314 THE ARTICULATIONS. 



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

 upon which it encroaches for a short distance, and to which it is firmly attached ; 

 it then invests the inner surface of the capsular or other ligaments enclosing 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. Hence the articular synovial membrane may be regarded as a short 

 wide tube, attached by its open ends to the margins of the articular cartilages, and 

 covering the inner surface of the various ligaments which connect the articular 

 surfaces, so that along with the cartilages it completely encloses the joint-cavity. 

 In some of the joints the synovial membrane is thrown into folds, which pass 

 across the cavity. They are called synovial ligaments, and are especially distinct 

 in the knee. In other joints there are flattened folds, subdivided at their margins- 

 into fringe-like processes, the vessels of which have a convoluted arrangement. 

 These latter generally project from the synovial membrane near the margin of the 

 cartilage and lie flat upon its surface. They consist of connective tissue covered 

 with endothelium, and contain fat-cells in variable quantities, and, more rarely, 

 isolated cartilage-cells. The larger folds often contain considerable quantities of 

 fat. They were described by Clopton Havers as mucilaginous glands, and as the 

 source of the synovial secretion. Under certain diseased conditions similar pro- 

 cesses are found covering the entire surface of the synovial membrane, forming a 

 mass of pedunculated fibro-fatty growths which project into the joint. Similar 

 structures are also found in some of the bursal and vaginal synovial membranes. 



The bursal synovial membranes 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 bursce mucosce and the bursce synovice. The bursce mucosce are large, 

 simple, or irregular cavities in the subcutaneous areolar tissue, enclosing a clear 

 viscid fluid. They are found in various situations, as between the integument and 

 the front of the patella, over the olecranon, themalleoli, and other prominent parts. 

 The bursce synovice are found interposed between muscles or tendons as they play 

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

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

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

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

 erally 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 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 consists, 

 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. 



1. Synarthrosis. Immovable Articulations. 



Synarthrosis includes all those articulations in which the surfaces of the bones 

 are in almost direct contact, fastened together by an intervening mass of connective 

 tissue, and in which there is no appreciable motion, as the joints between the bones 



