CAUSES AND PHENOMENA OF MOTION. 35 



A low shaking or rumbling sound is heard, the height and loudness of the 

 note being in direct proportion to the force and quickness of the muscular 

 action, and to the number of fibres that act together, or, as it were, in 

 time. 



(3.) Changes in shape. The mode of contraction in the transversely 

 striated muscular tissue has been much disputed. The most probable 

 .account is, that the contraction 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 trans- 

 verse striae seen on the surface of the fasciculus, and by its increased 

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

 was supposed the fibres are thrown in contraction, 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 to- 

 gether 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 contents. 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 attended 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 commu- 

 nicates. Any diminution of the bulk of the contracting muscle would 

 he 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 contracted or not. 



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

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

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

 tion 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 tissue is even 

 softer, more extensile, and less elastic than in its ordinary uncontracted 

 state. 



(4.) Chemical changes. (a) The reaction of the muscle which is nor- 

 mally alkaline or neutral becomes decidedly acid, from the development 

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

 up oxygen, the amount of the carbonic acid given out not appearing to be 

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

 arising upon some other source. (c) Certain imperfectly understood 

 chemical changes occur, in all probability connected with (a) and (b). 



