, so that we may use make or break induction shocks, which are applied 

 to the nerve by means of the small platinum electrodes. The tendon of the muscle 

 is connected by a thread with a lever, which is arranged to write on a smoked surface, 

 so that the height of the contraction can be recorded. 



