462 MOTION. 



is shown in that their nerves can communicate to the mind an 

 accurate knowledge of their states and positions when in action. 

 By this sensibility, we are not only made conscious of the mor- 

 bid sensations of fatigue and cramp in muscles, but acquire, 

 through muscular action, a knowledge of the distance of bodies 

 and their relation to each other, and are enabled to estimate 

 and compare their weight and resistance by the effort of which 

 we are conscious in measuring, moving, or raising them. Ex- 

 cept with such knowledge of the position and state of each 

 muscle, we could not tell how or when to move it for any re- 

 quired action ; nor without such a sensation of effort could we 

 maintain the muscles in contraction for any prolonged exer- 

 tion. 



The mode of contraction in the transversely striated muscular 

 tissue, has been much disputed. The most probable account, 

 which has been especially illustrated by Mr. Bowman, is that 

 the contraction is effected by an approximation of the constitu- 

 ent 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 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 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 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 dimi- 

 nution 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 ex- 

 periment is carefully performed, the level of the water in the 

 tube remains the same, whether the muscle be contracted or 

 not. 1 



In thus shortening, muscles appear to swell up, becoming 



1 Edward Weber, however, states that a very slight diminution 

 does take place in the bulk of a contracting muscle ; but it is so slight 

 as to be practical^ of no moment. 



