The essentials of healthA text-book of anatomy, physiology, hygiene, alcohol, and narcotics . hearm. Others cannot be controlled in this way; they dotheir work independent of any action of the will, hencethey are called involuntary. The muscles of the stom-ach and the heart are of this variety. The heart beats,the stomach contracts, and we are powerless to stoptheir action. As a rule, all those movements in the THE MUSCLES. 1S1 body most essentia] to life are not under the control ofthe will. Yet many of the involuntary muscles can be rout rolled, although for a short time only. An illus-trati
The essentials of healthA text-book of anatomy, physiology, hygiene, alcohol, and narcotics . hearm. Others cannot be controlled in this way; they dotheir work independent of any action of the will, hencethey are called involuntary. The muscles of the stom-ach and the heart are of this variety. The heart beats,the stomach contracts, and we are powerless to stoptheir action. As a rule, all those movements in the THE MUSCLES. 1S1 body most essentia] to life are not under the control ofthe will. Yet many of the involuntary muscles can be rout rolled, although for a short time only. An illus-tration of this is found inordinary breathing. Webreathe a certain number oftimes a minute and arc entirelyunconscious of it; still by aneffort of the will, we canbreathe faster, slower, ordeeper. Even the voluntarymuscles can be made to con-tract by a sharp blow, or bysome fright. While voluntarymuscles, therefore, are con-trolled by the will, they arenot invariably so all the voluntary mus-cles are attached to bone ateach end ; while the involun-tary are not attached to the. Fig. 71. Voluntary muscle,with its blood vessels : (1) the skeleton, but are found in the muscle fibers (2) the blood ves- n c i u x1 sels, magnified. walls or hollow organs, as the stomach and intestines, and in the walls of the arteries. The Uses of Muscle. The muscles are primarily theorgans of motion. They act as a protection to theblood vessels and nerves; they inclose the large tho-racic and abdominal cavities; they serve as cushionsto diminish the force of falls and blows; they fill upirregularities, and thus add to the symmetry of thewhole boch . Structure of Voluntary Muscle. If a piece of lean meat, 182 THE ESSENTIALS OF HEALTH. which is voluntary muscle, be boiled, it will appear as ifready to fall apart into little bundles of tissue. Thesebundles may be easily divided intostill smaller ones, by separatingthem carefully with needles. In thisway minute threads of tissue are ob-taine
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