THE DIGESTIVE TRACT. 



The large excretory duct of the parotid gland, or Stenson's 

 duct, contained within the interlobular connective tissue, is composed 

 of a fibre-elastic tunica 



propria, lined by a simple FIG. 227. 



low columnar epithe- 

 lium, and strengthened 

 externally by fibrous tissue. 

 Passing into the smaller 

 ducts, the salivary tubes, 

 the cylindrical epithelium 

 becomes slightly taller, and 

 exhibits a distinct vertical 

 radial striation in its outer 

 zone. On entering the 

 intralobular divisions of 

 the ducts, or interme- 

 diate tubes, the columnar 

 epithelium is replaced by 

 low flattened cells, which 

 finally pass into the dilated 

 terminal compartments, be- 

 coming directly continuous with the secreting cells lining the acini. 



The acini are limited by the basement-membrane, the prolonga- 

 tion of that of the smaller ducts, and almost 

 completely filled by the irregularly polyhedral 

 glandular epithelium, the narrow intercellular 

 cleft which remains representing the commence- 

 ment of the lumen of the system of ducts. 

 The appearance of the cells of the acini varies 

 with the stages of secretion ; when quies- 

 cent and filled with the serous secretion, the 

 cells appear larger, clearer, and coarsely granu- 

 lar, while after functional activity and in the ex- 

 hausted condition they are smaller, darker, and 

 finely granular, the threads of the protoplasm 

 lying closely packed, and not, as when the gland is at rest, sepa- 

 rated by the intervening particles of stored-up secretion. 



The sublingual gland possesses the general arrangement already 

 considered in connection with the parotid gland, its peculiarity being 

 the absence of the intermediate division of the duct, the intralobular 

 or " mucous" tubes passing at once into the acini. 



The cells lining the saccules are encountered in all stages of secre- 

 tion. During rest the majority are clear, being filled with homo- 

 geneous viscid mucus. After the discharge of this secretion, fol- 



Section of human parotid gland, exhibiting general ar- 

 rangement of lobules (a) ; b, interlobular connective tissue 

 containing large ducts (c) and blood-vessels (v) ; d, intra- 

 lobular ducts. 



FIG. 228. 



Section of human parotid 

 gland, including several 

 acini : d, cut intralobular 

 duct. 



