As already stated, the skeleton, the nervous system, and the muscular system are concerned in the production of motion. The skeleton and the nervous system, however, serve other purposes in the body, while the muscular system is devoted exclusively to the production of motion. For this reason it is looked upon as the special _motor_ system. The muscular tissue is the most abundant of all the tissues, forming about 41 per cent of the weight of the body. *Properties of Muscles.*—The ability of muscular tissue to produce motion depends primarily upon two properties—the property of irritability and the property of contractility. _Irritability_ is that property of a substance which enables it to respond to a stimulus, or to act when acted upon. _Contractility_ is the property which enables the muscle when stimulated to draw up, thereby becoming shorter and thicker (a condition called contraction), and when the stimulation ceases, to return to its former condition (of relaxation). The property of contractility enables the muscles to produce motion. Irritability is a condition necessary to their control in the body. *Kinds of Muscular Tissue.*—Three kinds of muscular tissue are found in the body. These are known as the _striated_, or striped, muscular tissue; the _non-striated_, or plain, muscular tissue; and the _muscular tissue of the heart_. These are made up of different kinds of muscle cells and act in different ways to cause motion. The striated muscular tissue far exceeds the others in amount and forms all those muscles that can be felt from the surface of the body. The non-striated muscle is found in the walls of the food canal, blood vessels, air passages, and other tubes of the body; while the muscular tissue of the heart is confined entirely to that organ. *Striated Muscle Cells.*—The cells of the striated muscles are slender, thread-like structures, having an average length of 1-1/2 inches (35 millimeters) and a diameter of about 1/400 of an inch (60 μ). Because of their great length they are called fibers, or fiber cells. They are marked by a number of dark, transverse bands, or stripes, called striations,(83) which seem to divide them into a number of sections, or disks (Fig. 108). A thin sac-like covering, called the _sarcolemma_, surrounds the entire cell and just beneath this are a number of nuclei.(84) [Fig. 108] Fig. 108—*A striated muscle cell* highly magnified, showing striations and nuclei. Attached to the cell is the termination of a nerve fiber.