Part 2

Compact Bone Tissue.—The compact bone tissue forms the hard outer layer of all bones. It is thickest in the shaft and becomes thin toward the extremities. Through the compact bone tissue approximately parallel with the longitudinal axis of the bone, run canals called Haversian canals, through which pass blood and lymph vessels for the nourishment of the bone and nerves. The Haversian canals are surrounded by concentric lamellæ. The spaces between the cylinders thus formed are filled with interstitial lamellæ; and both the exterior surface of the bone and the interior surface surrounding the medullary canal, are built up of peripheral, or circumferential lamellæ. Between the lamellæ, somewhat irregularly placed, are minute reservoirs, called lacunæ, which contain bone corpuscles. From the lacunæ radiate minute canals, or canaliculi, which maintain circulation through the bone substance, and which communicate with the Haversian canals. Complex anastomoses exist among the canaliculi. Still other channels for the passage of blood-vessels are Volkmann’s canals which pierce the peripheral lamellæ, thus allowing vessels to pass from the periosteum to the Haversian canals. Similar channels afford communication between the inner Haversian canals and the medullary cavity.

The entire structure composed of an Haversian canal, its surrounding lamellæ, lacunæ, and canaliculi, with their contained vessels, is called an Haversian system.

Cancellous Bone Tissue.—The cancellous bone tissue forms the bulk of the short, flat, and irregular bones and of the extremities of the long bones. It consists of delicate bony plates and spicules, which intercross in various directions. The spaces between these plates and spicules, called cancelli, are occupied by marrow except in the bones that are pneumatic. The blood-vessels, lymphatics, and nerves course through this marrow but are not arranged in an Haversian system.

FIG. 1.—Longitudinal section of compact bone of the femur of the hen. 1, Haversian canals. 2, Lacunæ with their canaliculi. ]

The Periosteum.—Covering the surface of bone, except at the articular surface where it is covered with cartilage, is a membrane, the periosteum, which consists of two layers: an outer, fibrous, protective layer, and an inner, cellular, osteogenic layer. The outer layer consists principally of white fibrous tissue. The inner layer contains many more connective-tissue cells, which gradually become more closely aggregated as we proceed toward the osseous surface; but there is no sharply defined line of demarcation between the two periosteal layers.

FIG. 2.—Transverse section of compact bone of the femur of the hen. 1, The lacunæ and canaliculi. 2, The periosteum. ]

FIG. 3.—Transverse section of compact bone of the femur of the hen showing the lacunæ and canaliculi under high magnification. ]

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.

=Composition of Bone.=—Bone consists of organic and inorganic matter. Organic matter gives toughness and elasticity to the bone, and inorganic matter hardness. The organic substance of bone is called ossein. When boiled in water ossein is resolved into gelatin. The following tables give the results of an analysis of the femur, fresh, of a mature hen.

Footnote 1:

Grateful acknowledgment is hereby made to Dan M. McCarty, Chemist, Animal Industry Division, North Carolina Agricultural Experiment Station, for this analysis.

Fresh femur: Water 18.23 per cent. Dry matter 81.77 per cent. Dry matter: Organic matter 63.09 parts Inorganic matter 18.68 parts Salts in dry matter: Calcium 6.970 per cent. Magnesium 0.283 per cent. Potassium 0.004 per cent. Sodium 0.276 per cent. Iron 0.020 per cent. Phosphorus 3.210 per cent. Sulphur 0.085 per cent. Chlorine 0.520 per cent. Carbon dioxid 0.550 per cent.

The inorganic matter of the femur of the hen consists of 18.68 parts or 22.84 per cent. of dry matter, and the organic matter of 77.16 per cent. Stated in other words the femur, including its contained marrow, consists of organic and inorganic matter in the ratio of 3.4, approximately, to 1.

=The Skeleton of the Fowl.=—The skeleton of a bird is remarkable for the rapidity of its ossification. It is worthy of note that other parts of the bodies of adult birds also become ossified. Among such parts are the tendons of the muscles of the legs, of the feet, and of the neck; the plates of the corneal margin of the sclerotic tunic of the eye; and the stapes of the ear. Ossification in birds at the attachments of the semi-lunar valves of the aorta and of the pulmonary artery has been reported by Owen.

The bony structure is compact, and the bones contain a greater proportion of phosphate of lime than do the osseous structures of mammals. Especially is this the case in those parts of the skeleton which are permeated by air.

THE DIVISIONS OF THE SKELETON

( The Skull ( Cranium ( ( Face The Axial skeleton ( ( Cervical region ( ( Dorsal region ( Ribs ( The Vertebral column ( ( Sternum ( Lumbar region ( Sacral region ( Coccygeal region

( Scapula ( Shoulder girdle ( Coracoid ( ( Clavicle ( ( ( Arm ( Humerus ( ( ( Fore limb ( Forearm ( Radius ( ( ( Ulna ( ( ( ( ( Carpus The Appendicular skeleton ( ( Hand ( Metacarpus ( ( Phalanges ( ( Pelvic girdle ( Ilium ( (Hip bone) ( Ischium ( ( Pubis ( ( ( Thigh ( Femur ( Hind limb ( ( Leg ( Tibia ( ( Fibula ( ( Foot ( Metatarsus ( Phalanges

The bodies of birds contain many air reservoirs to make them light that flying may be more easy. Many bones have their weight in proportion to size and strength thus greatly reduced. In very young birds the cavities of bones contain, instead of air spaces, loosely arranged red marrow, which is in most bones later absorbed. The air reservoirs in bones are most capacious in the best flyers. In the non-flyers more of the bones retain their red marrow.