MUSCLE.—(Syn. Musculus, Mus cular or Sarcous tissue; vulgo, Flesh, Meat.) This term is applied to certain fibrinous con tractile organs, either elongated and fixed at their two extremities, or hollow and enclosing a cavity, which in all the higher animals are the seat of the power by which locomotion, circulation, the prehension and passage of food, the expulsion of many of the excretions and of the young, as well as other diversified functions, are performed. It is also used to denote the peculiar contractile material or tissue, constituting the principal and essential portion of such organs. This tissue is always arranged in the form of fibres, which in many minute animals occur singly, each serving the purpose of a perfect muscle. But they are usually aggregated in very great numbers, sur rounded with a network of capillary vessels, and connected to one another by areolar tissue. The nervous tissue is universally associated with the muscular, however small may be the quan tity of the latter; it is through this that the stimulus to contract is ordinarily transmitted, and, when the mass is great, made to affect simultaneously many contiguous fibres. A muscle is the organ resulting from the union of these several parts.
Muscles are styled voluntary or involuntary, according as they are, or are not, subject to the influence of volition, and they have been usu ally so classified. But, however convenient these terms may be in the ordinary language of physiology, they cannot be applied, in a strict sense, to the purposes of classification without obvious objections. Many muscles, especially those under the immediate domi nance of reflex nervous action, (as the respi ratory and sphincter muscles,) partake of both characters, since volition can interfere only temporarily with their contraction ; and all muscles, even the most confessedly voluntary, are subject to emotional and instinctive influ ences, in which the will has no share. The attempt to introduce an intermediate or mixed class, which has been generally sanctioned, while it is an acknowledgment of the imper fection of the arrangement, does not appear to be sufficiently warranted either on anatomical or physiological grounds. If subjection or non-subjection to the influence of the will be made the basis of classification, all muscles should be accounted voluntary on which this can exercise a direct influence either in causing or controlling contraction, even though such influence be but momentary, and capable of being exerted only while the stimulus excitive of involuntary action is in abeyance.
The voluntary muscles are generally solid organs, while the involuntary are hollow; and, on recurring to the minute structure of their respective elementary fibres, we detect very striking differences between them, those of the former being striped crosswise with very deli cate and close parallel lines, which, with some exceptions, are altogether absent from the lat ter. But these exceptions are of so important
a kind as to demonstrate beyond doubt that there is no necessary connexion between the minute conformation of the fibres and their re lation to the influence of the will. The mus cular coat of the esophagus often displays the striped structure as far down as the stomach, though the will has no power whatever over its movements; and the heart itself is composed of striped fibres. As the structural differences between these two kinds of fibre are constant, well-marked, and therefore easily ascertained, and as they seem, moreover, to be related to varieties in the activity and mode of exercise of their contractile power, they will be employed as the ground of division in the present ar ticle.
I shall first describe the minute anatomy of these two kinds of elementary fibre, and the steps of their development; and, secondly, I shall advert to their mode of aggregation and to the arrangement of the tissues found in con nection with them.
a. Of the striped elementary fibres.—These have received the name of Primitive Fasciculi on the erroneous supposition of their being bundles of finer filaments. They may be sepa rated from the tissues associated with them in the compound organ by a variety of means, but as they always constitute the principal mass of the organ, they may he examined without any attempt at such separation. It was a favourite plan with the older anatomists to obtain the fibres apart by submitting them to a long boiling, which destroys the texture of the vessels and filamentary tissue, but at the same time considerably modifies the size, shape, and structure of the fibres. It is in general only requisite to take a small portion of a muscle, as fresh as possible, (but after its contractility has departed,) and to tear it, under water, into fine shreds, with needles. By these means the elementary fibres will be separated from one another, and being in parts irregularly broken, and torn, can be submitted to inspection under a high power of the microscope, in such a condi tion as to exhibit most of the important points in their structure. Many sedulous examinations of specimens from various sources are requisite for the acquirement of a correct idea of their organization and properties, but that this simple method of procedure is the one most likely to lead to a true insight and conclusion regarding the anatomy, not only of this but of all elemen tary structures, becomes every day more evi dent. Various subsidiary means may doubtless be employed with advantage, such as injections and physical and chemical agencies ; but the method which of all others is the least liable to admit of erroneous interpretations by the ad mixture of artificial elements in which the mind of the inquirer has had a share, is that of em ploying a power capable of reaching the utmost limits of organization, on examples the most nearly approaching to their natural state during life.