SEROUS MEMBRANES. 73 



Secretion : Mucous membranes are bathed and lubricated 

 in a mucinous fluid, secreted mostly by goblet-cells or special 

 small mucous glands. Most mucous membranes lined with 

 stratified squamous or stratified ciliated epithelium are pro- 

 vided with large numbers of mucous glands ; in the stomach 

 and intestines (lined by simple columnar epithelium) mucus 

 is secreted by large numbers of goblet-cells, no mucous glands 

 being present. Some mucous membranes lined with stratified 

 squamous epithelium, however, lack both goblet-cells and 

 mucous glands, as in the vagina and bladder, whose mucous 

 secretion is ^produced by the. mucinogenous action of the 

 squamous cells themselves. The mucous glands when present 

 have their alveoli in the submucosa and their ducts penetrate 

 the mucosa to empty on the surface ; the cells of the secreting 

 portions of these glands are of the mucous glandular type, 

 while the ducts are lined with a different form of cell, usually 

 cuboidal or columnar cells similar to or merging into those of 

 the epithelial surface. 



These are the general features of the structure of mucous 

 membranes, but the details vary greatly in different situations. 



Serous membranes, as the pleura, pericardium, peritoneum, 

 tunica vaginalis, and the allied synovial membranes, line 

 certain closed cavities or sacs. The portion of serous mem- 

 branes covering the viscera is called the visceral portion, that 

 reflected over the outer walls of the cavity being the parietal 

 portion. Serous membranes consist of an interlacing fibrous 

 connective-tissue membrane or basis, lined on the free surface 

 by endothelium, while beneath is a variable amount of areolar 

 and often adipose tissue, the subserous tissue, connected with 

 the subjacent structures and analogous to the submucosa. 

 They contain blood and lymphatic vessels. Between the 

 endothelium-cells in some localities are occasional minute 

 orifices or lymph-stomata, guarded by small spheroidal cells ; 

 these open into lymphatic vessels within the membrane. Thus 

 the serous sacs form large lymph-spaces, communicatingdirectly 

 with the lymphatic channels. The function of serous mem- 

 branes is partly associated with that of the lymphatic system, 

 and is largely to obviate friction and to permit and facilitate 

 the gliding movement of viscera upon opposing surfaces. 



