SECRETING GLANDS 193 



the free surface of a mucous membrane, and the branches and 

 twigs as the larger and smaller interlobular ducts reach out in all 

 directions to finally end in minute alveolar dilatations, the secret- 

 ing acini. 



Except for the ducts of certain mucous glands whose epithelial 

 coat resembles that of the mucous membrane to which they are 

 attached, the ducts of this type of secreting gland are lined by 

 columnar cells whose cytoplasm frequently presents a rodded ap- 

 pearance at the deeper end of the cell. The acini contain typical 

 serous or mucous, secreting epithelium. Occasionally the secret- 

 ing cells are also found for some distance beyond the acinus in the 

 lining membrane of the smallest ducts. 



The tubules and acini of these glands are invested with a base- 

 ment membrane and a delicate tunica propria. The acini are 

 united by the connective tissue into small groups which inclose a 

 central duct of the smallest type, the intercalary duct. These 

 acinar groups are again united into the lobules of the gland by fine 

 bands of connective tissue, and broader bands of loose connective 

 tissue cement the many lobules into one glandular mass. The in- 

 tercalary ducts by union within the lobule form numerous small 

 intralobular ducts which approach the periphery of the lobule and 

 at its margin open into the interlobular ducts; the latter are 

 found in the broader septa of connective tissue between the lob- 

 ules. The interlobular ducts by union with one another result in 

 progressively larger branches which finally form the main excretory 

 duct of the gland. 



Simple saccular glands occur as the smallest sebaceous glands 

 of the skin. These are small glandular pouches with a short duct, 

 a constricted neck, and a dilated fundus which, instead of having 

 a single coat of epithelium as in most of the tubular glands, is 

 more or less completely filled with a mass of epithelial cells. The 

 cells as they approach the duct of the gland show progressive 

 stages of degeneration and disintegration which culminate in the 

 formation of a thick, viscid, fatty secretion. Since these cells form 

 their secretion by disintegration they are obviously capable of 

 pussing through the various stages of secretory activity but once, 

 and hence they must be renewed by the repeated mitotic cell 

 division which occurs at the periphery of the saccule. 



The epithelium of the secreting saccule rests upon a distinct 

 iKisrniciit membrane and is invested with a very vascular tunica 

 propria. 



14 



