

DEVELOPMENT OF THE JOINTS 283 



is hence termed synovia. In the fetus this membrane is said, by Toynbee, to be 

 continued over the surfaces of the cartilages ; but in the adult such a continuation 

 is wanting, excepting at the circumference of the cartilage, upon which it encroaches 

 for a short distance and to which it is firmly attached. In some of the joints the 

 sy no vial membrane is thrown into folds which pass across the cavity; they are 

 especially distinct in the knee. In other joints there are flattened folds, subdivided 

 at their margins into fringe-like processes which contain convoluted vessels. 

 These folds generally project from the synovia! 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. 



Closely associated with synovial membrane, and therefore conveniently described 

 in this section, are the mucous sheaths of tendons and the mucous bursae. 



Mucous sheaths (vaginae mucosce) serve to facilitate the gliding of tendons in 

 fibroosseous canals. Each sheath is arranged in the form of an elongated closed 

 sac, one layer of which adheres to the wall of the canal, and the other is reflected 

 upon the surface of the enclosed tendon. These sheaths are chiefly found surround- 

 ing the tendons of the Flexor and Extensor muscles of the fingers and toes as they 

 pass through fibroosseous canals in or near the hand and foot. 



Bursse mucosae are interposed between surfaces which glide upon each other. 

 They consist of closed sacs containing a minute quantity of clear viscid fluid, and 

 may be grouped, according to their situations, under the headings subcutaneous, 

 submuscular, subfacial, and subtendinous. 



DEVELOPMENT OF THE JOINTS. 



The mesoderm from which the different parts of the skeleton are formed shows 

 at first no differentiation into masses corresponding with the individual bones. 

 Thus continuous cores of mesoderm form the axes of the limb-buds and a continu- 

 ous column of mesoderm the future vertebral column. The first indications of the 

 bones and joints are circumscribed condensations of the mesoderm; these condensed 

 parts become chondrified and finally ossified to form the bones of the skeleton. 

 The intervening non-condensed portions consist at first of undifferentiated meso- 

 derm, which may develop in one of three directions. It may be converted into 

 fibrous tissue as in the case of the skull bones, a synarthrodial joint being the 

 result, or it may become partly cartilaginous, in which case an amphiarthrodial 

 joint is formed. Again, it may become looser in texture and a cavity ultimately 

 appear in its midst; the cells lining the sides of this cavity form a synovial mem- 

 brane and thus a diarthrodial joint is developed. 



The tissue surrounding the original mesodermal core forms fibrous sheaths for 

 the developing bones, i. e., periosteum and perichondrium, which are continued 

 between the ends of the bones over the synovial membrane as the capsules of the 

 joints. These capsules are not of uniform thickness, so that in them may be 

 recognized especially strengthened bands which are described as ligaments. This, 

 however, is not the only method of formation of ligaments. In some cases by 

 modification of, or derivations from, the tendons surrounding the joint, additional 

 ligamentous bands are provided to further strengthen the articulations. 



In several of the movable joints the mesoderm which originally existed between 

 the ends of the bones does not become completely absorbed a portion of it 

 persists and forms an articular disk. These disks may be intimately associated in 

 their development with the muscles surrounding the joint, e. g., the menisci of the 

 knee-joint, or with cartilaginous elements, representatives of skeletal structures, 

 which are vestigial in human anatomy, e. g., the articular disk of the sterno- 

 clavicular joint. 



