

ACT OF MUSCULAR CONTRACTION. 209 



similar length of a non-contracted fibre. According to Mr. Bowman's 

 observations, the contraction usually commences at the extremities of 

 a fibre; but it may occur also at one or more intermediate points. 

 The first appearance of. contraction is a dark spot, caused by the ap- 

 proximation of the striae ; and this gradually extends itself, so as to 

 involve a greater or less proportion of the length of the fibre. The 

 approximation of the solid portions forces out the fluid, which was 

 previously contained amongst the fibrillse; and this is seen to lie in 

 bullse or blebs beneath the myolemma, which is drawn up into wrinkles. 



355. The successive stages of the act of contraction can only be 

 thus observed, when it takes place very slowly, as in the rigor mortis, 

 or slow contraction after death, the phenomena of which will be pre- 

 sently noticed ( 367). But the resulting change in muscular fibres, 

 which haye been made to contract by galvanism or any other stimulus, 

 is essentially the same. This may be best seen in transparent Entozoa, 

 Crustacea, and others among the lower Articulated Animals, whilst 

 alive. Again, in persons who have died from Tetanus, a considerable 

 number of the fibres are found to have been ruptured by violent spas- 

 modic action ; the contractile force, called into action by the powerful 

 stimulation of the nerves, having overcome the tenacity of the fibre: 

 and in such cases, the same approximation of the transveise striae, 

 and proportional increase in the diameter of the fibre, are to be 

 observed. 



356. It appears that, even when considerable force of contraction is 

 being exerted, the whole fibre is seldom or never in contraction at 

 once ; but that a continual interchange is taking place amongst its 

 different parts, some of them passing from the contracted to the 

 relaxed state, as shown by the separation of the transverse striae, 

 whilst others are taking up the duty, and passing from the relaxed to 

 the contracted condition, as shown by the approximation of the striae. 

 But it is not only among the different parts of the individual fibres, 

 that this interchange seems to take place. There is good reason to 

 believe, that, when a muscle is kept in a contracted state, by an effort 

 of the will, for any length of time, only a part of its fibres are in con- 

 traction at any one time ; but that a constant interchange of condition 

 takes place amongst them, some contracting while others are relaxing, 

 so that the entire muscle remains contracted, whilst the state of every 

 individual fibre may have undergone a succession of alterations. When 

 the ear is applied to a muscle in vigorous action, an exceedingly rapid, 

 faint, silvery vibration is heard, which seems to be attributable to this 

 constant movement in its substance. 



357. Thus it appears that the prolongation of the contraction of a 

 muscle, through any length of time, is not opposed to the fact that, in 

 the individual fibres, relaxation speedily follows contraction; but is 

 only a peculiar manifestation of it. The ordinary movements of the 

 Heart exhibit a different manifestation ; its fibres contracting simulta- 

 neously, and relaxing together, instead of alternating amongst them- 

 selves like those of a voluntary muscle. The occasional zigzag ar- 

 rangement of the fibres, which has been supposed to be their contracted 

 state, is really dependent upon the approximation of their extremities, 



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