EXTERNAL SECRETIONS 481 



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. 



The secreting glands are formed of the same histologic elements as 

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

 membrane or skin, the epithelium of which is variously modified structurally 

 and functionally in the various situations in which they are formed. Like 

 the membranes themselves, the glands are invested by capillary blood- 

 vessels and supplied with lymph-vessels and nerves, of which the latter 

 are in direct connection with the blood-vessels and epithelial cells. The 

 interior of each gland is in communication with the free surface by one or 

 more passageways known as ducts. 



These glands may be classified according as the involution is cylindrical 

 or dilated as 



1. Tubular. The tubular glands may be simple e.g., sweat-glands, 

 intestinal glands, fundus glands of the stomach; or compound e.g., kidney, 

 testicle, salivary, and lachrymal glands. 



2. Alveolar. The alveolar glands may also be simple e.g., the seba- 

 ceous glands, the ovarian follicles, meibomian glands; or compound, as 

 the mammary glands and salivary glands. 



For the production of a secretion it is necessary that the plasma of the 

 blood, the common material, be delivered to the lymph-spaces with which 

 the epithelial cells are in close relation. The processes involved in the pass- 

 age of the plasma across the capillary wall have already been considered 

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