MUSCLE AND MUSCULAR TISSUE. Muscular tissue is specially distinguished by its contractile power, and is the instrument by which all the sensible movements of the ani mal body are performed. When examined under a high magnifying power, the fibers of which it is composed are found to exist under two forms, which can be distinguished from one another by the presence or absence of very close and minute transverse bars or stripes. 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 example, the muscular fibers of the intestinal canal, the uterus, and the bladder, are unstriped.
Ou examining an ordinary voluntary muscle with the naked eye (a muscle from one of the extremities of any animal, for example), we observe that it presents a fibrous appear ance, and that the fibers are arranged with great regularity in the direction in which the muscle is to act or contract (for it is by their inherent power of contracting that muscles act). On closer examination it is found that these fibers are arranged in fasciculi, or bundles of various sizes, inclosed in sheaths of areolar tissue, by which they are at the same time connected with and isolated, from those adjoining them; and when the smallest fascicalus 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 'ultimate) fibers present two sets of markings or strite—viz., a longitudinal and a transverse set. The fibers, when separated from each other, frequently split longitudinally into fibrilla. Sometimes, however, when a fiber is extended, it separates in the direction of the trans verse strim into a series of disks. Either cleavage is equally natural, but the latter is the least common. Hence, observes Mr. Bowman, who has specially investigated the minute structure of voluntary muscle, "it is as proper to say that the fiber is a pile of disks as that it is a bundle of fibrilla; but, in fact, it is neither the one nor the other, but a mass in whose structure there is an intimation of the existence of both, and a tendency to cleave in the two directions. If there were a general disintegration along all the lines in both directions, there would result a series of particles, which may be termed p•imi tive particles or sarcous elements, the union of which constitutes the mass of the fiber.
These elementary particles are arranged and united together in the two directions, and the resulting disks, as well as fibrIllm, are equal to one another in size, and contain an equal number of particles. The same particles compose both. To detach an entire fibrilke is to extract a particle of every disk, and vice versa." The fibers are supplied with ves sels and nerves, which lie in the intervals between them, and are attached by their extremities through the medium of tendon or aponeurosis to the parts which they are intended to move. Aggregated in parallel series, of greater or lesser size, and associated with nerves,vessels,tendinous structures,etc.,they form the various museleswhich are for the most part solid and elongated, but are sometimes expanded into a membranous shape. The length of the fibers is usually about that of the muscle in which they may occur, and may vary from two feet or more (in the sartorius muscle) to less than two lines (in the stapedius muscle in the middle ear); while their width varies from to of an inch, being largest in crustaceans. fishes, and reptiles, where their irritability, or prop erty of contracting under the action of a stimulus, is most enduring, and smallest in birds where it is most evanescent. Their average width in man is about of of an inch, being about 3-b of an,inch ,in the ,male and - of an inch in the female. The average distance between the strim, or the size of the sarcous elements, in the human sub ject is an inch, the extremes being and of an inch, according to the contraction or relaxation of the fiber. The form of the fibers is polygonal, their sides being flattened against those of the adjoining fibers. Each fiber is enclosed in a transparent, very delicate, hut tough and elastic tubular sheath, which cannot always be readily seeu,but is distinctly ahown stretching between separated' fragments of a fiber which has been broken within it, for its toughness will often resist a force before which its brittle contents give way. This tubular sheath is known as the sarcolemma or myolemma —the former term being derived front the Greek words 8(112•, flesh, and lemma, a skin or husk; and the latter from the Greek words inns, a muscle, and lemma.