SEROUS AND SYNOVIAL MEMBRANES 99 



passing to which they form folds, within which blood-vessels, tymphatics, 

 and nerves pass to the viscera. 



The inner surface is lined by a continuous layer of pavement- 

 epithelium (fig. 122), which is very distinct in nitrate of silver prepa- 

 rations. In some places there are apertures in the epithelium which 

 lead direct into subjacent lymphatic vessels. These apertures are called 

 stomata, and are surrounded by small protoplasmic cells (fig. 122, s, s). 

 They are most numerous upon the peritoneal surface of the diaphragm, 

 but are present in all serous membianes, and they serve to prevent any 

 undue accumulation of lymph within the serous cavity during health. 

 The pavement-epithelium rests upon a homogeneous basement-mem- 

 brane, which is especially well marked in the serous membranes of 

 man. The rest of the thickness of the membrane is composed of con- 

 nective tissue, with a network of fine elastic fibres near the inner 

 surface. 



The cavities of the serous membranes are originally formed in the 

 embryo as a cleft in the mesoblast (pleuro-peritoneal split) which 

 becomes lined with epithelium, and its wall eventually becomes dif- 

 ferentiated into the serous membrane. 



The sy no vial membranes, which are often compared with the 

 serous membranes, and are indeed, like the latter, connective-tissue 

 membranes which bound closed cavities moistened with fluid, are not 

 so intimately connected with the lymphatic system, nor is the fluid 

 (synovia) which moistens them of the nature of lymph. Moreover, it 

 is only here and there that there is a lining of epithelium-like cells, in 

 place of the continuous lining of epithelium which we find in the 

 serous membranes. Curious villus-like projections occur in many 

 parts ; they are covered by small rounded cells, and probably serve- to 

 extend the surface for the secretion of synovia. The blood-vessels of 

 synovial membranes are numerous, and approach close to the inner 

 surface of the membrane. 



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