Muscle (Lat. musculus), the fibrous contractile tissue forming the flesh of man and animals, by which locomotion and the various functions of life requiring voluntary or involuntary movements are performed. Whether elongated or enclosing a cavity, this tissue is arranged in the form of fibres, usually in bundies connected by areolar tissue, surrounded by a vascular network, and supplied with nervous filaments. Muscles are so arranged as to produce great velocity, extent of motion, and strength, without injuring the beauty of proportions, by the obliquity of their fibres to the tendons and of the last to the bones on which they act, and by the proximity of their points of insertion to the axis of motion of the joints. Muscles are attached to bone by means of tendons, rounded or flattened fibrous cords, white and shining, inelastic, and very resisting; aponeuroses or fasciae are firm, shining fibrous membranes, enveloping the muscles, giving attachments to their fibres, and often fixed to bones like the tendons. Muscles occupy the whole distance between the skin and bones, and take an elongated, broad, or thin form, according to the necessities of the several parts of the body; their strength is in proportion to their length and thickness, and may be rapidly exhausted by continuous exertion.

Muscles are called voluntary or involuntary, according as they are or are not under the control of the will; the division is not strictly accurate, as all of the former at times contract independently of the will, and some of the latter are to a certain extent under the influence of volition. The former are generally solid, as in the muscles of the trunk and limbs, and the latter hollow, as in the heart or the muscular layers surrounding cavities and canals. The voluntary and involuntary muscles are also distinguished by their structure; the former consisting of striped, the latter of unstriped fibres. The fibres of voluntary muscles are generally cylindrical, though more or less prismatic or many-sided, being somewhat flattened against each other. They vary in length in different muscles, and in the human subject average 1/400 of an inch in diameter. Their color in man and the higher animals is ruddy, and they are elegantly marked by transverse or circular striations, giving them a very characteristic appearance, which has led to their being distinguished by the name of striped fibres.

They consist of a cylindrical or prismatic mass of contractile substance marked with the above mentioned striations throughout its entire thickness, and containing also minute elongated or oval bodies termed nuclei. Each fibre is invested by a delicate, transparent, structureless and colorless membrane, the sarcolemma, which supports the contractile material and limits its lateral expansion. The fibres are arranged side by side, parallel with each other, and united in small groups or bundles of 100 to 200 each. These bundles are again united into larger secondary bundles, connected with each other by areolar tissue, and so on; the entire muscle being invested with an external fibrous expansion of condensed areolar tissue, and abundantly supplied with blood vessels and nerves. The unstriped or involuntary muscular fibres are soft, pale, flattened bands, apparently homogeneous or finely granular, about 1/3000 of an inch in diameter, with an elongated nucleus in the central part of each one. The fibres are arranged in parallel layers, their pointed extremities interlocking with each other, so as to form membranous expansions surrounding the cavities of the internal organs.

Thus the oesophagus, the stomach, the intestines, the bladder and urinary passages, the uterus and Fallopian tubes, the excretory ducts of the glandular organs, and the arteries and veins, all have their muscular coat, composed of un-striped fibres, and liable to contraction and relaxation independently of the will. An exception to the rule that involuntary muscular organs are composed of unstriped fibres is found in the heart and in the great veins immediately contiguous to it. Here the muscular fibres belong to the striped variety, but they are smaller than those of voluntary muscles, their stri-ations are less distinct, and they also present the peculiarity of branching and inosculating with each other, which is not seen in other striated muscular fibres. In all probability the difference in structure between the two kinds of fibres, strictly speaking, has reference to their mode of contraction, rather than to its voluntary or involuntary character. The contraction of the striped muscular fibres is prompt, vigorous, and rapidly followed by relaxation, as in the voluntary muscles and the heart; that of the unstriped fibres is generally sluggish, gradual, and continued, as in the peristaltic action of the alimentary canal.

Striped fibres have been found in all vertebrates and in articulates; as we descend the animal scale the movements become more and more automatic, until complex muscular action gives place to simple ciliary vibration. - The contractility of muscle depends on an inherent property, independent of, though capable of modification by, nervous influence. The stimuli which induce contraction are volition, emotion, impressions conveyed to the nervous centres and involuntarily reflected thence, and various physical and chemical agents applied to any portion of the course of a motor nerve or to the muscular fibres. A muscle in action becomes shorter and thicker, changing its relative proportions without any actual change in bulk. After death muscles become fixed and rigid, a condition constituting the rigor mortis. In the active contractions which characterize muscles on the application of stimulus, force is exerted against some opposing power; this is attended with exhaustion or fatigue, and requires intervals of rest. Sustained contraction consists of an infinite number of partial momentary contractions acting in succession.

There are altogether in the human body 527 distinct muscles, of which 261 are in pairs, and 5 single on the median line; of these there are in the head and face 83, the orbicularis oris being single; in the neck 49, the arytenoid of the larynx being single; in the thorax 78, the triangularis sterni and the diaphragm being single; in the abdomen 33, the sphincter ani being single; in the back 78; in the upper extremities 98, and in the lower 108. Yet, with all this complex apparatus, everything is in perfect order and harmony. Matteucci and Du Bois-Reymond have investigated the electric currents of muscles. The combination of the muscular movements is in most cases so far independent of the will, that we are apt to lose sight of their perfection; but let paralysis affect one side of the body or contraction draw up a muscle, and the fact becomes at once evident, as may be seen every day in palsy of one side of the face, or strabismus with the turning in or out of the eye. The simple process of walking, performed it may be unconsciously, with its nice adjustments executed by the automatic guidance of the senses rather than by any act of the will, is what the most ingenious mechanician can never effect in an automaton, from the impossibility of harmonizing the many acts which constitute walking. - The energy and rapidity of muscular contraction is more remarkable in the lower animals than in man.

