In turning into spongy limestone, or animal concrete, they have become one of the deadest tissues in the body. They are tools of the muscles, the levers by which the muscles move the limbs and body about; they never do anything of their own accord. On account of their lifelessness and lack of vitality, they are rather easily attacked by disease, or broken by a blow or fall. There are such a large number of bones (two hundred and six, all told), and they resist decay and last so much longer after death than any other parts of the body, that they fill our museums and text-books of anatomy, form most of our fossils, and have thus given us rather an exaggerated idea of their importance during life. [Illustration: THE HUMAN SKELETON] The Frame-Work of the Body. Just look at any part of the body and imagine that it has a bony core of about the same general shape as itself, and you can reason out all the bones of the skeleton. To begin at the top, the _skull_ is a box of strong, plate-like bones, which have hardened to protect the brain as it grew; and the shape of its upper, or brain, part is exactly that of the head, as you can easily feel by laying your hands upon it. Then come bony shells, or sockets, for the eyes and nose; and, below these, two heavy half-circles of bone, like the jaws of a steel trap, to carry the teeth. The thickness of the lower jaw and the size and squareness of the angle where it bends upward to be hinged to the skull, below the ear, are what give the appearance of squareness and determination to the faces of strong, vigorous men or women. If we want to imply that a person has a feeble will, or weak character, we say he has a "weak jaw." The skull rests upon the top of the backbone, or _spinal column_, which, instead of being one long solid bone, is made up of a number of pieces, or sections, known as _vertebrae_. Each one of these vertebrae has a ring, or arch, upon its back. These, running one after the other, form a jointed, bony tube to protect the _spinal cord_, or main nerve-cable of the body, which runs through it. [Illustration: THE SPINAL COLUMN _V_, vertebra; _C_, cartilage protecting spinal cord; _A_, point of articulation on the right side.] Although the backbone can bend forward or backward, or twist from side to side a little, by the little pieces of bone of which it is built up gliding and turning upon one another, it is really very stiff and rigid, so as to protect the spinal cord and prevent its being stretched or pinched. Most of the movements which we call bending the spine are really movements of other joints which connect the body or head with it. When we bend our necks, for instance, we hardly bend the backbone at all, as most of the movement is made in the joint at the top of it, between it and the skull. Similarly, when we bend our backs, we really bend our backbones very little; for most of the movement comes at the hip joints, between the thighs and the hip bones.