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 Chemical Composition of Bone.—Experiments (2 and 3) show that the bones contain a mineral or earthy substance, which makes them hard and stiff, and a certain amount of animal matter,called gelatine, which binds the mineral matter together and makes the bones tough and somewhat elastic. The fire burned out the animal matter of the first bone, and the acid dissolved out the mineral matter of the second bone. The mineral matter is chiefly lime, and makes up about two thirds of the weight of the bone. (Why is more mineral than animal matter needed?) The animal gelatine is a gristly substance. As the body grows old, the animal matter of the bones decreases, and they become lighter. They are more easily broken and do not heal so readily as the bones of young persons.

The skeleton is subdivided into the bones of the head, trunk, and limbs. The bones of the trunk are those of the spine, the chest, the shoulder blades, collar bone, and hip bones.

Side view.

The spinal or vertebral column is made up of twenty-six bones (Fig. 31). It is the axis of the human skeleton, to which all other bones are directly or indirectly attached. Animals with inside skeletons have this column, and are called vertebrates. Fish, reptiles, birds, beasts, apes, and man are vertebrates. The spine, as this column is some