The muscular power of insects is seen in the rapid flight of the dragon fly, the leap of the flea and the cricket, the fixed attitudes of some larvae, and the strength of beetles. It is very great in the flight of birds, though their whole structure is organized for aėrial motion; the power of the wings is three times as great as that of the legs in ordinary birds, and their absolute power in proportion to the weight of the body is as 10,000 to 1; in small birds the movements of the wings are so rapid that they cannot be counted by the eye; the muscular force of the hawk can propel it 150 miles an hour, and the albatross can fly across the ocean without fatigue. Dragons, flying fish, pha-langers, and squirrels (pteromys), though well organized in some respects for aerial progression, cannot fly for want of sufficient muscular power; but the extinct pterodactyl shows evidence of having possessed, like the existing bats, extensive powers of flight. The amount of muscular force necessary for flight is so great, that if man could concentrate all the strength employed in a day's labor, he could not support himself in the air for more than five minutes; the accomplishment of flight in man, even with the assistance of any contrivance thus far suggested, may be safely considered an impossibility.

The energy of the muscular system of fishes, considering the rapidity with which they move in their dense medium, must be very great. Other familiar examples of muscular power are seen in the constrictions of the boas; the leap of the frog, kangaroo, jerboa, and hare; the speed of the antelope; the spring of the lion; and the strength of the ox and elephant. The muscular power of man is more advantageously displayed by the extent and variety of motion than by actual force; but by scientific training great strength may be obtained from naturally feeble persons. The rapidity of muscular action is familiarly seen in the ventricular contractions of a child's heart, each of which occupies a little more than half a second; in the movements of the vocal cords in rapid singing or speech; and most remarkably in the flight of insects, whose wings strike the air sometimes thousands of times in a minute, by a muscular mechanism and arrangement of elements mentioned under Gnat. Muscle may be hypertrophied from excess of nutrition arising from abundance of formative material, from increased supply of blood, but principally from preternatural formative capacity; the opposite conditions lead to atrophy of muscle.

A remarkable change in muscle consists in its fatty degeneration, to which the flbres of the heart are very subject; the muscles of the limbs after paralysis are occasionally thus affected. Throughout the animal kingdom the development of the muscular system is in conformity with that of the nervous system. The vertebral system of muscles is most developed in fishes, the costal in serpents, the hyoid in fishes, the masticatory in vertebrates, the tegumentary in those mammals armed with spines (like the hedgehog and porcupine), and in the unpaired or vertical fins of fishes; those of the voice are most developed in birds, mammals, and man; those of the limbs inversely as those of the spine, and feeblest in fishes; the diaphragm exists in mammals only. The muscles of the hand reach their highest perfection in man, while those of the tongue, eyes, ears, and nose show that many groups of muscles which are complete in the lower mammals, exist in man in a comparatively rudimentary condition. - Muscles which move a limb in opposite directions are called antagonist muscles. The flexor muscles of the arm, for instance, bend the limb at the elbow joint, and the extensor muscles draw it back, or extend the arm in a direct line; thus these muscles antagonize each other.

There is a sort of passive action in the different muscles of the body, constituting what is termed the natural tone of the system; and when this is lost or partially enfeebled in one set of muscles, their natural antagonists have an undue action on the parts, and cause disfigurement by destruction of the natural balance. The form and position of the muscles of the face, for instance, keep up a balance of feature in the natural expression of immobility or stillness; those of one side antagonize those of the other. In paralysis of one side of the face, the muscles of that side are deprived of their natural tone and power of action, while those of the other side retain their tone and power as before; the consequence of which is, that the latter draw the mouth to their side of the face, while the others are unable to counterbalance this action from want of power to act in the opposite direction. Certain muscles are antagonized by the natural elasticity of the parts to which they are attached; the elasticity of the ribs and that of the elastic ligaments of the spinal column may be considered as antagonistic to the natural tone and power of the muscles attached to them, or acting in a contrary direction.

Muscles and Tendons of the Arm and Hand.

Fig. 1. - Muscles and Tendons of the Arm and Hand.

Striped Muscular Fibre, crushed at one end and breaking up into fibrillae.

Fig. 2. - Striped Muscular Fibre, crushed at one end and breaking up into fibrillae.

Striped Muscular Fibre, highly magnified, torn across, and showing the Sarcolemma.

Fig. 3. - Striped Muscular Fibre, highly magnified, torn across, and showing the Sarcolemma.

Transverse Section of a Voluntary Muscle, showing the bundles of Muscular Fibres and intervening layers of Areolar Tissue, and the external Fibrous Expansion.

Fig. 4. - Transverse Section of a Voluntary Muscle, showing the bundles of Muscular Fibres and intervening layers of Areolar Tissue, and the external Fibrous Expansion.

Unstriped Muscular Fibres, highly magnified, from the walls of the Kenal Vein.

Fig. 5. - Unstriped Muscular Fibres, highly magnified, from the walls of the Kenal Vein.