THE MUSCULAR SYSTEM. 417 



is due to the relaxation of a fibre which has been recently contracted, 

 and is not at once stretched again by some antagonist fibre, or whose 

 extremities are kept close together by the contractions of other fibres. 

 The contraction is therefore a simple, and, according to Ed. Weber, a 

 uniform, simultaneous, and steady shortening of each fibre and its con- 

 tents. What each fibril or fibre loses in length, it gains in thickness: 

 the contraction is a change of form not of size; it is, therefore, not at- 

 tended with any diminution in bulk, from condensation of the tissue. 

 This has been proved for entire muscles, by making a mass of muscle, 

 or many fibres together, contract in a vessel full of water, with which a 

 fine, perpendicular, graduated tube communicates. Any diminution of 

 the bulk of the contracting muscle would be attended by a fall of fluid 

 in the tube; but when the experiment is carefully performed, the level 

 of the water in the tube remains the same, whether the muscle be con- 

 tracted or not. 



In thus shortening, muscles appear to swell up, becoming rounder, 

 more prominent, harder, and apparently tougher. But this hardness of 



FIG. 294. The microscopic appearances during a muscular contraction in the individual fibrillae 

 after Engelmann, 1. A passive muscle fibre; c to d = doubly refractive discs, with median disc a b 

 in it; h and g are lateral discs; f and e are secondary discs, only slightly doubly refractive; fig. 

 on right same fibre in polarized light; bright part is doubly refracted, black ends not so. 2. Transi- 

 tion stage; and 3. Stage of entire contraction; hi each case the right-hand figure represents the 

 effect of polarized light. (Landois after Engelmann.) 



muscle in the state of contraction, is not due to increased firmness or 

 condensation of the muscular tissue, but to the increased tension to 

 which the fibres, as well as their tendons and other tissues, are subjected 

 from the resistance ordinarily opposed to their contraction. When no 

 resistance is offered, as when a muscle is cut off from its tendon, not 

 only is no hardness perceived during contraction, but the muscular tis- 

 sue is even softer, more extensile, and less elastic than in its ordinary 

 uncontracted state. During contraction in each fibre it is said that the 

 anisotropous or doubly refractive elements become less refractive and the 

 singly refractive more so (Fig. 294). 



(4.) Chemical changes. (a) The reaction of the muscle which is 

 normally alkaline or neutral becomes decidedly acid, from the develop- 

 ment of sarcolactic acid, (b) The muscle gives out carbonic acid gas 

 and takes up oxygen, the amount of the CO, given out not appearing to 

 be entirely dependent upon the taken in, and so doubtless in part 



