THE ARTICULATIONS, OR JOINTS. 



fTlHE various bones of which the skeleton consists are connected at different 

 JL parts of their surfaces, and such connections are designated by the 

 name of joints, or articulations. Certain joints are immovable, as those between 

 the cranial bones (with the exception of the mandible). In an immovable joint 

 the adjacent margins of the bones are applied in close contact, a thin layer of 

 fibrous membrane, the sutural ligament, or, as at the base of the skull, in certain 

 situations, a thin layer of cartilage, being interposed. Where slight movement is 

 required, combined with great strength, the osseous surfaces are united by tough 

 and elastic fibro cartilages, as in the joints between the vertebral bodies and 

 in the interpubic articulation; but in the movable joints the bones forming the 

 articulation are generally expanded for greater convenience of mutual connection, 

 covered by hyaline cartilage, held together by strong bands or capsules of fibrous 

 tissue called ligaments, and partially lined by a membrane, the synovial membrane, 

 which transudes a fluid to lubricate the various parts of which the joint is formed; 

 so that the structures which enter into the formation of a joint are bone, hyaline 

 cartilage, fibrocartilage, ligament, and synovial membrane 



Bone. Bone constitutes the fundamental element of all the joints. In the 

 long bones the extremities are the parts which form the articulations; they are 

 generally somewhat enlarged, and consist of spongy, cancellous tissue, with a thin 

 coating of compact substance. The layer of compact bone which forms the 

 articular surface, and to which the cartilage is attached, is called the articular 

 lamella. It is of a white color, extremely dense, and varies in thickness. Its 

 structure differs from ordinary bone tissue in this respect, that it contains no 

 Haversian canals, and its lacunae are much larger than in ordinary bone and 

 have no canaliculi. The vessels of the cancellous tissue, as they approach the 

 articular lamella, turn back in loops, and do not perforate it; this layer is conse- 

 quently more dense and firmer than ordinary bone, and is evidently designed to 

 form a firm and unyielding support for the articular cartilage. In the flat bones 

 the articulations usually take place at the edges, and, in the short bones, at various 

 parts of their surface. 



Cartilage. There are three varieties of cartilage A, hyaline; B, fibrocartilage; 

 and C, yellow elastic cartilage; of these, but two, hyaline and fibrocartilage are 

 utilized in the structure of a joint. 



In general, cartilage consists of a genetic investing membrane, the perichondrium, surrounding 

 the cartilage substance proper. The latter consists of the cellular elements, or chondroblasts, 

 and the intercellular substance, or matrix. 



The perichondrium is composed chiefly of white fibrous connective tissue with a few added 

 yellow elastic fibres and cellular elements. The outer part contains few cells, and is called the 

 fibrous layer. The inner part, or genetic layer, contains the bloodvessels, and is rich in flat- 

 tened, elongated, or spindle-shaped cells, the chondroblasts. 



The chondroblasts vary in shape; those immediately beneath the perichondrium are flat 

 and elongated, while those farther in become larger and oval in form. Each cell contains a 

 prominent nucleus embedded in a clear protoplasm that may contain one or more vacuoles. 

 Each cell is sharply outlined, and lies in a space called the lacuna, but two or more cells may be 

 seen to occupy the same lacuna. The matrix immediately surrounding the lacuna is somewhat 

 differentiated from the remaining matrix, and is called the capsule of the lacuna. 



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