254 THE DIGESTIVE ORGANS. 



branches enter the lobules and join the tubules and alveoli. The 

 ducts of the human submaxillary glands have been carefully inves- 

 tigated by Flint ; his account is here followed. The submaxillary 

 duct (Wharton's duct) generally divides into three primary ducts, 

 which extend in various directions and are usually relatively short, 

 dividing into the interlobular ducts, which often run for relatively 

 long distances before giving off individual branches. They run in 

 the connective tissue between the lobules, and give off branches 

 and end in ducts which ramify between the lobules and are known 

 as sublobular ducts, which in turn give rise to lobar ducts, which 

 generally ramify through three or four divisions which follow in 

 close succession, forming the intralobular ducts. These radiate 

 from the centre toward the periphery of the lobules, without, how- 

 ever, reaching the periphery. The terminal branches of the intra- 

 lobular ducts are the intermediate ducts (intercalary), which are in 

 communication with the secretory compartment, the tubules. 



The epithelium lining the different portions of the large excre- 

 tory ducts varies somewhat. For a short distance from their oral 

 end they are lined by a stratified columnar epithelium consisting 

 of two layers of cells (Wharton's ducts are now and then lined 

 for a short distance by a stratified pavement epithelium continuous 

 with that lining the mouth). Beyond this stratified columnar epi- 

 thelium, which extends for a variable distance along the large 

 excretory ducts, the interlobular ducts and the sublobular ducts are 

 lined by a pseudostratified columnar epithelium, possessing two 

 rows of nuclei (Steiner). The intralobular ducts are lined by a 

 single layer of columnar cells, the basal half of each cell showing 

 a distinct striation. The intermediate portions of the ducts are 

 lined by a low, cubic, or flattened epithelium. The epithelium of 

 the ducts rests on a basement membrane, consisting of very fine, 

 closely woven connective-tissue fibrils (Flint). External to this 

 there is a sheath of areolar connective tissue, which shows external 

 to the basement membrane a layer of closely woven elastic fibers. 

 The larger divisions of the duct have nonstriated muscle-cells in 

 their walls. 



Between the membrana propria and the secreting epithelium of 

 the tube, and more especially in the acini, are branched cells which 

 anastomose with each other, the so-called basket cells. The origin 

 of these cells has not been fully determined ; their existence even 

 has been questioned. Their processes penetrate between the glan- 

 dular cells and form a supporting structure for them. The mem- 

 brana propria surrounding the entire glandular tube is in close 

 relationship to these cells. 



We shall now consider more in detail the structure of the 

 alveoli, tubules, and of the salivary glands. 



