64. Motion in Animals. All motion of our bodies is produced by means of muscles. Not only the limbs are moved by them, but even the movements of the stomach and of the heart are controlled by muscles. Every part of the body which is capable of motion has its own special set of muscles. Even when the higher animals are at rest it is possible to observe some kind of motion in them. Trees and stones never move unless acted upon by external force, while the infant and the tiniest insect can execute a great variety of movements. Even in the deepest sleep the beating of the heart and the motion of the chest never cease. In fact, the power to execute spontaneous movement is the most characteristic property of living animals. 65. Kinds of Muscles. Most of the bodily movements, such as affect the limbs and the body as a whole, are performed by muscles under our control. These muscles make up the red flesh or lean parts, which, together with the fat, clothe the bony framework, and give to it general form and proportion. We call these muscular tissues voluntary muscles, because they usually act under the control of the will. The internal organs, as those of digestion, secretion, circulation, and respiration, perform their functions by means of muscular activity of another kind, that is, by that of muscles not under our control. This work goes on quite independently of the will, and during sleep. We call the instruments of this activity involuntary muscles. The voluntary muscles, from peculiarities revealed by the microscope, are also known as striped or striated muscles. The involuntary from their smooth, regular appearance under the microscope are called the unstriped or non-striated muscles. The two kinds of muscles, then, are the red, voluntary, striated muscles, and the smooth, involuntary, non-striated muscles. 66. Structure of Voluntary Muscles. The main substance which clothes the bony framework of the body, and which forms about two-fifths of its weight, is the voluntary muscular tissue. These muscles do not cover and surround the bones in continuous sheets, but consist of separate bundles of flesh, varying in size and length, many of which are capable of independent movement. Each muscle has its own set of blood-vessels, lymphatics, and nerves. It is the blood that gives the red color to the flesh. Blood-vessels and nerves on their way to other parts of the body, do not pass through the muscles, but between them. Each muscle is enveloped in its own sheath of connective tissue, known as the fascia. Muscles are not usually connected directly with bones, but by means of white, glistening cords called tendons. Illustration: Fig. 30.—Striated (voluntary) Muscular Fibers. A, fiber serparating into disks; B, fibrillæ (highly magnified); C, cross section of a disk