MUSCLE AND MUSCULAR TISSUE, tissue specially distinguished by its con tractile power, the instrument by which all the sensible movements of the animal body are performed. If examined under a high magnifying power the fibers of which it is composed are found to exist under two forms, which can be distin guished from one another by the pres ence or absence of very close and minute transverse bars or The fibers of the voluntary muscles, or those whose movements can be influenced by the will, as well as the fibers of the heart, are striped; while those of the involuntary muscles, the muscular structures over which we have no control, as, for ex ample, the muscular fibers of the intes tinal canal, uterus, and the bladder, are unstriped. On examining an ordinary voluntary muscle with the naked eye, we observe that it presents a fibrous ap pearance, and that the fibers are ar ranged with great regularity in the direction in which the muscle is to act or contract. On closer examination it is found that these fibers are arranged in fasciculi, or bundles of various sizes, in closed in sheaths of areolar tissue, by which they are at the same time con nected with, and isolated from, those adjoining them; and when the smallest fasciculus visible to the naked eye is examined with the microscope, it is seen to consist of a number of cylindrical fibers lying in a parallel direction, and closely bound together. These primitive (or, as some writers term them, the ulti mate) fibers present two sets of mark ings or stria;, viz., a longitudinal and a transverse set.
Muscles vary extremely in their form. In the limbs they are usually of consid erable length, surrounding the bones and forming an important protection to the joints; while in the trunk they are flattened and broad, and contribute very essentially to form the walls of the cavi ties which they inclose. Muscles derive
their names (1) from their situation as the temporal, pectorals, glutals, etc.; or (2) from their direction — as the rectus, obliquus, etc., of which there may be several pairs; or (3) from their uses — as the masseter, the various flexors and extensors; or (4) from their shape — as the deltoid, trapezius, rhom boid, etc.; or (5) from the number of their divisions — as the biceps and triceps; or (6) from their points of attachment— as the sterno-cleidomastoid, the genio hyo-glossus, the sterno-thyroid, etc. In the description of a muscle we express its points of attachment by the words origin and insertion; the former being applied to the more fixed point, or that toward which the motion is directed, while the latter is applied to the more movable point. The skeleton, which may be termed the locomotive frame work, may be regarded as a series of levers, of which the fulcrum is, for the most part, in a joint, viz., at one extremity of a bone; the resistance (or weight) at the further end, and the force (or muscle) in the intermediate portion. The great and characteristic property of muscular tissue, that of shortening it self in a particular direction when stimu lated, is called contractility. The stimu lus may be direct irritation by mechan ical means, or by galvanism, or by some chemical substances, but in the living body the muscular fibers are, in most cases, made to contract by the immediate influence of the nerves distributed among them, which are consequently termed motor nerves, and are under the influence of the will.