It was for a long time believed that the contraction of a muscle was associated with a change in the directidn of each fiber from a straight line to a sinuous or zigzag course. The investigations of Mr. Bowman, have, however, shown that this view is erroneous. He has proved that in a state of contraction there is an approximation of the transverse 'strife, and a general shortening with a simultaneous thickening of the fiber, but that it is never thrown out of the straight line, except when it has ceased to contract and its extremities are acted on by the contraction of adjacent fibers.
Muscles grow by an increase, not of the number, but of the bulk of their elementary fibers; and ,1r. Bowman believes "that the number of fibers remains through life as it was in the foetus, and that the spare or muscular build of the individual is determined by the mold in which his body was originally east," The structure of the i,evolantary or unstriped muscles must now be considared, This form of muscular tissue most commonly occurs in the shape of flattened bands of con siderable length, but of a width not exceeding or of en inch. These bands are translucent, and sometimes slightly granular, and are usually marked at intervals by elongated nuclei, which become much more apparent on the addition of acetic acid. Xffiliker has shown that every one of these bands or fibres is either a single elongated cell (a fiber-cell) or is a fascicuhis of such cells. These fibres have not usually fixed points of attachment like the striated fibres, but form continuous investments around cavities within the body—such as the intestinal canal, the bladder, the uterus, the blood-vessels, etc.—or are dispersed through the substance of tissues, such as the skin, to which they impart a contractile property.
The chemical composition of ordinary (or voluntary) muscle is described in the article FLESH. It is only necessary to add that the fibrilla, or the sarcous elements of which they are composed, consist of a substance termed SYNTONINE (q. v.), which closely resembles the fibritie or coagulating constituent of the blood; and that the same syntonine is also the main constituent of time unstriped muscles, or at all events of their fibre-eells. Like the blood-fibrine, it exists in a fluid form in the living tissue, and only coagulates or solidifies after death.
Our limited space prevents even an allusion to the arrangement and distribution of blood-vessels, nerves, and areolar-tissue in muscular structures; and we therefore pass ou to the consideration of the muscles and their functions.
Muscles vary extremely in their form. In the limbs they are usually of considerable 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 cavities which they inclose. There is unfortunately no definite rule regard ing the nomenclature of muscles. Muscles derive their names (1) from their situation— as the temporal, pectorals, &Weals, etc. ; or (2) from their direction—as the rectus, °Whom. etc., of which there may be several pairs—as, for example, rectus fervoris, rectus abdominalis, rectus capitis, etc.; or (3) from their uses—as the masseterr, the various flexors, extensors; or, (4) front their shape—as the deltoid, trapezius, rhomboid, etc. ; or (5) from the number of their divisions—as the biceps and triceps; or (6) from their points of attachment—as the sterno-cleido-mastoid, 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 towards which the motion is directed, while the latter is applied to the more movable point. The application of these terms is, however, in many cases arbitrary, as many muscles pull equally towards both attachments. Muscles opposed in action are termed antagonists, this antagonism being in most cases required by the necessity that exists for an active moving power in opposite directions. Thus, by one set of muscles, the/lavers, the limbs are bent; while by a contrary set, the extensors, they are straightened. One
set, termed the muscles of mastication. closes the jaws, while another set opens them; and probably every muscle in the body has its antagonists in one or more other muscles.
The skeleton, which may lie termed the locomotive framework, 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 intermedthte portion. In most cases, in order to preserve the necessary form of the body, muscles are applied at a great mechanical disadvantage as retards the exercise of their power; that is to say, a much huger force is employed than would suffice, if differently applied, to overcome the resistance. The two main sources of tills disad vantage lie in the obliquity of the insertion. and consequently of time action of most muscles, and in the muscles being usually inserted very near the fulcrum. The first of these disadvantages is in many eases diminished by the enlargements of the bones at the joints. The tendons of the muscles situated above the joint are usually inserted immediately below the bony enlargement, and thus reach the hone that is to be moved in a direction somewhat .approaching the If this enlargement did not exist, the contraction of the muscle, instead the lower bone to turn upon the upper one with comparatively little loss of power, would do little more than cause the two ends of the bones to press upon each other. The second mechanical disadvantage is compensated for by gain in the extent and velocity of movement, and by the avoidance of the great inconvenience of having the muscles extended in straight lines between the ends of jointed continuous levers. Thus the bones of the forearm are bent upon the bone of the arm by the biceps muscle which arises close to the head of the latter, and is inserted at a short distance from the elbow-joint, which acts as the fulcrum of the lever. By this arrangement, a con traction of a single inch in the muscle moves the • hand, in the same time, through the extent of about twelve inches, but then the hand moves through every inch with only about the twelfth part of the power exerted by the muscle. By the junction of two or more levers in one direction, as in the different segments of the extremities, the extent and velocity of their united actions :ire communicated to the extreme one. This a blow of the fist may be made to include the force of all the muscles engaged in extending the shoulder, elbow, and wrist.
The great and characteristic property of muscular tissue—that of shortening itself in a particular direction when stimulated—is called contractility. The stimulus may be direct irritation by mechanical means, or by galvanism, or by some chemical substance, but in the living body the muscular fibres are, in most cases, made to contract by the immediate influence of the nerves distributed among them, which are consequently termed motor nerves (see NERVOUS SYSTEM), and are under the influence of the will. By an exertion of volition, we can contract more or fewer muscles at once, and to any degree, within certain limits; and as a matter of fact, there is hardly any ordinary move ment perform( d in which several muscles are not called in play. But every voluntary muscle is also subject to other influences more powerful in their operation than the will. The movement of the features under the impulses of passion and emotion are more or less involuntary, as is shown by the very partial power the will has of restraining them, and the extreme difficulty of imitating them.
Many movements ensue involuntarily when certain impressions, which need not neces be attended with consciousness, are made on the surface of the body, or on any part of its interior, either by external or internal causes Such movements are termed reflex, and are noticed in the article :NERVOUS SYSTEM. Our space precludes us from noticing the individual groups of muscles in the human body. Several important groups are, however, noticed under Arai, EYE, FOOT, HAND, LEG. etc.