CH. xxix.] SECRETING GLANDS. 469 



A mucous membrane consists of two parts ; the epithelium 

 on its surface, and the coriuni 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 muein, the chief constituent of mucus ; this may be 

 formed from the surface epithelium cells breaking down into 

 goblet cells (see p. 29), or an analogous process may occur in 

 the cells of little glands called mucous glands, situated more or 

 less deeply under the epithelium, 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 communicate with the exterior, and by which either matters 

 are eliminated from the body or foreign substances taken into 

 it. The principal tracts are G astro-pulmonary and Genito-urinary ; 

 the former is sub-divided into the Digestive and Respiratory 

 tracts. 



Secreting glands may be classified according to certain types, 

 which are the following : i. The simple tubular gland (A, fig. 392), 

 examples of which are furnished by the crypts of Lieberkiihn, 

 in the intestinal wall. They are simple tubular depressions of 

 the mucous membrane, the wall of which is formed of a basement 

 membrane and is lined with secreting cells arranged as an 

 epithelium. To the same class may be referred the elongated 

 and tortuous sudoriferous glands. 



2. The compound tubular glands (D, fig. 392) form another 

 division. These consist of main gland-tubes, which divide and 

 sub-divide. Each gland may be made up of the subdivisions of 

 one or more main tubes. The ultimate subdivisions of the tubes 

 are generally highly convoluted. They are formed of a basement- 

 membrane, lined by epithelium of various forms. The larger 

 tubes may have an outside coating of fibrous, areolar, or 

 muscular tissue. The kidneys and testes are examples of this 

 type. 



3. The racemose glands are those in which a number of vesicles 

 or acini are arranged in groups or lobules (c, fig. 392). The Meibo- 

 mian follicles are examples of this kind of gland. Some glands like 

 the pancreas are of a mixed character, combining some of the cha- 

 racters of the tubular with others of the racemose type ; these are 

 called tubulo-racemose or tubulo-acinous glands. These glands differ 

 from each other only in secondary points of structure, but all have 

 the same essential character in consisting of rounded groups of 

 vesicles containing gland-cells, and opening by a common central 



