298 HANDBOOK OF PHYSIOLOGY. 



Every secreting apparatus possesses, as essential parts of its structure, 

 a simple and almost textureless membrane, named the primary or base- 

 ment-membrane; certain cells; and blood-vessels. These three structural 

 elements are arranged together in various ways; but all the varieties may 

 be classed under one or other of two principal divisions, namely, mem- 

 branes and glands. 



Organs and Tissues of Secretion. 



The principal secreting organs are the following: (1) the serous and 

 synovial membranes; (*2) the mucous membranes with their special 

 glands, e.g., the buccal, gastric, and intestinal glands; (3) the salivary 

 glands and pancreas; (4) the mammary glands; (5) the liver; (G) the 

 lachrymal gland ; (7) the kidney and skin ; and (8) the testes. 



The structure and functions of the glands secreting materials used in 

 digestion will be considered when we study the alimentary tract. The 

 functions of the kidney and skin will be described in a future chapter. 



The lachrymal gland will be considered with the rest of the optic 

 apparatus and the testes in the Chapter on Generation. There remain, 

 then, the serous and mucous membranes and the mammary gland to be 

 here described. 



(1.) Serous and Synovial Membranes. Serous membranes are of 

 two principal kinds: 1st. Those which line visceral cavities, the arach- 

 noid, pericardium, pleurae, peritoneum, and tnnicce vaginales. 2d. The 

 synovial membranes lining the joints, and the sheaths of tendons 

 and ligaments, with which, also, are usually included the synovial lursce, 

 or burscB mucosw, whether these be subcutaneous, or situated beneath 

 tendons and glide over bones. 



The serous membranes form closed sacs, and exist wherever the free 

 surfaces of viscera come into contact with each other or lie in cavities 

 unattached to surrounding parts. The viscera invested by a serous 

 membrane are, as it were, pressed into the shut sac which it forms, 

 carrying before them a portion of the membrane, which serves as their 

 investment. To the law that serous membranes form shut sacs, there 

 is, in the human subject, one exception, viz. : the opening of the Fal- 

 lopian tubes into the abdominal cavity, an arrangement which exists 

 in man and all Yertebrata, with the exception of a few fishes. 



The serous membranes are especially distinguished by the characters 

 of the endothelium covering their free surface : it always consists of a 

 single layer of polygonal cells. The ground substance of most serous 

 membranes consists of connective-tissue corpuscles of various forms 

 lying in the branching spaces which constitute the lymph canalicular 

 system, and interwoven with bundles of white fibrous tissue, and nu- 



