CHANGE IN SHAPE DURING MUSCULAR CONTRACTION 



449 



muscle. The second part of the record shows the contraction phase proper. 

 The lever is raised by the sudden shortening of the muscle. The contrac- 

 tion is at first very rapid, but then progresses more slowly to its maximum. 

 It occupies on an average 0.04 of a second in the frog's gastrocnemius. The 

 third stage is the relaxation phase. After reaching its highest point, the lever 

 begins to descend, in consequence of the elongation of the muscle. At first 

 the fall is rapid, but it then becomes more gradual until the lever reaches the 

 abscissa or base line, when the muscle has attained its precontraction length. 

 The stage occupies 0.05 of a second. Usually after the contraction proper 

 is over the lever oscillates below and above the base line in a series of dimin- 

 ishing waves, the elastic rebound following movement of the simple contrac- 

 tion. These are, of course, wholly passive and would occur equally well if 

 we should lift the weight to the height of the contraction, then simply let it 

 fall while taking a record. 



Change in Shape during Muscular Contraction. There is a consider- 

 able difference of opinion as to the mode in which the transversely striated mus- 

 cular fibers contract. The most probable account is that the contraction is 



FIG. 321. The Microscopic Appearances During a Muscular Contraction in the Individual 

 Fibrillae, after Engelmann. i. A passive muscle-fiber; c to c/=doubly refractive discs, with median 

 disc a & in it; k and g are lateral discs; f and e are secondary discs, only slightly doubly refractive; 

 figure on right same fiber in polarized light. The bright part is doubly refracted, black ends not 

 so. 2. Transition stage. 3. Stage of entire contraction. In each case the right-hand figure repre- 

 sents the effect of polarized light. (Landois, after Engelmann.) 



effected by an approximation of the constituent parts of the fibrils, which, at the 

 instant of contraction, without any alteration in their general direction, become 

 closer, flatter, and wider, a condition which is rendered evident by the approxi- 

 mation of the transverse striae seen on the surface of the fasciculus, and by its in- 

 creased breadth and thickness. The appearance of the zigzag lines into which 

 it was supposed the fibers are thrown in contraction is due to the relaxation 

 of a fiber which has been recently contracted and is not at once stretched again 

 by some antagonist fiber, or whose extremities are kept close together by the 

 contractions of other fibers. The contraction is therefore a simple and, ac- 

 cording to Edward Weber, a uniform, simultaneous, and steady shortening 

 of each fiber and its contents. What each fibril or fiber loses in length, it gains 

 in thickness. The contraction is a change of form, not of size; it is, therefore, 

 not attended with any diminution in bulk from condensation of the tissue. 

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