MUSCULAR SYSTEM, Development of. Two main forms of muscle-tissue (see MUSCLES) exist in the human body, the striated muscle-tissue, which makes up the muscles of the bony framework of the body, and which responds to willed or wished impulses passing through the brain structures, and also to vege tative stimuli of the unstriated muscle-tissue, which chiefly responds to automatic stimuli of unconscious nature by means of the vegetative or sympathetic nervous system. Some control of vegetative functions may be exercised by conscious processes. (See NERVOUS SYSTEM), The heart-muscle is regarded as an interme diate form, resembling striated muscle-tissue, but in its development more nearly allied to the more primitive unstriated muscle-tissue. Non striated muscle-tissue is formed by a direct transition of certain cells in the middle ger minal layers (see EMBRYOLOGY) or mesenchyme At first these muscle-fibres are irregularly dis tributed, hut later they are collected into small bundles .or into layers, and become associated with the individual organs with which they functionate. Striped muscular tissue develops from the same layer, but the details of develop ment, as is the case in the histogenesis of the heart-muscle, are extremely complicated. The cells at first develop a mesh-like structure or reticulum. This reticulum develops small discs, which later become small columns of muscular tissue. The columns at first develop at the periphery of the cell, and gradually fill in around the nucleus, which in the heart muscle lies in the centre of the cell, whereas in the muscles of the skeleton the nucleus is pushed to one side, or disappears, new nuclei appearing just beneath the sarcolemma-sheath. The skeletal muscles develop in regular order from the different segments (somites) of the mesoderm. In the early stages the distribution is very symmetrical, but later it becomes ex tremely uniform by reason of the irregular welding of different segments of the bony skeleton. There remains, however, a regular association of the muscular myotomes and their embryonic nerve supply, and the homologies of structure may be traced by the nerve-supply, although the muscles themselves may have shifted from their original position. This is a question of highly technical nature, hut has many practical hearings in modern medicine. Thus the great broad muscle of the hack, the latissimus dorsi, which arises from the seventh and eighth cervical segments, but later migrates and is fastened all the way down the spine as far as the crest of the hip-bone, is supplied by a nerve which also develops from the seventh and eighth cervical nerves. The de velopment history of each skeletal muscle can thus be traced by means of its nerve-supply. The whole process is one of extreme intricacy and should be studied in special monographs.
Evolution of Muscular in the development of the muscular system in man there has been a gradual evolution of the plan of muscular arrangement, so in the animal series there has been a gradual development of a muscular system from the very simplest types of contractile protoplasm. Even in plants definite movements may occur which may be very slow or very rapid. Yet no muscular tissue proper has ever been found in plants.
Many of the lowest plants, the Algce, are motile and are provided with vibratory cilia, but these minute hairs, although capable of rapid motion, cannot be regarded as muscular organs. In the contracting protoplasm of the lowest animals, the rhizopods, although movements take place, there is no muscle-tissue. Nor is muscle-tissue found in the next higher group, the Infu soria, although very actively moving forms are known, for example, the familiar microscopic animal Paramecium. The bell-animalcules, Vorticella, Stentor, etc., have stalks that coil and uncoil with great rapidity, but they con tain no muscle-tissue proper. They do, how ever, contain what are termed myronemes, and spironemes, which are longitudinally striated and have as many functionate as muscular organs. In another genus of infusorians (Bursaria) there is a contractile band about the body of the animal. It has been regarded as a true sphincter muscle. It has not the structure of the developed unstriped muscle-cell. In the closely allied sponges (Porifera) certain elon gated cells with rod-shaped nuclei are found, and may be considered the ancestral forms of the unstriped muscle-cell, although it is not un til the group of the C'rlenterates is reached that true unstriped muscular tissue is present in its more advanced forms. In many of the hydroids a form of external neuromuscular cell is found. This is a type of cell half nerve, half muscle, but not resembling true muscular tissue. In the jelly fishes of this order both neuromuscular tissue and true unstriped muscle-fibres are found. In the sea-anemones unstriped muscle is abundant. It is mostly developed, however, from the ex ternal layers of the body, and thus embryologi cally is not comparable to the muscle-tissue that in practically all the animals higher than the coelenterates is formed in the middle ger minal layers of the developing animal. In one of the higher coelenterates, the common water hydra, some muscle-cells are found imbedded in the deeper tissues of the body, thus fore shadowing the higher type of muscular tissue. In the next great family of animals, the Fchinodermata, to which the sea-urchins, star fishes, and sea-cucumbers belong, unstriped muscular tissue is common, but no evidence of striated or striped muscle is yet present. In the worms the muscular tissue is unstriped and abundant. In the mollusks, the foot of the soft clam, the muscle of the oyster, are made up of unstriped muscle fibres. A higher order, the Arthropods or Crusiaceatts, including the crabs, lobsters, etc., contains a well-developed muscular system, which is made up of striated muscle, practically the first appearance of this type of muscle in the animal kingdom. In these animals, moreover, there is a type of de velopment of the muscles that anticipates the regular segmented type, metameres or myo meres, of higher animals.
From the crustaceans onward both types of muscle-tissue are found. In the low verte brates, selachians and fishes, typical heart muscle cells, striated and with central-lying nuclei, are found. Consult McMurrich, Devel opment of the Human Body,) with full bibliog raphy (1902) ; Parker, The Elementary Ner vous System) (1919) ; Bayliss, cGeneral Physi See ANATOMY.