STRUCTURE OF JOINTS. 261 



sy no vial membranes found in the body admit of subdivision into three kinds, 

 articular, bursal, and vaginal. 



The articular synovial membranes are found in all the freely movable joints. 

 In the foetus, this membrane is said, by Toynbee, to be continued over the 

 surface of the cartilages; but in the adult it is wanting, excepting at their 

 circumference, upon which it encroaches for a short distance ; it then invests 

 the inner surface of the capsular or other ligaments inclosing the joint, and is 

 reflected over the surface of any tendons passing through its cavity, as the 

 tendon of the Popliteus in the knee, and the tendon of the Biceps in the shoulder. 

 In most of the joints, the synovial membrane is thrown into folds, which project 

 into the cavity. Some of these folds contain large masses of fat. These are 

 especially distinct in the hip and the knee. Others are flattened folds, subdi- 

 vided at their margins into fringe-like processes, the vessels of which have a 

 convoluted arrangement. The latter generally project from the synovial mem- 

 brane near the margin of the cartilage, and lie flat upon its surface. They 

 consist of connective tissue, covered with epithelium, and contain fat cells in 

 variable quantity, and, more rarely, isolated cartilage cells. They are found in 

 most of the bursal and vaginal, as well as in the articular synovial membranes, 

 and were described, by Clopton Havers, as mucilaginous glands, and as the 

 source of the synovial secretion. Under certain diseased conditions, similar 

 processes are found covering the entire surface of the synovial membrane, 

 forming a mass of pedunculated fibre-fatty growths, which project into the 

 joint. 



The bursss are found interposed between surfaces which move upon each 

 other, producing friction, as in the gliding of a tendon, or of the integument, 

 over projecting bony surfaces. They admit of subdivision into two kinds, the 

 bursse mucosse, and the synovial bursse. The former are large, simple, or irregu- 

 lar cavities in the subcutaneous areolar tissue, inclosing a clear viscid fluid. 

 They are found in various situations, as between the integument and front of 

 the patella, over the olecranon, the malleoli, and other prominent parts. The 

 synovial bursss are found interposed between muscles or tendons as they play 

 over projecting bony surfaces, as between the Glutei muscles and surface of the 

 great trochanter. They consist of a thin wall of connective tissue, partially 

 covered by epithelium, and contain a viscid fluid. Where one of these exists 

 in the neighborhood of a joint, it usually communicates with its cavity, as is 

 generally the case with the bursa between the tendon of the Psoas and Iliacus, 

 and the capsular ligament of the hip, or the one interposed between the under 

 surface of the Subscapularis and the neck of the scapula. 



The vaginal synovial membranes (synovial sheaths) serve to facilitate the 

 gliding of tendons in the osseo-fibrous canals through which they pass. The 

 membrane is here arranged in the form of a sheath, one layer of which adheres 

 to the wall of the canal, and the other is reflected upon the outer surface of the 

 contained tendon; the space between the two free surfaces of the membrane 

 being partially filled with synovia. These sheaths are chiefly found surrounding 

 the tendons of the flexor and extensor muscles of the fingers and toes, as they 

 pass through the osseo-fibrous canals in the hand or foot. 



Synovia is a transparent, yellowish-white, or slightly reddish fluid, viscid like 

 the white of egg, having an alkaline reaction, and slightly saline taste. It con- 

 sists, according to Frerichs, in the ox, of 94.85 water, 0.56 mucus and epithelium, 

 0.07 fat, 3.51 albumen and extractive matter, and 0.99 salts. 



The Articulations are divided into three classes: Synarthrosis, or immovable; 

 Amphiarthrosis, or mixed; and Diarthrosis, or movable joints. 



