THE SECRETING MEMBRANES 291 



External secretions are fluids of a definite consist- 

 ency and known function which, when secreted by 

 the epithelial cells and poured from the organ by 

 means of a duct or ducts on to the membrane they are 

 to bathe, etc., perform this given activity. Such 

 secretions are: the saliva, mucus, milk, gastric juice, 

 sebaceous matter, etc. 



The epithelium lining the secretory organs have 

 a general similar histologic arrangement, and resem- 

 blance; however, the difference in the constituents of 

 the secretion is supposed to be based upon their ulti- 

 mate chemic structure. 



The epithelial secretory organ consists of a thin, 

 delicate membrane lined on its functionating surface 

 by a layer of epithelial cells and on the outer side 

 by a net- work of capillary bloodvessels, lymph vessels, 

 and nerves. 



The epithelial secretor organs are subdivided into: 

 (1) secreting membranes; (2) secreting glands. 



THE SECRETING MEMBRANES 



These are the membranes lining the pulmonary 

 and gastro-intestinal tracts, the genito-urinary tracts, 

 and the serous membranes lining closed cavities, such 

 as the pleural, pericardial, peritoneal, and synovial 

 membranes. 



The secretion from the various epithelial cells 

 lining mucous membranes possesses different com- 

 position, according to the locality. It is called mucus, 

 a pale, semitransparent, alkaline fluid containing 

 white cells and epithelial cells. Chemically it consists 

 of water, mineral salts, and a protein mucin. Most 

 of the mucus is secreted by the goblet cells. Mucus 

 is classified according to where secreted, as nasal, 

 bronchial, vaginal, urinary, and gastro-intestinal. 



The serous membranes are practically large lymph 



