SECRETION IN GENERAL 



sometimes occur in which the secretion continues to be formed by the natural 

 organ, but, not being able to escape toward the exterior, on account of some 

 obstruction, is reabsorbed and accumulates in the blood. It may be dis- 

 charged from the body in other ways; but these are not instances of true 

 vicarious secretions, and must not be so regarded. 



Organs and Tissues of Secretion. The principal secreting organs 

 are the following: i, The serous and synovia! membranes; 2, The mucous mem- 

 branes with their special glands, e.g., the buccal, gastric, and intestinal glands; 

 3, The salivary glands and pancreas; 4, The liver; 5, The mammary glands; 

 6, The lachrymal glands; 7, The kidney and skin; and 8, the testes and 

 ovaries. 



The special structure and functions of the secreting organs will be given 

 in greater detail in the chapters which immediately follow. The general 

 types of structure and general conditions that influence the functions are 

 introduced at this point. 



Structural Types of Secreting Organs. Serous and Synomal Type. 

 The serous membranes form closed sacs lining visceral cavities like the 

 abdominal, pericardial, or pleural cavities. The organs are, as it were, pushed 

 into this s:ic and carry before them an investment of membrane. The serous 

 membranes consist of a single layer of flattened polygonal cells resting on a 

 supporting membrane of connective tissue, supporting a ramification of blood- 

 vessels, lymphatics, and nerves. 



In some instances, i.e., synovial membranes, the secreting layer is in- 

 creased by finger-like elevations. This type of secreting organ producer 

 ordinarily only enough secretion to keep the surface moist. 



The Mucous Type. The mucous tracts, and different portions of e::ch 

 of them, present certain structural peculiarities, adapted to the functions 

 which each part has to discharge; yet in some essential characters the mucous 

 membrane is the same, from whatever part it is obtained. In all the princi- 

 pal and larger parts of the several tracts it presents an external layer of epithe- 

 lium, situated upon a basement membrane, and beneath this a stratum of 

 vascular tissue of variable thickness, containing lymphatic vessels and nerves. 

 The vascular stratum, together with the basement membrane and epithelium, 

 in certain cases is elevated into minute papillae and villi, in others depressed 

 into involutions in the form of glands. But in the invaginations of the secreting 

 membrane of gland cells, the supporting basement membrane and the network of 

 capillaries are still reta ined in their relative position. With increasing complexity 

 of involution the simple mucous membrane becomes packed away in an ap- 

 parently solid mass. The equivalent of a large amount of secreting surface 

 is thus condensed into a small space. In the process of invagination some 

 differentiation occurs in that certain of the gland tubes become conducting 

 and have their secretory activity somewhat reduced. But there is no distinc- 

 tion that can be drawn between simple mucous membranes and gland cells. 



