.HAP. ii CHANGE OF FORM IN MUSCLE DURING ACTIVITY 55 



equally rapid contraction, with much slower subsequent elongation. 

 A rapid contraction of this kind, which is especially characteristic 

 of striated muscle, is termed a " twitch." This elementary form 

 of activity is, however, by no means peculiar to muscle, since, on 

 the one hand, the rhythmical, or even non-rhythmical, movements 

 of a ciliated element may be regarded as consisting of single, con- 

 secutive twitches, many flagella also " twitching " in contraction ; 

 and, on the other, many muscles the uninuclear, smooth muscle- 

 cells in particular contract so slowly that it is as impossible to 

 speak of " twitch " in these as in the far more sluggish contraction 

 of the pseudopods in most Ehizopoda. 



A " twitching " contraction seems invariably to denote the 

 presence of fibrillated structure (more especially with cross- 

 striation), although, as we see in smooth muscle-cells, the differ- 

 entiation of fibrils does not on the other hand invariably produce 

 a very rapid contraction. 



In the most characteristic case of the " twitch," contraction, 

 as far as can be seen, begins simultaneously with the cause of 

 excitation, reaches its maximum as quickly as possible, and then 

 dies out again in slow relaxation. The very marked difference 

 which appears between the duration of contraction on the one 

 hand, and elongation on the other, in the contractile, twitching 

 parts of the lowest animal-forms (stem of vorticella, spirostomum, 

 myopodia, etc.), is due in great part to the peculiar mechanical 

 relations which here govern contraction and relaxation. The 

 twitching fibrils behave more or less like an unloaded muscle, 

 swimming in mercury, which only recovers its normal length 

 when an extending force is acting upon it. The course of a 

 single twitch is usually so rapid that it is impossible, from direct 

 observation, to detect any minutiae in regard to the time-relations 

 of the contraction, and behaviour of the contractile fibres, in 

 the individual stages of shortening. Finer artificial means of 

 measuring time are necessary in order to ascertain the relations 

 of the different phases within the brief act of a single contrac- 

 tion. 1 



As a means of measuring such minute intervals as are here 

 under consideration, preference must undoubtedly be given to the 



1 Cf. v. Bezold, Uiitcrsuclivncjcn fiber <Uf rj^lirisclic Erregung der Nerven u. Mus- 

 A-cfn, 1861, p. 31. (Historical survey of attempts to measure the minute time- 

 intervals occupied by nerve and muscle action.) 



