The periosteum is firmly attached to the bone by trabeculæ of fibrous tissue, called the _fibers of Sharpey_. These fibers of Sharpey penetrate the bone at right angles to its surface and carry blood-vessels. _Marrow._—There are two kinds of marrow: yellow, or medulla ossium flava, and red, or medulla ossium rubra. The _yellow marrow_ occurs in all bones except the femur and proximal portion of the tibia of adult fowls. It is composed of a network of fibrous tissue carrying blood-vessels, fat cells, and myelocytes, or marrow cells. The _red marrow_ is found throughout the femur and the proximal portion of the tibia, and in a few of the pelvic bones and vertebræ in the adult fowl, and in certain other bones of the baby chick. Red marrow consists of a delicate network of connective tissue supporting a dense capillary plexus, a small amount of fat, and numerous cells. The cellular elements of red marrow consist of marrow cells which contain large nuclei and possess ameboid movement, red blood cells, giant cells containing one or more nuclei, and various kinds of leucocytes, including eosinophiles, mast cells, and also osteoclasts. _Growth of Bone._—In the baby chick, only the shaft and a portion of the extremities of the long bones are thoroughly ossified, the extreme ends, and of the femur most of the articular head, being cartilaginous. The bones grow in length by an increase in the cartilage, the cartilage gradually becoming ossified. Growth in diameter is accomplished by the constant deposition of new layers of bone beneath the periosteum. During this process the osteoclasts absorb the bone from within. The formation of the marrow cavity is thus effected. =Classification of Bones.=—The bones of the fowl are classified as long, short, flat, and irregular. _Long Bones._—The long bones occur in the legs and wings, where they serve as levers to sustain weight and make locomotion possible. A long bone consists of a shaft and two extremities. The superior is called the _proximal_ and the inferior the _distal extremity_. The expanded articular surfaces in forming joints with adjoining bones afford ample space for the attachment of ligaments. The shaft is cylindrical and hollow. _Short Bones._—Short bones occur in the feet and in the wings. Their structure is similar to that of the long bones. _Flat Bones._—The flat bones occur where extensive protection is needed, as in the cranial region; or where large surface for muscular attachments is needed, as in the costal and pelvic regions. Flat bones are made up of two thin layers of compact bone with a variable amount of cancellous tissue interposed. _Irregular Bones._—The irregular bones include the vertebræ, the patellæ, and the carpal bones.