versian canal. These spaces are known as la cuna•, and each one has passing off from it, in a radiate manner, numerous small canals or canaliculi, which anastomose with canaliculi from other belonging to the same system. and probably serve as lymph-channels for the nutrition of the bone. The are not empty, but contain hone cells or corpuscles, which are nucleated bodies and send out fine processes into the canaliculi. In addition to the Haversian systems are irregular lame/b•. or lay ers of bone, which fill in the spaces left between the systems, and are known as interstitial la mell(r. Along the surface of the bone, and also along the side of the central canal, the /fimciicr are arranged parallel to the surface, and are known as circumferential lamella-. The cavities of bones and the various nutrient channels which run through them are filled with a substance called marrow. In all bones of young animals the marrow is red in color, and is known as red marrow. As the bone grows older the marrow becomes lighter in color from the accumulation of fat, and is known as yellow marrow.
harrow consists of a delicate eonnective-tissue net work. in which ramify numerous small blood vessels. In the meshes of this network are found various kinds of cells—mar•ow-cells, or rather large oval cells: giant cells, or myeloplaxes; large cells containing several nuclei; red-blood cells; nucleated red-blood cells: white-blood cells; connective-tissue cells; and fat-cells. The periosteum is a connective - tissue membrane, rich in blood-vessels, which invests the surface of all bones except where they articulate with other hones. Connective-tissue fibres ('Sharpey's fibres') extend from the periosteum into the superficial layers of the bone.
With the exception of the flat bones of the head, all hones are first represented by masses of hyaline cartilage which resemble in shape the future bone. The cartilage is covered over with
the primitive periosteum, or perichondrium. From this membrane blood-vessels extend into the cartilage at some point, usually near the centre of a long bone, and a rearrangement of the car tilage-eells occurs. This point is known as the centre of ossification. New cells. probably de rived from the white-blood cells, appear and are known as osteoblasts. Each osteoblast sur rounds itself with a little layer of bone, and is then known as a hone-cell. In compact bone the formation of the Haversian systems is due to the laying down of the bone in concentric lay ers around the blood-vessels. During the devel opment of bone within the cartilage (intra-carti laginous development), hone is also being formed the periosteum 1 subperiosteal develop ment). Here, also, the bone is formed by the osteohlasts surrounding themselves by hone. The flat bones like those of the skull are not laid down in cartilage like the long bones, hut de velop in a membranous connective-tissue matrix (intramenthranous development). Except for the absence of the prei;xisting cartilage, the process is quite similar to that which occurs in intracartilaginons development.
Nerves may be seen entering bone, and the acute pain felt in some of its diseased conditions proves their existence; but they have not yet been actually demonstrated in the osseous tis sue; neither have lymphatics, though we sup pose, from analogy, that hones are supplied with them. The several bones composing the animal frame will be treated of under the head SKELE TON ; any important peculiarities in the bones of different classes of animals, under the heads of these classes.