THE PROPERTIES OF MUSCULAR TISSUE. 139 



its thinner ends die more quickly than its central parts, or 

 the ends are directly injured when the muscle is cut across to 

 remove it from the animal; and in that way the currents so 

 usually observable arise. When all of a muscle is dead, its 

 surface is isoelectric; no currents can be led oif from it. 



Even a quite uninjured muscle is however, capable, of giv- 

 ing rise to currents when it contracts, and these currents 

 pass in such direction as to show that a portion of muscle 

 in contraction is electronegative to a portion at rest. If a 

 curarized muscle be stimulated at one point, its contraction 

 commences at that point and travels from it over the remainder 

 of the muscle; so that by the time a distant portion is in con- 

 traction the part which just contracted has come to rest. By 

 electrodes suitably applied it can be observed that immedi- 

 ately after the stimulation the region of muscle close to the 

 point of stimulation is electro-negative to a more distant part; 

 but that afterwards, when a distant portion is in contraction 

 and the stimulated region has returned to rest, the reverse is 

 the case. Electrically, therefore, any contracting part of a 

 muscle has to any resting part a relation similar to that of a 

 dying or injured part of a muscle to an uninjured. The cur- 

 rents which arise in consequence of the changes going on in 

 contracting muscle are known as the action currents to dis- 

 tinguish them from the resting currents due to unequal rates 

 of death usually found between different parts of an exposed' 

 muscle in rest. 



When a muscle is stimulated through its nerve the action 

 current is less easy to demonstrate, because the nerve fibres 

 branch all through the muscle and stimulate all parts of it at 

 once, and throw all simultaneously into contraction. The cur- 

 rent may, however, be shown indirectly. A muscle is removed 

 with its nerve attached and electrodes put on it one, for ex>- 

 ample, on the middle of the belly and the other on the tendon, 

 so as to show on the galvanometer a resting current. If the 

 muscle be now made to contract by stimulating its nerve the 

 current is diminished, or, as is said, shows a negative varia- 

 tion. The cause of this is as follows : The amount of resting 

 current depends on the difference between the less injured 

 belly of the muscle and the injured end; anything which 

 makes these two less different electrically must diminish this 

 current; and as contracted muscle is electrically like dying 

 muscle, when we throw the whole into activity the previously 



