The bones are composed of two substances, one animal, and the other mineral. The animal part is a very fine network, called _cellular membrane._ In this are deposited the harder mineral substances, which are composed principally of carbonate and phosphate of lime. In very early life, the bones consist chiefly of the animal part, and are then soft and pliant. As the child advances in age, the bones grow harder, by the gradual deposition of the phosphate of lime, which is supplied by the food, and carried to the bones by the blood. In old age, the hardest material preponderates; malting the bones more brittle than in earlier life. The bones are covered with a thin skin or membrane, filled with small blood-vessels which convey nourishment to them, Where the hones unite with others to form joints, they are covered with _cartilage,_ which is a smooth, white, elastic substance. This enables the joints to move smoothly, while its elasticity prevents injuries from sudden jars. The joints are bound together by strong, elastic bands called _ligaments,_ which hold them firmly and prevent dislocation. Between the ends of the bones that unite to form joints are small sacks or bags, that contain a soft lubricating fluid. This answers the same purpose fur the joints as oil in making machinery work smoothly, while the supply is constant and always in exact proportion to the demand. If you will examine the leg of some fowl, you can see the cartilage that covers the ends of the bones at the joints, and the strong white ligaments that bind the joints together. The health, of the bones depends on the proper nourishment and exercise of the body as much as that of any other part. When a child is feeble and unhealthy, or when it grows up without exercise, the bones do not become firm and hard as they are when the body is healthfully developed by exercise. The size as well as the strength of the bones, to a certain extent, also depend upon exercise and good health. [Illustration: Fig. 61] The chief supporter of the body is the spine, which consists of twenty-four small bones, interlocked or hooked into each other, while between them are elastic cushions of cartilage which aid in preserving the upright, natural position. Fig. 61 shows three of the spinal bones, hooked into each other, the dark spaces showing the disks or flat circular plates of cartilage between them. The spine is held in its proper position, partly by the ribs, partly by muscles, partly by aid of the elastic disks, and partly by the close packing of the intestines in front of it. The upper part of the spine is often thrown out of its proper position by constant stooping of the head over books or work. This affects the elastic disks so that they grow thick at the back side and thinner at the front side by such constant pressure. The result is the awkward projection of the head forward which is often seen in schools and colleges.