MUCOUS MEMBRANES AND GLANDS. 



139 



ducts, and the excretory ducts, which latter usually unite to form 

 a single common duct of large size. 



At the open end of the acinus the lining cells of the latter become 

 flattened or cuboidal, and, together with the basement-membrane, 

 are directly continuous with the similar structures forming the walls of 

 the narrow intermediate tubule ; the latter succeeds the acinus as 

 the continuation of the narrow intercellular clefts of several adjacent 

 acini, and, after a longer or shorter course as a delicate narrow- 

 lumened canal, passes into the intralobular tube. The distinctive 

 characters of the latter are its larger lumen and the columnar epithe- 

 lium, many cells of which exhibit a distinct vertical striation through- 

 out the peripheral zone next the basement-membrane. The branch- 

 ing intralobular tubes, on emerging from the lobular tissue, join to 

 form the interlobular duct which occupies the connective tissue lying 

 between and holding together the divisions of the glandular sub- 



FIG. 165. 



FIG. 166. 



Section of racemose gland showing relation 

 of glandular tissue to origin of duct : x, acini 

 lined with secreting cells which are directly 

 continuous with those of the intermediate 

 tubule (z) ; y, interlobular connective tissue. 



I 



Section of the human parotid gland 

 showing the interlobular tissue : s, s t se- 

 creting cells of surrounding acini ; d, inter- 

 lobular duct ; v, blood-vessels within the 

 fibrous tissue ; g, group of ganglion-cells. 



stance. The interlobular ducts are clothed with simple columnar 

 cells, which form a passive lining to the canal for the conveyance of 

 the secretions of the more active parts of the gland. Towards the 

 free surface of the mucous membrane the interlobular ducts unite to 

 form the chief, often single, excretory duct of large lumen, whose 

 walls for a variable distance from the point of discharge are covered 

 with epithelium similar to that covering the adjoining mucous surface ; 

 this is soon replaced, however, by the columnar cells which then 

 continue into the smaller tubes. In the large ducts the subepithelial 

 tissue is strengthened by net-works of elastic fibres. 



The saccules or alveoli are limited by a basement-membrane 



