SECONDARY PULSATION A^D TETANUS. 211 



If the nerve of one muscle is placed on a second 

 muscle in such a way that no observable part of the 

 current passes through the former (as shown in the 

 nerve of the muscle (7, in fig. 58), no secondary pul- 

 sation takes place in the muscle. 



If the nerve of the first muscle is repeatedly irri- 

 tated in such a way that the muscle B passes into a 

 state of tetanus, then the muscle A assumes the con- 

 dition of secondary tetanus. This important experi- 

 ment shows that variations of electric activity take 

 place in rapid succession in tetanised muscle. For it 

 is only owing to such rapidly succeeding variations in 

 the strength of the current that a persistent, tetanising 

 irritation can occur in the second nerve. Just as the 

 phenomenon of muscular tone led us to the conclusion 

 that muscle-tetanus, though the similarity in external 

 form is apparently complete, is not a state of rest, but 

 that the molecules of the muscle must be in constant 

 internal motion during tetanus, so we now find from 

 the phenomenon of secondary tetanus that throughout 

 its duration a continual variation occurs in its elec- 

 tric condition ; and from this we may infer that elec- 

 tric variation is connected with the motion of the 

 molecules which causes contraction. 



More detailed study of negative variation has also 

 shown that it occurs even in the stage of latent irri- 

 tation, that is, at a time at which the muscle has not 

 yet altered its external form in any way. It has also 

 been found that the electric change which occurs on 

 irritation propagates itself when the muscle-fibre is 

 partially irritated at a rate equal to that of the propa- 

 gation of the contraction (from 3 to 4 m. per second : 

 cf. ch. vi. § 5, p. 100). When, therefore, a muscle- 



