384 THE DIGESTIVE SYSTEM 



which, in the larger duets, is invested with a fibre-elastic coat containing 

 a few longitudinal smooth muscle fibers. 



The ducts divide and subdivide in an arborescent manner, the larger 

 branches lying in the connective tissue which invests the lobules into 

 which the gland is subdivided, while the smaller branches are found 

 within the lobule. The duct system is thus divisible into interlobular 

 and intralobular ducts; the latter include generally an excretory por- 

 tion continuous with the interlobular duct, a modified 'salivary' se- 

 cretory portion, and a constricted intercalary or intermediate portion 

 connecting with the acinus. 



In the smaller glands of the mouth the number of subdivisions 

 of the duct system is relatively small, but in the larger salivary glands 

 the small ducts are practically innumerable. Thus, in the submaxillary 

 gland, Flint (Amer. Jour, of Anat., 1902) found that the interlobular 

 duct system formed 1,500 terminal branches, each of which entered 

 a lobule and was further subdivided into intralobular and intercalary 

 ducts before terminating in the secreting acini. The larger glands 

 may therefore be said to bear to the smaller ones represented in Fig. 

 353, a relation which is comparable with that of a full-grown tree to the 

 youngest sapling. 



The larger salivary glands are enveloped by a fibre-elastic capsule 

 continuous with the adjacent areolar tissue. From this capsule coarse 

 trabecula? enter to divide the gland into groups of lobules, the lobes. 

 The lobules are invested by more delicate septa from the interlobar 

 trabeculse. The ducts of the interlobar connective tissue may be desig- 

 nated interlobar ducts to distinguish them from the interlobular ducts 

 between the lobules. These are the excretory ducts of the system ; the 

 'salivary' intralobular ducts have a secretory function. The glandular 

 tissue is known as the parenchyma, the connective tissue as the inter- 

 stitial tissue of the gland. 



The smaller intralobular ('salivary') ducts are lined by columnar 

 epithelium whose cells contain two zones, one on either side of the 

 centrally situated nucleus. The distal zone or free extremity of the 

 cell is finely granular, the proximal zone or base presents a character- 

 istic striated appearance which is apparently due to a fibrillar structure 

 of the cytoplasm in this portion of the cell. The basal fibrillas are 

 probably, in part at least, the mitochondria which are present in all 

 functionally active cells. The epithelium is easily detached from its 

 basement membrane by the artificial contraction of the tissues during 

 fixation and hardening. 



