Page:Structure and functions of the body; a hand-book of anatomy and physiology for nurses and others desiring a practical knowledge of the subject (IA structurefunctio00fiskrich).pdf/25

 *ing various salts of calcium, magnesium, and sodium. In young children the animal matter predominates and the bones are soft and often bend instead of breaking, only the outside shell on one side giving way, as in "green-*stick" fracture. In rickets there is a deficiency of lime salts, but the increased brittleness of the bones in old age is due, not to increase of mineral matter, but to the less spongy texture of old bones.

Classification and Function of Bones.—There are in the body some two hundred bones, which may be classified as long, short, flat, and irregular. Occasionally an irregular bone develops in a fontanelle, the membranous opening at the juncture of the sutures of the skull. This is known as a Wormian bone. It is not, however, included in the two hundred, as are not the sesamoid bones or bones developed in tendons, with the exception of the patella or knee-cap.

Long bones are developed in cartilage and consist of a shaft, two extremities, and various processes. They are more or less curved to give them strength and grace. They serve as supports and act as levers for purposes of motion and the exercise of power. Since a hollow cylinder is just as strong as a solid one of the same size, the weight coming only on the outer shell, the great bones which are accountable for weight and which need to be light themselves have hollow shafts, composed chiefly of compact tissue with a central medullary canal. The ends, however, are expanded in order to make better connection at the joints and to afford broad surfaces for muscular attachment, cancellous tissue being used in them for lightness and strength. The large spongy ends also give elasticity and lessen jar, and by bringing the tendons to the bone at a greater angle increase their effectiveness. Blood is brought to the long bones not only by the vessels of the periosteum but by the medullary artery, which penetrates the compact tissue by the nutrient foramen and divides into an ascending and a descending branch.