CHANGE IN SHAPE DURING MUSCULAR CONTRACTION 



479 



It will be observed that after the stimulus has been applied, as indicated 

 by the vertical line St, there is an interval before contraction commences. 

 This interval, termed the latent period, when measured by the number of vi- 

 brations of the tuning-fork directly beneath, is found to be about o.oi of a 

 second. The latent period is longer in some muscles than in others and 

 differs also according to the condition of the muscle and the kind of stimulus 

 employed. During the latent period there is no apparent change in the 

 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 o . 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 o . 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 

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



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

 Individual Fibrillae, after Engelmann. i. A passive muscle fiber; c to d = doubly refractive 

 discs, with median disc a b in it; h and g are lateral discs;/ 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 contrac- 

 tion. In each case the right-hand figure represents the effect of polarized light. (Landois, 

 after Engelmann.) 



is 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 approximation of the transverse striae seen on the surface of the fasciculus, 

 and by its increased breadth and thickness. The appearance of the zigzag 



