20 ANATOMY OF THE DOMESTIC FOWL 



The periosteum is firmly attached to the bone by trabeculse. of 

 fibrous tissue, called the fiers 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 meduUa 

 ossium flava,.and red, or meduUa ossium rubra. 



The yellow marrow occurs in all bones except the femur and proxi- 

 mal portion of the tibia of adult fowls. It is composed of a network 

 of fibrous tissue carrying blood-vessels, fat ceUs, and myelocytes, or 

 marrow cells. 



The red marrow is found throughout the femur and ihe proximal 

 portion of the tibia, and in a few of the pelvic bones and vertebrae 

 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 move- 

 ment, 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 ex- 

 treme ends, and of the femur most of the articular head, being 

 cartUagiaous. The bones grow in length by an increase in the carti- 

 lage, 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 ab- 

 sorb 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 hgaments. The shaft is 

 cylindrical and hoUow. 



Short Bones. — Short bones occur in the feet and in the wings. 

 Their structure is similar to that of the long bones. 



