SECRETION. 439 



The Epithelial Secretor Organs. The epithelial secretor organ 

 consists primarily of a thin delicate homogeneous membrane, one side of 

 which is covered with a layer of epithelial cells and the other side of which 

 is closely invested by a network of capillary blood-vessels, lymph-vessels, 

 and nerves. Though the epithelial cells have a general histologic resem- 

 blance one to another, their physiologic function varies in different situations, 

 in accordance probably with their ultimate chemic structure, a fact which 

 determines the difference in the character of the secretions. 



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

 group of cells, and may be subdivided into 



1. Secreting membranes. 



2. Secreting glands. 



The secreting membranes are the mucous membranes lining the 

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

 serous membranes lining closed cavities, such as the pleural, pericardial, 

 peritoneal, and synovial membranes. 



The mucous membranes are soft and velvety in character and are com- 

 posed 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 physiologic 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, semi-transparent, 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 

 viscidity. 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, and 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 membranes enclose 

 what are practically large lymph sacs or spaces, and the fluid they contain 

 resembles lymph in all respects and is practically 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 a mucin-like substance, a nucleo- 

 albumin, which imparts to it considerable viscidity. This synovial fluid 

 serves to diminish friction between the opposing surfaces of the bones as 

 they glide over one another during movement. 



Other secretions, such as the aqueous and vitreous humors of the eye, 

 the fluid of the internal ear, the cerebrospinal fluid, etc., will be considered 

 in connection with the organs with which they are associated, as have been 

 the digestive secretions. 



The secreting glands are formed of the same histologic elements as 

 the secreting membranes. They are formed by an involution of the mucous 



