See Mytilus and Cetus. In ana-tomv from Musculus 5063 , to draw or contract. Lacertuli, q. v.

Muscles consist of those bundles of fleshy fibres by which the motions of all animal bodies are performed, and each is divided into the head, belly, and tail. The head is the part fixed on the immovable joint, called its origin, and is usually tendinous; the belly is the middle, fleshy part, which consists of the true muscular fibres; the tail is the tendinous portion inserted into the part to be moved, called the insertion; but in the tendon the fibres are more compact than in the belly of the muscle, and do not admit the red globules. The number and their nature are supposed not to differ.

The arteries, veins, and nerves, generally enter the middle of muscles, and ramify alike throughout their whole substance. The large arteries and veins run according to the direction of the muscular fibres; the less anastomose and run transversely: but a muscle seems to have a greater proportion of blood than is required for its nourishment, so that it probably assists in the performance of its functions.

The muscles are commonly attached to the bones, and the tendons are inserted into the substance of the bone; but when a muscle is fleshy at its insertion, it is only fixed to the periosteum. The muscles fixed to cartilages are attached to the perichondrium. Some are fixed to ligaments, as those in the fore arm; others into membranes, as those of the eye; and others again into fleshy parts, as those of the tongue.

Muscles are either oblong, hollow, or mixt; the oblong are divided into the rectilinear, half penniform, the penniform, the complex penniform, and the radiated. The muscular fibres are united very firmly in tendons, to lessen the bulk near the joint, which would have obstructed motion, and been otherwise inconvenient, as in the hand. By this means also the fleshy part of the muscle is nearer the centre of motion; and injurious friction is avoided. The appendices of muscles are, the fascia, or aponeurosis, the annular ligament, by which the tendons of some muscles are confined, and the sacculi mucosi.

The more intimate structure of the muscles has not yet been ascertained. The appearance of fleshy fibres is well known; and these, we have said, terminate in white, shining, firm cords, called tendons. The tendinous are seemingly the continuation of muscular fibres, losing, with their more compact structure, the peculiar muscular appearance. Yet this is contradicted on authority that we cannot lightly pass by, and it is said that they are obviously distinct; that the muscular fibres are attached to, without being continued in, them; that they are arranged in a different manner, and inserted at angles more or less obtuse. On the other side, tendinous aponeuroses, diminishing in thickness, constitute internal aponeuroses, giving tension to the fibres, as external aponeuroses sometimes give points of attachment, and almost an origin, to muscles; nor can we deny that tendinous fibres may be inserted into muscles on one side, as they are to ligament or bone on the other. At the same time it is certain that tendons partake of none of the peculiar structure of muscles, and that they are more nearly allied to the simple solid.

Muscular fibres are, when carefully washed, white, solid, and parallel. It is said that they are indefinitely ramified; an opinion, observes Mr. Carlisle, which an hour's labour at the microscope will refute. On these fibres arteries very minutely ramify; it is said, also, with the same indefinable minuteness: but it is at least certain, that before they escape the assisted sight they cease to give off branches. The minuter ramifications of the nerves cannot be ascertained. They enter into the muscle often at right angles, at least considerable ones; and when they spread on the fibres they lose their external coverings, and become transparent, so that we can no longer follow them. The fibres are connected by a cellular substance of different fineness in different muscles, but apparently of a more delicate texture, as the muscular fibre itself grows more minute, and has very seldom any adipose accumulations.

From these facts, which are now well established, we may reject the opinion of the primitive muscular fibre being globular, spheroidal, or rhomboidal, of its being wholly nervous or arterial. We own that we have indulged the language of calling muscular fibres the sentient extremities of nerves, and occasionally spoken of them as such; but if we have at any time expressed such an opinion, we beg leave, on mature reflection, to disclaim it. If there were no other arguments, the proportion of bulk in the vessel and nerve, compared with that of the muscle, oppose it: the peculiar structure and other properti«s of the muscles are equally adverse.

Though muscles are evidently fibrous and red, we must not deny a muscular structure where we do not find red fibres. The lymphatic system is probably muscular; for the fibrous structure is discoverable in the thoracic duct of a horse: and Mr. Home has rendered it probable that the fibres of the crystalline lens are equally so. At least we know, in numerous instances, that muscular contraction takes place where fibres are scarcely, if at all, discoverable, and where the red muscular structure is not found. Yet, as a part of the fibrous structure, the muscles form a part of the primordial germ. We see their influence at the first dawn of life in the punctum saliens: their action is coeval with animation.

These facts at once disprove the theory of Girtan-ner, who attributes muscular contraction to a kind of explosion, or effervescence, arising from the oxygen in the blood uniting with the azot, the hydrogen, and carbon of the muscles; for contraction exists when azot is not yet formed. We admit that azot, which furnishes