_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. [Illustration: 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. [Illustration: FIG. 2.—Transverse section of compact bone of the femur of the hen. 1, The lacunæ and canaliculi. 2, The periosteum. ] [Illustration: FIG. 3.—Transverse section of compact bone of the femur of the hen showing the lacunæ and canaliculi under high magnification. ]