336 SECRETION IN GENERAL 



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 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 liver; 5. the mam- 

 mary glands; 6. the lachrymal glands; 7. the kidney and skin; 8. the 

 testes and ovaries, and 9. thyroid, supra-renal, etc. 



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 Synovial 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 sac 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 rami- 

 fication 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 produces 

 ordinarily only enough secretion to keep the surface moist. 



The Mucous Type. The mucous tracts, and different portions of each 

 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 secret- 

 ing membrane of gland cells, the supporting basement membrane and the net- 

 work of capillaries are still retained in their relative position. With increas- 

 ing complexity of involution the simple mucous membrane becomes packed 

 away in an apparently solid mass. The equivalent of a large amount of 

 secreting surface is thus condensed into a small space. In the process of in- 

 vagination some differentiation occurs in that certain of the gland cells be- 

 come conducting and have their secretory activity somewhat reduced. 



Secreting Glands. The secreting glands present, amid manifold 

 diversities of form and composition, a general plan of structure; but all are 



