THEORY OF NERVE-ACTIVITY. 259 



are, that the muscle is contractile, which the nerve is 

 not, and that electrotonus, which intervenes in nerve, 

 cannot be shown in muscle), may also be entirely simi- 

 lar in the matter of irritability, and that those who dis- 

 pute this quality are forced to assume the existence of 

 a substance intermediate between that of the nerve 

 and of the muscle, and which differs almost more from 

 the nerve than from the muscle. 



8. Summing up, it appears that the independent 

 irritability of muscle-substance has not been proved ; 

 nor has it been disproved. To understand how the 

 nerve acts on the muscle one must assume that the 

 latter is irritated by the former, and therefore there is 

 no sufficient reason, remembering the similarity in all 

 other points between nerve and muscle, to dispute that 

 it may also be irritated by other irritants (electric, 

 chemical, mechanical, or thermic). In the theory above 

 explained as to the nature of the influence on the 

 muscle, we have assumed that this irritation takes 

 place electrically. We have therefore tacitly presup- 

 posed that the muscle is electrically excitable. Except 

 on this assumption, all that can be said is that the 

 molecular process originating in the nerve is trans- 

 ferred to the muscle : which explains nothing, but rather 

 renounces all explanation. Our hypothesis, on the other 

 hand, has the undeniable advantage that it is based 

 on the well-known process of the negative variation of 

 the nerve during its activity. That the negative varia- 

 tion, when it has once originated in the nerve, propa- 

 gates itself to the nerve-ends, can only be regarded as 

 natural, and, provided that it is of sufficient strength, 

 it can then act as an irritant on the muscle. 



We have already seen that the nerve must be 



