MUSCLES. The organs called muscles are made up of a collection of muscle-cells, which have for their function the accomplishment of bodily movements. These movements may be gross, as in walking, or very minute, such as take place in the contracting movements of the arteries or veins; they may be voluntary, brought into action by wishes and called "willed" impulses made possible by the brain structures, or they may be involuntary and brought about by acting through the vegetative nervous system (q.v.). The muscular move ments necessary in writing are illustrative of the voluntary type; the heart-beat, of the in voluntary variety. In point of evolutionary de velopment the involuntary movements may be said to have preceded the voluntary, and hence in regard to complexity of structure the invol untary muscle-cells are simpler than the muscle cells that perform voluntary movements, and to these two types of cells students of minute anatomy have given the names of unstriped or involuntary muscle, and striped or voluntary muscle. The muscular tissue of the heart is of intermediary character. It is a striped involun tary muscle. Muscle-tissue, like protoplasm, is strongly contractile, but unlike ordinary pro toplasm, which can contract in all directions, muscle-substance can contract in only one di rection. The simplest type of muscle-substance is seen in many lower animals. In these the muscle-cells are elongated spindles with sharp ened points and with a single elliptical to rod shaped nucleus, situated about the middle of the fibre. They show a faint longitudinal striation, vary in length from 40 to 500 microns 1-50 of an inch) in length, and 3 to 8 microns (1-8000, 1-3000 inch) in diameter. These muscle cells in man correspond to the unstriped muscle cells, are bound together in bundles or flattened plates by an inter-cellular cement-substance and are found more particularly in the walls of the intestine, the trachea, bronchi, blood-vessels, bladder, ureter, uterus, in many of the organs of the body, and in glandular structures. They
have a rich blood-supply and a nervous net work of sensory terminal filaments, as well as fibres from the vegetative (old term was sympa thetic) nervous system. The pains of an intes tinal colic, of a gall-stone, of childbirth, are all due to forcible contractions of these involun tary muscle-cells in the respective organs. The more prominent muscles of the body, such as those that move the various bones, the muscles proper and such as are eaten as roast beef are of the striped variety. These are a modifica tion of the simple spindle-cells. In man they originate in the mesoderm, muscle-sub!tance commences to form in the interior of some of the cells; these elongate, many nuclei are formed and the final result is a bundle of much elongated cylindrical cells 12 centimeters (2 inches) long and from 10 to 100 microns (1-2500 to 1-250 inch) broad. Each cell is covered by a special sheath, the sarcolemma, and within is made up of very intricate and minutely structured protoplasm, the most strik ing feature of which is its banded or striated appearance; hence the name striated. Just within the sarcolemma a number of flattened elliptical muscle-nuclei are found. In insects striped muscle-tissue can be studied to best ad vantage. Occasionally branched forms of striated muscle are found. Blood-vessels are numerous in striped muscle and lymphatics and nerves are also abundant. Special types of nerve-endings and muscle-plates, are cha-acter istic of voluntary muscle. Striped muscle is probably a combination of types of muscle. The sarcoplastic substance is thought of as of um. striped muscular origin, and it is probably in nervated by the vegetative nervous system. This type of innervation is that which keeps its metabolism in order, and also contrib utes to what is known as muscle-tonus. The emotions act through this part of the muscle probably. The other part of the muscle, called the anisotropic disc, is probably innervated by the sensori-motor system and is that part of the muscle which carries out willed or wished actions.