Muscle (mŭs′’l), an animal tissue endowed with the power of contraction. It is the part called flesh or lean meat in the higher animals. Through its action motion and locomotion are accomplished in the animal kingdom. Muscles have not been developed in the simple, microscopic animals, like the amœba and its relatives, and, therefore, their movements are not dependent upon muscles, but upon the powers of the protoplasm of which they are composed. In that, however, lies the germ from which muscular tissue is developed. Muscles are composed of modified protoplasm, in which the power of contraction has been highly exalted, while the other qualities of protoplasm are undeveloped or held in check. Muscles first make their appearance in animals of the grade of hydra and the jelly-fishes, but in them they are imperfectly developed. In all animals of a higher grade than jellyfish muscular tissue is fully developed. It arises in the middle germ layer (mesoblast). In its formation the cells elongate into fibers.
As an example of muscular tissue take the muscles of the arm. These are bundles of flesh, which can be felt under the skin running lengthwise in the arm. Each muscle is covered with a smooth, shining membrane, and is made up of a number of bundles also covered with a thin membrane. These bundles are further subdivided into smaller ones, and the microscope shows that these are made of thread-like fibers lying side by side. All are surrounded by sheaths and united together. The microscopic fibers are crossed by stripes, and this kind is called striated muscular tissue. It is the kind usually controlled by the will, and is therefore called voluntary. There is another variety of muscular tissue found in the walls of the alimentary canal and blood-vessels (and in other situations), the action of which is not directed by the will, and, therefore, is called involuntary. Under the microscope this is made up of spindle-shaped cells, each with a round nucleus in the middle. These are not striped, and this kind is called smooth muscular tissue. The muscles of the heart are striated, but are branched and different from ordinary muscles. Therefore we have the three varieties: striated, smooth and heart-muscle.
There are about 400 muscles in the human body, most of them in pairs. All receive distinct names, and are connected with nerves and blood-vessels. The blood-vessels for their nourishment form a network around the fibers, while the nerves which control them form a closer connection. Most of the muscles attached to the bones have an enlarged middle (belly) and two ends tapering off into tendons by means of which they are grown to the bones. The bones are roughened where the muscles are attached. The more rigid attachment is called the origin, and the more movable one the insertion, of the muscle. Other muscles are flat, and some surround cavities. They are named in various ways, according to their position, as the temporals, in the region of the temple; the pectorals, on the chest; or the abdominals, etc.; from their direction, as a rectus or straight muscle, an obliquus or oblique muscle; from their uses, as flexors, which bend a joint; extensors, which extend it; levatores, which lift, etc.; according to attachment by tendons, as sternomastoid, mylohyoid etc.; and also in other ways.
The parts of the skeleton that move usually act as levers. The illustration shows, for example, the chief muscle which bends the lower arm upon the elbow joint as a fulcrum. This muscle is a flexor, as it flexes the arm; its origin is at the shoulder, where it has two heads, and is, therefore, called the biceps; its insertion is on one of the bones of the forearm. The muscle which extends the arm is not shown in the illustration. It should be kept in mind that the action of a muscle is to contract, not to expand. In contracting it gets thicker and shorter, and it returns to its original state of extension. Other muscles perform the contrary action. Ordinarily, muscles contract in direct response to
nerve stimuli, but many causes influence them, as their condition of nutrition, mental states etc. A good illustration is found in the muscles of the face. They vary with the emotions and mental states, and the expression of the face is the result of the action of a number of muscles.