The muscles are all symmetrical, and with the exception of the sphincters and one or two others are in pairs. Each muscle constitutes a separate organ, composed chiefly of contractile fibrous tissue, which is called muscular, and of other tissues and parts which may be regarded as accessory. Thus muscular fibers are connected together in bundles or fasciculi, and these fasciculi are again embedded in and united together by a quantity of connective tissue, forming the perimysium; and the whole is usually inclosed in an external sheath of the same material. Many of the muscles are connected at their more or less tapering extremities with tendons by which they are attached to the bones or hard parts; and the tendinous bands frequently run to a considerable length either on the surface of the muscle or between its fibers. There are two chief kinds of muscular tissue, the striped, and the plain or unstriped, and they are distinguished by structural peculiarities and mode of action. The striped form of muscular fibers is sometimes called voluntary muscle, because all muscles under the control of the will are constructed of it. The plain or unstriped variety is often termed involuntary, because it alone is found in the greater number of muscles over which the will has no power. The involuntary or unstriped muscles are made up of elongated, spindle-shaped fiber cells, which in their most perfect form are flat, from about 1/4500 to 1/3500 of an inch broad, and about 1/600 to 1/300 of an inch in length; very clear, and granular and brittle so that when they break they often have abruptly rounded or square extremities. The fibers of involuntary muscles form the proper muscular coats of the digestive canal, æsophagus, urinary bladder, trachea, bronchi, gall-bladder, blood-vessels, lymphatics, etc. To this kind of fiber, muscular fiber, the term organic is often applied. The sympathetic or ganglionic portion of the nervous system, which consists of a chain of ganglia connected by nervous cords, extends from the cranium to the pelvis, along each side of the vertebral column, and from which nerves with ganglia proceed to the viscera in the thoracic, abdominal, and pelvic cavities. By its distribution, as well as by its peculiar mode of action, this system is less immediately connected with the mind, as conducting either sensation or the impulses of the will; it is more closely connected than the cerebro-spinal system is with the processes of organic life.