178 



ELECTRO-PHYSIOLOGY 



CHAP. 



Then, after permanently fixing one half of the shell, 11011- 

 polarisable brush electrodes can be applied on both sides, as near 

 as possible to the insertion of the muscular band (which usually 

 consists of parallel fibres), while, in order to prevent any shifting 

 of the electrode corresponding with the other movable half of the 

 shell, the current at this point is best led in by a short loop of 

 thread. 



If a current of adequate strength is then sent through the 

 relaxed muscle, changes of form may be observed which, apart 

 from the sluggishness of reaction more or less characteristic of 

 all smooth muscle, concur on the whole with those exhibited by 

 striated muscle under analogous conditions. As regards form 



and process of contraction during 

 closure of the current, the resulting 

 curve will of course rarely corre- 

 spond with the process designated in 

 re the time-distribution of the contrac- 

 tions of striated muscle, the " make 

 twitch." Apart from the slowness 

 with which the whole process occurs, 

 the difference of duration between 

 the contraction and relaxation phases 

 (periods of rising and falling energy) 

 is much more marked in smooth 

 molluscan muscle, which gives a dis- 

 tinct and peculiar character to its 

 curve of contraction. Two cases 

 must here be distinguished, that in which the current is opened 

 before, or as soon as, the muscle has reached its maximum of 

 shortening, and that in which there is a long period of closure. 

 In the first case, at least under certain conditions, e.g. with 

 warmed and therefore quickly reacting preparations, curves are 

 obtained, which from their form and process might be regarded as 

 extended twitch curves, since not only does the shortening rapidly 

 rise to a considerable height, but the relaxation also occupies a 

 comparatively short time (Fig. 74). 



In other cases a longer closure of the circuit produces 

 curves which rise abruptly at the moment of closure, without 

 sinking down again, corresponding with a persistent and uniform 

 shortening of the muscle. Under these conditions the closure 



FIG. VS. Schema of electrical excitation 

 in adductor muscle of molluscs. 



