THE FREEING OF FORCES. 249 



excite not only the muscle-fibre in which the nerve 

 ends, but the adjacent fibres also. For in the muscle 

 and its envelopes no electric isolators are present, and 

 an electric shock, occurring at any point, can and must 

 spread throughout the whole muscle mass. But from 

 the law of the distribution of currents in irregular con- 

 ductors, the essential outlines of which are given in the 

 twelfth chapter, it appears that the strength of the cur- 

 rent in the immediate neighbourhood of the spot at 

 which the discharge actually takes place may be con- 

 siderable, though it decreases so rapidly with increasing 

 distance, that it is easy to believe that it may be quite 

 unnoticeable, even in a muscle-fibre which stands side 

 by side with the fibre directly irritated. It is this 

 very circumstance which lends especial weight to the 

 fact that the nerve penetrates within the muscle-fibre, 

 and there comes into immediate contact with the muscle- 

 substance. Only in this way is it intelligible that a 

 discharge occurring in the nerve can irritate the muscle. 

 When the excitement has once arisen at any point 

 within the muscle-substance, it can, as we have seen, 

 spread within the muscle-fibre. It is possible that this 

 may result without any co-operation of the nerve-sub- 

 stance ; so that the spreading of the nerve within the 

 muscle-substance, as claimed by Gerlach, is not required 

 to explain the processes within the muscle. 1 



4. We therefore assume that the excitement aris- 

 ing in the nerve itself becomes an irritant, which 

 then irritates the muscle. The forces which are gene- 

 rated, in consequence of this, in the muscle are, as we 

 know, able to accomplish considerable labour, which 

 bears no relation to the insignificant forces which act 



1 See Notes and Additions, No. 15. 

 12 



