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Structure of Bone.-Osseous tissue, as distinguished from bone in the anatomical sense, belongs to the connective-tissue group (page 29), in which the fundamental substance is permeated with insoluble lime salts, the phosphate and carbonate being the most abundant. With dilute solutions of hydrochloric acid, these can be converted into soluble salts and dissolved out. The osseous matrix left behind is soft and pliable, and yields gelatin on boiling. The surfaces of all bones in the recent state, except where they are covered with cartilage, are invested with a fibrous membrane, the periosteum. The inner surface of this membrane is loose in texture, and supports a fine capillary plexus of blood vessels and numerous protoplasmic cells, the “osteoblasts.” As this layer is directly concerned in the formation of bone, it is spoken of as the osteogenetic layer.

A section of any bone shows that it is composed of two kinds of tissue, compact and cancellated. The compact resembles ivory, and is found on the outer surface ; the cancellated is spongy, and to the naked eye appears to be made up of thin, bony plates, which intersect each other in all directions. It is found in great abundance in the interior of bones. The shaft of a long bone is hollow, the cavity extending almost from one extremity to the other. This central cavity, as well as the interstices of the cancellated tissue, is filled in the recent state with marrow. The marrow or medulla is a vascular tissue, the capillaries of which are supported by a delicate connective-tissue framework. In its meshes are to be found characteristic marrow cells, or osteoblasts, engaged in the formation of bone. The marrow in long bones is yellow, from the presence of fat resulting from a transformation of the protoplasm of connective-tissue cells. In the cancellated tissue, especially near the extremities, this fatty transformation does not take place, and the marrow remains red. The cells are supposed to give birth to red corpuscles.

Examined microscopically, a thin, transverse section of a bone reveals numerous small oval or round openings, which are the transverse sections of canals which run, for the most part, in a longitudinal direction. These are the Haversian canals. In the living state, they are partly filled with blood vessels and lymphatics. The canals are connected with each other and with the surfaces of the bones by numerous anastomosing branches. Around each Haversian canal is a series of concentric lamina, composed of fibers. Between every two laminæ are found small cavities (lacunæ) from which radiate in all directions small canals (canaliculi), which communicate freely with each other. The Haversian canals, with their associated lacunæ and canaliculi, form a series of intercommunicating passages, through which lymph passes for the nourishment of bone. In the cancellated tissue the blood-vessels pass through its interstices, and are supported by

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connective tissue. Bone cells, protoplasmic and nucleated, are found in each lacuna. When young, they are branched, sending their prolongations into the canaliculi.

Cartilage is a modified form of connective tissue. It is opaque, bluishwhite in color, though in thin sections translucent. In some situations it is firm in consistence, in others soft and elastic. All cartilage consists primarily of a ground or fundamental substance throughout which are scattered cells. There are two principal varieties in the human body, viz : hyaline and fibro-cartilage.

Hyaline cartilage is the most typical form, the matrix of which being translucent and homogeneous. It is found on the ends of bones entering into the formation of joints, where it forms articular cartilage, between the ribs and sternum, forming the costal cartilages. It is also found in other situations.

Microscopically examined, the ground substance reveals the presence of oval or spherical corpuscles containing one or more nuclei. The cell substance is frequently marked off from the ground substance by concentric lines, or fibers, which form a capsule for the cell. Repeated division of the cell substance frequently takes place, until the whole capsule is fully occupied with cells. The cell protoplasm is granular, and frequently contains drops of fat. According to some investigators the cell spaces are not isolated, but connected by fine channels, which in turn communicate with lymphatics. By these means nutritive fluid permeates the entire structure.

Fibro-cartilage consists of two varieties, white and yellow.

White fibro-cartilage consists of the usual ground substance pervaded by white fibers. It is firm and resistant, and found wherever strength and fixedness are required. Hence it is present between the vertebræ, forming the intervertebral discs, between the condyle of the lower jaw and glenoid fossa, in the knee joint, around the margins of cup-shaped cavities,

In these situations it assists in maintaining the apposition of the bones, in giving a certain degree of motility to joints, and in diminishing the effect of shock and pressure. The fibers of wbite cartilage are arranged in bundles and layers, the ground substances being relatively less abundant. Between the layers are the usual cartilage corpuscles.

Yellow fibro-cartilage is found in the epiglottis, the external ear, Eustachian tube, and larynx. Primarily hyaline in character, the ground substance becomes pervaded with yellow fibers which branch and interlace in all directions, forming a dense network, but are so arranged as to form small spaces in which are found cartilage corpuscles surrounded by a soft matrix. These fibers are very elastic, and give to this form of cartilage a considerable degree of elasticity.

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STRUCTURE AND MECHANISM OF JOINTS.

The various bones comprising the skeleton do not form a rigid framework, but are united by a variety of structures and in such a manner as to admit of varying degrees of movement. The points of union are termed articulations, or joints.

The structures entering into the formation of joints are : 1. Bones, the articulating surfaces of which are often expanded and variously modified, as in the case of long bones. 2. Hyaline cartilage, which covers the articulating surfaces. Owing to its smoothness it facilitates the gliding movements of opposed surfaces and by its elasticity diminishes the force of jars and shocks imparted to the bones. White fibro-cartilage in the form of interarticular discs is found in many joints. Placed between the ends of bones it subdivides the articulation and adjusts dissimilar surfaces. 3. A synovial membrane, consisting of connective tissue mixed with elastic tissue. Its inner surface is lined with endothelium, which secretes the synovial fluid, a colorless, viscid alkaline fluid containing much mucin, albumin, and fat. Its function is to lubricate the articular surfaces and diminish friction. 4. Ligaments, tough bands of white fibrous tissue which pass from bone to bone in various directions. White fibrous tissue, being inextensible but pliant, maintains the bones in apposition and prevents displacement, but permits of easy movement within certain limits.

Modes of Articulation.-All articulations are divided, according to the extent of movement, into

1. Synarthrodial, comprising those joints endowed with little or no motion; e..., joints or sutures uniting the bones of the skull.

2. Amphiarthrodial, comprising those joints endowed with a slight degree of mobility in consequence of an intervening plate of fibro.cartilage and tough, unyielding ligaments; e. g., vertebral and pelvic joints.

3. Diarthrodial, comprising those joints which are freely movable, the extent of movement, however, being variable. In all such joints the articulating surfaces are generally adapted to each other, are covered with smooth cartilage, lubricated with synovial fluid, and surrounded by ligaments which, while not preventing, yet limit the extent of movement. The diarthrodial joints may be divided according to the character of the movement into (a) arthrodial, or gliding joints, (6) ginglymus, or hinge joints, (c) enarthrodial, or ball and socket joints, (d) trachoidal, or rotatory joints.

The articulations may also be grouped in accordance with the fundamental divisions of the skeleton into (1) axial, (2) appendicular.

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