4 INTERNAL SECRETION 



ith tlu nu'inbruiK' are found blood capillaries and lymph 

 spaces. When such a layer of epithelial cells becomes 

 invaginated, we have a tubule or saccule possessing a lumen, 

 and forming a simple tubular or saccular gland. Such glands 

 may be coiled, as in the case of the sweat glands, or the secretory 

 portions of the glands may divide, forming branched tubular 

 glands. This branching may occur again and again, until a 

 complicated structure is produced (compound tubular and 

 compound saccular for racemose] glands). In these glands 

 the terminal portion of the tubes or " alveoli " are the secre- 

 tory portions, while the tubes leading to the exterior are the 

 " ducts." The gland cell varies in its microscopic appearance, 

 according to its functional condition. In the submaxillary 

 gland and in the pancreas the variations are well known and 

 easily observed the discharge of the zymogen granules and 

 the resulting changes in the appearance of the secreting cells. 

 Similar functional changes may be observed in the epithelium 

 of the intestine. 



But it was discovered that some of these glands possessed no 

 duct, and they were therefore called " ductless glands," or in 

 German more usually " Blutgefassdriisen," or " Blutdriisen." 

 The latter names are, however, rapidly falling into disuse, and 

 the terms, " ductless glands," or " glands with an internal 

 secretion," are replacing them. The assumption was at once 

 made that, since these structures had the characters of glands, 

 they must " secrete." But since there was no communication 

 with a free surface, the hypothesis soon arose that in these 

 cases the specific secretion is passed into the blood-stream, 

 and both the process and the product were termed " internal 

 secretion." Thus a new conception in regard to the 

 physiological nature of secretion sprang into existence, and 

 tin- definition of a gland was extended so as to apply to any 

 structure made up of one or more cells of a special epithelial 

 character which form a produci^the secretion which is 



urged upon a free epithelial surface, such as the skin or 

 mucous membrane, or upon the closed epithelial surface of the 

 blood cavities. In some cases the material secreted by the 

 ductless glands has been supposed to be passed away not 

 directly into the blood-stream, but indirectly by means of the 



fiatics. This was formerly believed to apply to the 

 specific secretion of the thyroid gland (see, however, p. 307). 



