THE GLANDS. 



125 



account of its tall and sometimes striated epithelium. The further path of 

 the excretory passages lies within the connective tissue separating the 

 divisions of the glandular substance and embraces the interlobular and the 

 interlobar duds. The latter join to form the usually single main excretory 

 duct which opens on the free surface of the mucous membrane. The excre- 

 tory duct is lined for some distance by cells resembling those covering the 

 adjoining mucous membrane; where these are stratified squamous in type, 

 this character is retained for only a short distance within the duct, gradually 

 giving place to the simple, sometimes at first double, layer of columnar epi- 

 thelium which extends as far as the intralobular ducts. The walls of the 

 larger ducts consist of a fibrous coat, containing much elastic tissue and lined 



Mucous alveoli 



Serous alveoli 



FIG. 164. Section of tongue, showing alveoli of serous and mucous types of glands. X 60. 



by epithelium; in the large glands, as in the parotid, liver, pancreas, or 

 testicle, the walls are strengthened externally by a layer of unstriped muscle. 



The glandular epithelium lining the alveoli rests upon the basement 

 membrane and usually consists of a single layer of spherical or polygonal 

 secreting cells. The latter do not completely fill the alveolus, but leave an 

 intercellular cleft-like lumen into which the product of the cells is poured and 

 from which the secretion passes into the beginning of the duct. Depending 

 upon the peculiarities of the cells and the character of their secretion, glands 

 are divided into serous and mticous. It should be noted, however, that in 

 many glands both serous and mucous cells occur, either within adjoining 

 primary lobules, or, indeed, within the same alveolus. 



The serous glands are distinguished by cells which are distinctly 

 granular, somewhat pyramidal, and provided with nuclei situated near the 

 centre. The secretion elaborated by such glands is thin and watery. The 



