Before entering upon the diseases that are peculiar to bones, it will be advantageous to the reader to understand the structure and composition of the material that supports the weight of the animal, and protects the internal organs.

Bone, then, is composed of two distinct parts, namely, earthy and animal matter, in the following proportions: -

The earthy and animal matters are so completely blended together that they appear like a homogeneous mass. Such, however, is not the case, as they can be separated in the following manner. If you steep a bone in hydrochloric acid, you can dissolve out the earthy matter, and by burning you leave the earthy matter, while you destroy the animal. These earthy salts are found in all animal structures. If you make a transverse section, thus , in a hone, you find an opening called the haversion canal, which ramifies into the compact structure. Around these canals the osseous matter is deposited in layers or laminæ. Then, again, around these canals are spaces of an oval shape, termed lacunæ, or empty spaces. There are also numerous fine thread-like canals which run outwards from these haversian ones, and joining the lacunas are termed canaliculi, or little canals. Inside of these are the true hone-eells, extending from the lacunas to the haversian canal, which conveys the plasma of the blood, thereby affording nourishment to the whole bone.

Bones are covered with a material called periosteum, which is nothing more or less than fibrous tissue adhering firmly to the bona This periosteum is the chief source of supply for the compact tissue. It is from its inner surface that the blood vessels pass into the cancellated tissue, while the bone also receives support from the nutrient arteries which traverse the internal canal, and then break up into fine plexuses of bloodvessels.