120 THE ESSENTIALS OF HISTOLOGY. 



the serous membranes of man. The rest of the thickness of the mem- 

 brane is composed of connective 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 differ- 

 entiated into the serous membrane. 



SYNOVIAL MEMBRANES. 



The synovial 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, there is either no 

 epithelial lining, or it occurs only in patches, in place of the continuous 

 lining which we find in the serous membranes. Curious villus-like pro- 

 jections 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. 



