_ | 1. Uses. _ | 2. Composition. | 1. FORM, STRUCTURE, | 3. Structure. | ETC., OF THE BONES | 4. Growth. | | 5. Repair. THE SKELETON | |_6. The Joints. | _ | 2. CLASSIFICATION OF | 1. The Head. |_ THE BONES. | 2. The Trunk. |_3. The Limbs. THE SKELETON. I. FORM, STRUCTURE, ETC., OF THE BONES. (_See page 269_.) THE SKELETON, or framework of the "House we live in," is composed of about 200 bones. [Footnote: The precise number varies in different periods of life. Several which are separated in youth become united in old age. Thus five of the "false vertebræ" at the base of the spine early join in one great bone--the sacrum; while four tiny ones below it often run into a bony mass--the coccyx (Fig. 6); in the child, the sternum is composed of eight pieces, while in the adult it consists of only three. While, however, the number of the bones is uncertain, their relative length is so exact that the length of the entire skeleton, and thence the height of the man, can be obtained by measuring a single one of the principal bones. Fossil bones and those found at Pompeii have the same proportion as our own.] USES AND FORMS OF THE BONES.--They have three principal uses: 1. To protect the delicate organs; [Footnote: An organ is a portion of the body designed for a particular use, called its _function_. Thus the heart circulates the blood; the liver produces the bile.] 2. To serve as levers on which the muscles may act to produce motion; and 3. To preserve the shape of the body. Bones differ in form according to the uses they subserve. For convenience in walking, some are long; for strength and compactness, some are short and thick; for covering a cavity, some are flat; and for special purposes, some are irregular. The general form is such as to combine strength and lightness. For example, all the long bones of the limbs are round and hollow, thus giving with the same weight a greater strength, [Footnote: Cut a sheet of foolscap in two pieces. Roll one half into a compact cylinder, and fold the other into a close, flat strip; support the ends of each and hang weights in the middle until they bend. The superior strength of the roll will astonish one unfamiliar with this mechanical principle. In a rod, the particles break in succession, first those on the outside, and later those in the center. In a tube, the particles are all arranged where they resist the first strain. Iron pillars are therefore cast hollow. Stalks of grass and grain are so light as to bend before a breath of wind, yet are stiff enough to sustain their load of seed. Bone has been found by experiment to possess twice the resisting property of solid oak.] and also a larger surface for the attachment of the muscles.