39 6 TEXT-BOOK OF PHYSIOLOGY. 



between the two classes of fluids, no sharp line can be drawn between 

 the cell processes which take place in secretory and excretory organs. 

 All secretory organs may be divided into 



1. Epithelial. 



2. Reticular and vascular, the latter term indicating merely their re- 



lation to blood-vessels. 



The Epithelial Secretory Apparatus. The apparatus essential 

 to the production of a secretion is a delicate homogeneous mem- 

 brane, on one side of which and in close contact is a layer of capillary 

 blood-vessels, lymphatics, and nerves; on the other side, a layer of 

 epithelial cells, the physiologic function of which varies in different 

 situations. 



The epithelial organs may consist of a single layer of cells or a 

 group of cells, and may be subdivided into 

 1 1. Secreting membranes. 

 2. Secreting glands. 



The secreting membranes are the nrur^ms membranes lining 

 the gastro-intestinal, the pulmonary, and the genito-minary tracts, 

 and the seTous membranes lining closed cavities, such asfThe pleural, 

 pericardial, peritoneal, and synovial membranes. 



The mucous membranes are soft and velvety in character and 

 are composed of a condensed connective tissue forming a basement 

 membrane beneath which is a layer of blood-vessels and muscle-fibers, 

 and on which is a layer of epithelium, the histologic as well as physio- 

 logic characters of which vary in different situations. The mucus 

 secreted by the various epithelial forms will very naturally possess a 

 somewhat different composition, according to the locality in which it 

 is formed. In a general way it may be said that -mucus is a pale, 

 semitransparent, alkaline fluid, containing leukocytes and epithelial 

 cells. It is composed chemically of water, mineral salts, and an 

 albuminoid body, mucin, to the presence of which it owes its vis- 

 cidity. Much of the mucus is secreted by the goblet cells on the 

 surface of the mucous membranes. The principal varieties of mucus 

 are the nasal, bronchial, vaginal, urinary, gastro-intestinal. 



The serous membranes are composed of thin membrane formed 

 by a condensation of connective tissue and covered by a single layer 

 of large, flat, nucleated cells with irregular margins. These mem- 

 branes enclose what are practically large lymph sacs or spaces, and 

 the fluid they contain resembles lymph in all respects and is prac- 

 tically identical with it. It serves to diminish friction when the 

 viscera they enclose move over one another. The most important 

 of the serous membranes are the pleural, pericardial, and peritoneal. 



The synovial membranes in and around joints resemble serous 

 membranes! The cells covering them, however, secrete a clear 

 colorless fluid resembling lymph, but differing from it in containing 



