296 PHYSIOLOGY 



electrode by which the current leaves the muscle rises before the 

 other lever. On the other hand, on breaking the current the lever 

 at the anode rises first, showing that the anodic half of the muscle 

 contracts before the cathodic half. 



(6) The irritability of a muscle, i.e. its power of responding to a 



stimulus by contracting, is inti- 

 mately dependent on the life of 

 the muscle. If the muscle be 

 injured or killed at any spot, its 

 irritability at this spot will be 

 therefore diminished or de- 

 Fio.112. Sartorius clamped in middle and stroyed. Hence, if we stimulate 



attached to levers at either end. a muscle at the injured Spot, no 



contraction will ensue. This fact 



may be used to demonstrate 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. 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 ineffective, 

 and in the latter, when the 

 cathode is on the injured sur- 

 face, that the make stimulus 

 is ineffective, showing that 

 the part excited corresponds 



kathode 



anode 



nn 



anode(injured) 



contraction at make. 



kathode (injured) 

 no contraction at make. 



FIG. 113. Diagram to show the effect of local 

 injury on the excitability of a muscle. 

 b, battery ; m, muscle. The arrows indicate 

 the direction of the current. 



to the cathode at make and 

 to the anode at break. 



With a current of very 

 short duration no excitation 

 is produced at break. Every 

 induction shock can be therefore regarded as a 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 



