CHAPTER V. 

 GLANDS; MUCOUS AND SEROUS MEMBRANES; ORGANS. 



BEFORE beginning the consideration of the organs and sys- 

 tems in detail, it will be convenient to consider briefly : (1) 

 certain composite structures of wide distribution, namely, 

 glands, mucous membranes, and serous membranes ; and (2) 

 organs in general. 



Glands are of two entirely distinct types : (a) lymphoid and 

 blood-glands ; and (6) secretory or epithelial glands. 



The lymphoid glands, as the lymphatic glands, thymus, 

 tonsils, and lymphoid tissue generally, consist of an open net- 

 work of retiform tissue the meshes of which are filled with 

 lymph-corpuscles and lymph. The lymph-stream passes 

 directly through them. They have no proper secretion, 

 possess no outlet-ducts, and communicate only with lymphatic 

 vessels. Their function is not completely understood, but 

 they partly serve to develop some of the corpuscular elements 

 of the blood and lymph, and partly to arrest noxious sub- 

 stances in the circulation. 



The blood-glands, as the spleen, bear the same relation to the 

 blood-stream that the lymphoid glands do to the lymph- 

 stream. In general structure they resemble the lymphoid 

 glands. 



The secretory glands remove or secrete from the blood 

 various substances and fluids needed for special purposes. 

 The active agent in this process is the glandular epithelium, 

 which forms a layer supported on a basement-membrane ; 

 beneath and in close relation with the epithelium is a rich 

 capillary blood-supply ; on the free surface of the epithelium 

 is an open space or cavity in which the secreted product col- 

 lects and is carried away. Such is the general structure of 

 all secretory glands : a sac of epithelium with blood-supply 

 on the outside and a collecting cavity inside. The epithelium 



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