2. The more delicate the organ, the more completely does Nature shield it. For example: the brain, which is soft in structure, is enclosed on all sides by a complete box of bone; the eye, though it must be near the surface of the body to command an extensive view, is sheltered from injury within a deep recess of bone; the lungs, requiring freedom of motion as well as protection, are surrounded by a large case of bone and muscle. The bones serve other useful purposes. They give permanence of form to the body, by {16} holding the softer parts in their proper places. They assist in movement, by affording points of attachment to those organs which have power of motion--the muscles. [Sidenote: 3. Their shape and size? Of what composed? Possibility of being separated? Effect of fire? Of dilute acid?] 3. THE FORM AND COMPOSITION OF THE BONES.--Their shape and size vary greatly in different parts of the body, but generally they are arranged in pairs, one bone for each side of the body. They are composed of both mineral and animal substances, united in the proportion of two parts of the former to one of the latter; and we may separate each of these substances from the other for examination. First, if we expose a bone to the action of fire, the animal substance is driven off, or "burned out." We now find that, though the shape of the bone is perfectly retained, what is left is no longer tough, and does not sustain weight as before. Again, we may remove the mineral portion, which is a form of lime, by placing a bone into a dilute acid. The lime will be dissolved out, and the shape of the bone remain as before; but now its firmness has disappeared, and it may be bent without breaking. [Sidenote: 4. Effect of deficiency of ingredient? Usefulness of the lime? Of the animal substance? Effect of their union? Condition, in youth? Old age?] 4. If, for any reason, either of these ingredients is disproportionate in the bone during life, the body is in danger. The lime is useful in giving rigidity of form, while the animal substance insures toughness and elasticity. By their union, we are able to withstand greater shocks and heavier falls than would be possible with either alone. In youth, the period of greatest activity, the animal constituent is in excess: a bone then does not break so readily, but, when broken, unites with great rapidity and strength. On the other hand, the bones of old persons are more easily broken, and in some cases fail to unite. The mineral matter being then in excess, indicates that the period of active exertion is drawing to a close. {17} [Illustration: FIG. 1.--SECTION OF BONE.] [Illustration: FIG. 2. Structure of bone enlarged.] [Sidenote: 5. In what respect admirably fashioned? Its formation? Microscopic examination? The inference? "Line of beauty?"]