We are not less interested in tracing the formation of bone through its several stages, than in considering other parts of the human system. The formation of the Haversian canals for the passage of blood-vessels to nourish the bones, the earlier construction of bony tissue by a metamorphosis of cartilaginous substance, and also the commencement of ossification at distinct points, called _centers of ossification_, are all important subjects, requiring the student's careful attention. The bones are protected by an external membranous envelope, which, from its situation is called the _periosteum_. The bones are divided into four classes, _long, short, flat_ and _irregular_, being thus adapted to subserve a variety of purposes. The Long Bones are found in the limbs, where they act as levers to sustain the body and aid in locomotion. Each_long_ bone is composed of a cylinder, known as the _shaft_, and two _extremities_. The shaft is hollow, its wails being _thickest_ in THE middle and growing thinner toward the extremities. The _extremities_ are usually considerably enlarged, for convenience of connection with other bones, and to afford a broad surface for the attachment of muscles. The clavical, humerus, radius, ulna, femur, tibia, fibula, the bones of the metacarpus, metatarsus and the phalanges, are classed as long bones. Where the principal object to be attained is strength, and the motion of the skeleton is limited, the individual bones are short and compressed, as the bones of the carpus and tarsus. The structure of these bones is spongy, except at the surface, where there is a thin crust of compact matter. [Illustration: Fig. 17. Anatomy of a joint, _1, 1_. Bones of a joint. _2, 2_. Cartilage. _3, 3, 3, 3_. Synovial membrane.] [Illustration: Fig. 18. Anatomy of knee joint. _1._ Lower end of thigh-bone. _3._ Knee-pan. _2, 4_ Ligaments of the knee-pan. _5_. Upper end of the tibia, or shin-bone. _6, 12_. Cartilages.] When protection is required for the organs of the body, or a broad flat surface for the attachment of the muscles, the bones are expanded into plates, as in the cranium and shoulder-blades. The _irregular_ or _mixed_ bones are those which, from their peculiar shape, cannot be classed among any of the foregoing divisions. Their structure is similar to the others, consisting of cancellar tissue, surrounded by a crust of compact matter. The vertebræ, sacrum, coccyx, temporal, sphenoid, ethmoid, malar, two maxillary, palate, inferior turbinated, and hyoid are known as irregular bones.