Fig. 442.—Cartilage taken from a diseased finger, in which both cartilage and bone were in a state of degeneration.
Bone.—Bone is a hard unyielding structure, and which in the vertebrata forms the skeleton of the adult. It is the framework for the support of the soft tissues of the body, and forms various cavities for the reception of important organs, as the brain, spinal cord, eyes, heart and lungs, and acts as levers for the action of the muscles and joints. The partial elasticity of bone is seen in the ribs, and the rebound when the skull is dropped on the ground. Bone consists of earthy and animal matters intimately combined; the removal of either, however, does not destroy the form of the bone, if the process of separation be carefully conducted. The earthy constituents may all be dissolved out by hydrochloric acid, but the form of the bone is preserved in its minute particular, and in this state sections may be cut for microscopical examination. If allowed to become dry it shrivels, and assumes the density of horn. The interior of a bone is of a spongy or cancellated structure, particularly at the ends. The outer portion of the bone is more dense than the internal part. The study of bone should commence with sections of the softened structure. Directions for making sections of bone are given in the chapter on Practical Microscopy.
PLATE XX
VERTEBRATA, BONE STRUCTURE.
The intimate structure of bone will be studied in connection with [Plate XX]. Two series of lamellæ may be demonstrated in bone after maceration in acid, a larger system surrounding the medullary canal, and a smaller surrounding the Haversian canals, both of which are seen in Nos. 1 and 2. In macerating bones, the lamellæ of the layer concentric system may be peeled off in layers; these are seen to be pierced with fine apertures, caused by the canaliculi. In some parts larger apertures are seen through which bundles of fibres pass, pinning, as it were, the several layers together; these are the perforating fibres. The outermost of the layers, being near the periosteum, the membrane covering the bone, are termed periosteal layers; the innermost, being close to the canal, are called medullary layers. No. 1 is a transverse section of a flat bone, the clavicle, and it shows the Haversian canals, varying in size from 1⁄2000th to 1⁄200th of an inch in diameter, the largest being near the medullary canal. In shape they are round, oval, or oblong, according to the line of section. Each canal is surrounded by rings, none of which are complete, and running one into the other at various parts. Under a higher power, those irregular shaped bodies termed lacunæ, with fine radiating fibres, are seen to be smaller canals, canaliculi.
By means of this complete and intricate distribution of the canals of the Haversian system, the nutritive fluids pass into the most compact parts of the osseous tissue. Longitudinal sections of the long bones show these canals as continuous branching-out cells.
In many of the lower animals the bony structure differs from those of man, as will be seen in [Plate XX]. No. 3 shows a transverse section of the femur, or leg-bone of an ostrich, magnified ninety-five times, in which the Haversian canals are much smaller and more numerous, and many of them run in the transverse direction. No. 4, again, is a transverse section of the humerus, or fore-arm bone of a turtle (Chelonia mydas). This exhibits traces of Haversian canals, with a slight tendency to a concentric arrangement of bone-cells around them, the bone-cells being large and numerous, and occur, for the most part, in parallel rows. In No. 5, a horizontal section of the lower jaw-bone of a conger-eel exhibits a single plane of bone-cells arranged in parallel lines. There are no Haversian canals present, and when this specimen is contrasted with that of No. 4, it will be noticed that the canaliculi given off from each of the bone-cells of this fish are very few in number in comparison with that of the reptile. No. 6 is a section of a portion of the cranium of a siren (Siren lacertina), remarkable for the large size of the bone-cells, and of the canaliculi, which are larger in this animal than in any other yet examined; and as in the preceding specimen, no Haversian canals are present. No. 7 is a section of bone taken from the exterior of the shaft of the humerus of a Pterodactyle; this exhibits the elongated bone-cells characteristic of the order Reptilia. No. 8 is a horizontal section of a scale, or flattened spine, from the skin of a Trygon, or sting ray; this exhibits large Haversian canals, with numerous wavy parallel tubes, like those of dentine, communicating with them. This specimen shows, besides wavy tubes, numerous bone-cells, whose canaliculi communicate with the tubes, as in dentine.
The following points may be noted with regard to the several sections of bone described. That of the bird, for instance, contrasted with that of the mammal, exhibits the following peculiarities: the Haversian canals are more abundant, much smaller, and often run in a direction at right angles to that of the shaft, by which means the concentric laminated arrangement is in some cases lost; the direction of the canals follows the curve of the bone; the bone-cells are much smaller and more numerous; while the number of canaliculi sent off from the cells is less than in those of mammals. No. 3 is the average length of a bone-cell of the ostrich, 1⁄2000th of an inch, in breadth 1⁄6000th.
In the Reptilia, the bones may be either hollow, cancellated, or solid; and their specific gravity is less than that of birds or mammals. The short bones of most of the chelonian reptiles are solid, and the long bones are either hollow or cancellated; the ribs of the serpent-tribe are hollow, the medullary cavity performing the office of a Haversian canal; the bone-cells are accordingly arranged in concentric circles around their canals. The vertebræ of these animals are solid; and the bone, like that of certain birds, is remarkable for density and whiteness. When a transverse section is taken from one of the long bones, and contrasted with that of a mammal or bird, the difference will be noticed; there are very few, if any, Haversian canals, and these are large; and at one view, in the section, No. 7, the canals and bone-cells are arranged both vertically and longitudinally. The bone-cells are remarkable for the great size to which they attain; in the turtle they are 1⁄375th of an inch in length, the canaliculi are extremely numerous, and are of a size proportionate to that of the bone-cell.