_Action of the Muscles of the Abdomen._—The trunk is maintained from falling backward by the action of those huge muscles on its anterior surface. The space between the pelvis and the thorax is called the abdominal cavity. Its walls are almost wholly composed of muscles. There are several important facts to be noted about these muscles. First, that they extend from the brim of the pelvis, into which they are inserted, to the ribs and breast bone, to which the other ends of these muscles are attached; that there are three layers of these muscles; and, lastly, that the fibers of the different layers run in different directions, so that they cross each other, as shown in the figure. The reënforcement of the layers, the arrangement of their fibers, and the manner in which they dovetail into the adjacent groups of fibers give a structure of the greatest possible strength. [Illustration: Fig. 24.—Muscles of the anterior surface of the trunk (left side, superficial; right side, deep).] _Action of the Muscles of the Back._—The trunk is kept from falling forward by the action of the muscles of the back. These are arranged in six layers. The cut shows the direction of the fibers. The first, or outside layer, consists of the trapezius and latissimus dorsi, or, in other words, the broad muscle of the back. On the one side these muscles are attached to the spines of the vertebræ; the sharp ridge which is felt in the middle of the back, and the broad attachment to the pelvis afford a firm base of support. There are other muscles which run parallel with the spinal column, whose function it is to hold the spinal column erect. [Illustration: Fig. 25.—Muscles of the posterior surface of the trunk (left side, superficial; right side, deep).] Standing erect calls into action almost all the muscles of the lower extremities, trunk, and neck. So long as the line of gravity falls within the line of the feet, the muscular effort required is so slight that it is little more than the tonicity contained in all living muscle. The greater the displacement of the line of gravity, the greater the muscular effort required to maintain the equilibrium of the body. _Muscular Energy._—The muscles of the body, even when at rest, are under a slight degree of tension. When stimulated, the muscle contracts—that is, it becomes shorter and thicker. A muscle can only remain in a state of contraction for a few seconds, because the force of the muscular fibers is more or less exhausted during contraction. The more rapid the contractions, the sooner does fatigue manifest itself. Like the steam engine, the muscles of the body, in performing their work, produce heat and motion. The fuel which supplies this force is taken into the body in the shape of food; it is prepared for use in the intestinal tract, and from there carried by the blood, to be stored up in the muscles and various tissues as latent force. The muscles contain one-fourth of all the blood in the body.