THE LYMPHATIC SYSTEM. j^I 



within the ground-substance ; by means of the stomata and the minute 

 passages leading from them the lymphatics communicate with the 

 serous cavities, while, on the other hand, they join with the wide, 

 irregular lymph-channels within the subserous tissue. 



The nerves supplying these membranes are limited, those which 

 are present being largely derived from the sympathetic system, com- 

 posed of pale, non-medullated fibres destined chiefly for the blood- 

 vessels. The few fibres passing into the substance of the membrane 

 form a loose reticulum throughout its deeper layers, from which finer 

 fibrillae extend beneath the surface. 



The synovial membranes, which constitute a second group of 

 serous membranes, include the lining of the clefts developed within 

 the connective tissue (mesoderm) surrounding opposed movable 

 surfaces, embracing the capsules enclosing the articulating surfaces 

 of the various joints, the synovial sheaths in which the tendons glide, 

 and the bursal sacs interposed between surfaces ; these varieties of 

 synovial membranes are known respectively as the articular, the 

 vaginal, and the vesicular. Synovial membranes differ from 

 the serous in the character of their secretion ; that of the former 

 the synovia is a glairy, viscid fluid, resembling the white of egg, 

 well adapted for the lubrication of the opposed parts, and contains 

 fat particles, lymphoid cells, and degenerated endothelial plates. 



FIG. 159. 



^"~s/ ^ ' , ^N^-^jS 



Section of synovial membrane at edge of articular surface : j, j, tissue of synovial membrane bearing 

 villous projections (v, v) ; x, position at which tissues of membrane become continuous with those of 

 periphery of cartilage ; f t group of fat-cells ; /, fibrous tissue constituting peripheral zone of cartilage (c ). 



The secretion moistening serous membranes is thinner, watery, and 

 less suited to the reduction of friction. 



The articular synovial membranes surround the joints, tightly 

 embracing the bones and enclosing them within their sacs, but do 

 not extend over the articulating surfaces, which are composed of 



