GLANDS. 



1533 



The interlobar tissue and its interlobutar continuations contain the blood-vessels, 

 lymphatics, and nerves supplying the gland and, in addition, the major portion of 

 the excretory ducts. In the larger glands the latter form an elaborate system of pas- 

 sages arranged after the general plan shown in the accompanying diagram (Fig. 

 1285). Traced from the terminal compartments, or alveoli, of the gland, the duct- 

 system begins as a narrow canal, the intermediate duct, lined by low cuboidal or flat- 

 tened cells directly continuous with the glandular epithelium of the alveoli. After a 

 short course the tube increases in diameter and becomes the intralobular duct, which 

 is often conspicuous on account of its tall and sometimes striated or rod-epithelium. 

 The further path of the excretory, tubules lies within the connective tissue separating 

 the divisions of the glandular substance, and embraces the interlobular and the inter- 

 lobar ducts, the latter joining to form a single main excretory duct which opens upon 

 the free surface of the mucous membrane. The last-named passage is lined for some 

 distance by cells resembling those covering the adjacent mucous membrane ; where 

 these are stratified squamous in type, this character is maintained for only a limited 



Mucous aheol 



Serous alveoli 



Section of posterior part of tongue, showing alveoli of serous and mucous types of glands. X 60. 



extent, before the interlobar ducts are reached gradually giving place to a simple, 

 sometimes at first double, layer of columnar epithelium which extends as far as the 

 intralobular tubules. The walls of the larger ducts consist of a fibro-elastic coat, lined 

 by epithelium, and sometimes, in the case of the large glands, as the parotid, liver 

 pancreas, or testicle, are strengthened externally by a layer of involuntary muscle. 

 In the case of the large ducts the latter is usually disposed as a transverse and longi- 

 tudinal layer, to which, as in the hepatic duct (Hendrickson), a third oblique one 

 may be added. Differential stains show the presence of a large amount of elastica. 



The glandular epithelium lining the alveoli rests upon the limiting basement 

 membrane as a single layer of irregularly spherical or polygonal secreting cells ; these 

 do not completely fill the alveolus, but leave an intercellular cleft into which the 

 product of the cells is poured and in which the system of excretory ducts begins. 

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

 glands are divided into serous and mucous. 



