STRUCTURE OF JOINTS. . 103 



ion of the cartilaginous incrustation of the inferior extremity of the 

 radius. 



The fibro-cartilages of circumference contain a large proportion 

 of fibrous tissue ; they are situated upon the margins of the glenoid 

 and cotyloid cavities, and serve to deepen those articulations, and 

 at the same time to protect the edges from injurious pressure. 



The inter vertebral substance is composed of concentric lamellae of 

 fibrous structure, surrounding, towards the centre, a soft cartilagi- 

 nous and almost pulpy mass. The fibres of which the lamellae are 

 formed, interlace with each other obliquely; the intervening meshes 

 being filled with a soft cartilaginous substance, and becoming larger 

 towards the central pulp. 



Ligament is composed of numerous straight fibres collected 

 together, and arranged into short bands (fasciculi) of various 

 breadth, or interwoven so as to form a broad layer which com- 

 pletely surrounds the articular extremities of the bones, and consti- 

 tutes a capsular ligament. 



All the ligaments of the joints consist of fibres of this kind, which 

 are white, glistening and inelastic. But besides these there are 

 other ligaments, which are composed of yellow elastic tissue, and 

 serve to connect parts which are subject to frequent and conside- 

 rable separation, as the arches of the vertebrae, where they form 

 the ligamenta subflava. 



The synovial membrane is a thin membranous layer, which invests 

 the articular cartilages of the bones, and is thence reflected upon 

 the surfaces of the ligaments which surround and enter into the 

 composition of a joint. It resembles the serous membranes in being 

 a shut sac, and secretes a transparent and viscous fluid, which is 

 named synovia. Synovia is an alkaline secretion, containing 

 albumen, which is coagulable at a boiling temperature. The con- 

 tinuation of this membrane over the surface of the articular carti- 

 lage, a much agitated question, has lately been decided by the inte- 

 resting discoveries of Henle, who has ascertained the existence of an 

 epithelium upon cartilage identical with that secreted by the reflected 

 portion of the membrane. In some of the joints the synovial mem- 

 brane is pressed into the articular cavity by a cushion of fat, which 

 serves the purpose of facilitating the movements of the surfaces. This 

 mass was called by Havers the synovial gland, from an incorrect 

 supposition that it was the source of the synovia. It is found in the 

 hip and in the knee-joint. In the knee-joint, moreover, the synovial 

 membrane forms folds, which are most improperly named ligaments, 

 as the mucous and alar ligaments, the two latter being an 

 appendage to the cushion of fat. Besides the synovial membranes 

 entering into the composition of joints, there are numerous smaller 

 sacs of a similar kind interposed between surfaces which move 

 upon each other so as to cause friction ; they are often associated 

 with the articulations. These are the burses mucoscs; they are 

 shut sacs, analogous in structure to synovial membranes, and 

 secreting a similar synovial fluid. 



