398 SECRETION. 



It is also probable that the formation of synovial fluid 

 is a process of more genuine and elaborate secretion, by 

 means of the epithelial cells on the surface of the mem- 

 brane, and especially of those which are accumulated on 

 the edges and processes of the synovial fringes ; for, in its 

 peculiar density, viscidity, and abundance of albumen, 

 synovia differs alike from the serum of blood and from 

 the fluid of any of the serous cavities. 



The mucous membranes line all those passages by which 

 internal parts communicate with the exterior, and by 

 which either matters are eliminated from the body or 

 foreign substances taken into it. They are soft and 

 velvety, and extremely vascular. Their general structure 

 resembles that of serous membranes. It consists of 

 epithelium, basement membrane, and fibro- cellular or 

 areolar tissue containing blood-vessels, lymphatics, and 

 nerves. The structure of mucous membranes is less 

 uniform, . especially as regards their epithelium, than that 

 of serous membranes ; but the varieties of structure 

 in different parts are described in connection with 

 the organs in which mucous membranes are present, 

 and need not be here noticed in detail. The external 

 surfaces of mucous membranes are attached to various 

 other tissues ; in the tongue, for example, to muscle ; 

 on cartilaginous parts, to perichondrium ; in the cells of 

 the ethmoid bone, in the frontal and sphenoid sinuses, 

 as well as in the tympanum, to periosteum ; in the 

 intestinal canal, it is connected with a firm submucous 

 membrane, which on its exterior gives attachment to the 

 fibres of the muscular coat. 



The mucous membranes are described as lining certain 

 principal tracts. I . The digestive tract commences in the 

 cavity of the mouth, from which prolongations pass into 

 the ducts of the salivary glands. From the mouth it passes 

 through the fauces, pharynx, and oesophagus, to the 

 stomach, and is thence continued along the whole tract of 



