Muscle, in animal economy, a fleshy, fibrous part, destined to be the instrument or organ of motion.
A muscle is composed of a great number of thin parallel plates, each of which is divided into smaller fleshy threads or fibres, and inclosed in its proper cellular membrane.- The muscles are divided into three parts, namely, the head, belly, and tail: the first and third are firmly attached to the bones; whereas the belly adheres loosely to other parts, by means of the cellular membrane which swells, when the muscle acts. Their substance is fleshy in the middle, but tendinous or sinewy in the extremities : the fibres of the former are sensible and irritable, while those of the latter are destitute of all sensation.
All the muscles act, by the inflation of their bellies ; in consequence of which they are compressed or shortened. Thus, in muscular contraction, the moveable bone only is drawn in the direction of the fibres, while the other bone, to which the muscle is attached, remains fixed. Hence they operate in some measure as a lever; the two ends of every rnuscle being fastened to the bones by mean9 of the tendons; but, if any fibre or nerve of the muscle be divided, or only obstructed by a ligature, the power of contraction instantly ceases.
Various denominations have been given to the muscles, according to the different parts where they are situated ; but, as these particulars belong to the province of anatomy, we shall only mention, that their number in the human body amounts to about 450, while some of the lower animals are provided with several thousand.
Lastly, the colour of the muscles, in healthy individuals, is deep red; but, in dropsical, or those persons whose humours are in a vitiated state, it is of a paler hue. In the aged, the muscles contract and be come rigid : - to retard this inactive and feeble state, there appears to be no better preventive than the frequent resort to the tepid bath ; and the constant wearing of flannel next the skin, with a view to promote an uniform degree of insensible perspiration.