CH. XXIX.] 



SECRETING MEMBRANES 



471 



which bathes them the necessary material, and transform it into the 

 secretion which they pour at high pressure into the cavity. 



The principal secreting organs are the following : (1) the serous 

 and synovial membranes; (2) the mucous membranes with their 

 special glands, e.g., the buccal, gastric and intestinal glands ; (3) the 

 salivary glands and pancreas; (4) the mammary glands; (5) the 

 liver ; (6) the lacrimal gland ; (7) the kidney and skin ; and (8) the 

 testes. > 



Serous membranes. We have already discussed the structure 

 of serous membranes (p. 206), 

 and also the question whether 

 the lymph is a true secretion 

 (pp. 318-320). 



The synovial membranes 

 line the joints and the sheaths 

 of tendons and ligaments with 

 which we may include the 

 synovial bursce. The contents 

 of these sacs is called synovia ; 

 it lubricates the surfaces of the 

 joint, and so ensures an easy 

 movement. Synovia is a rich 

 lymph plus a mucinoid mate- 

 rial ; and it is this latter 

 constituent which gives the 

 secretion its viscidity. It is 

 thus a true secretion; and is 

 formed by the epithelial cells 

 which form an imperfect lining 

 to the sac, and which are 

 especially accumulated on the 

 processes of the synovial 

 fringes (fig. 391). 



A mucous membrane consists of two parts : the epithelium on 

 its surface, and the corium of connective tissue beneath. The 

 epithelium generally rests on a basement membrane which is usually 

 composed of clear flattened cells placed edge to edge. 



The name mucous is derived from the fact that these membranes 

 all secrete mucin, the chief constituent of mucus ; this may be formed 

 from the surface epithelium cells breaking down into goblet cells (see 

 p. 26), or an analogous process may occur in the cells of little glands 

 called mucous glands, situated more or less deeply under the epi- 

 thelium, and opening on the surface by ducts. Many mucous 

 membranes (e.g., that of the stomach) form other secretions as well. 



Mucous membranes line all those passages by which internal parts 



FIG. 391. Section of synovial membrane, a, epithelial 

 covering of the elevations of the membrane ; 

 b, underlying tissue containing fat and blood- 

 vessels ; c, ligament covered by the synovial mem- 

 brane. (Cadiat.) 



