260 MUCOUS MEMBEANES GLANDS 



smallest sebaceous glands of the skin. These are small glandular pouches 

 with a short duct, a constricted neck, and a dilated fundus which, in- 

 stead 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 passing 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 periph- 

 ery of the saccule. 



The epithelium of the secreting saccule rests upon a distinct base- 

 ment membrane and is invested with a very vascular tunica propria. 



Branched Saccular Glands. Branched saccular glands include the 

 larger of the sebaceous glands of the skin, 'in which several sacqules 

 pour their secretion into a common duct, and the tarsal (Meibomian) 

 glands of the eyelids in which a considerable number of saccules open 

 into an axial canal by which the secretion is conveyed to the terminal 

 duct. The structure of each glandular saccule of this type is identical 

 with that of a simple saccular gland. 



Compound Saccular Glands. This type includes only the mam- 

 mary gland. It consists of a system of tubular ducts which possess 

 ampullary dilatations and many branches. Its ducts terminate in small 

 saccular alveoli which have a thin epithelial lining. During the period 

 of their inactivity the lining epithelial cells are much flattened and 

 the acini appear shrunken. The epithelium of the lactating gland, on 

 the other hand, is cuboidal or columnar, the height being more or less 

 dependent upon the accumulation of secretion within the cell. 



The secretion is formed in the same manner as in the tubular glands 

 with an additional process of fatty infiltration by which fat droplets 

 are formed within the cytoplasm. These droplets collect in the central 

 portion of the cell and are finally discharged into the lumen of the acinus 

 with apparent rupture of the cell membrane and the escape of a portion 

 of its superficial cytoplasm. The epithelium is thus capable of repeated 

 secretion. 



The mammary glands may be considered as offering an inter- 

 mediate type between the branched saccular and the tubulo-acinar 

 types. 



Ductless Glands (Endocrine Glands). Under the head of glands 

 it is necessary to consider certain structures which apparently contain 



