GENERAL HISTOLOGY. 



77 



the goblet cells; here the secretion, usually mucus, is collected as 

 a clear mass in the interior of the cell, the cytoplasm being com- 

 pressed into a thin external wall, reminding one of a goblet con- 

 taining the nucleus at its base (fig. 26, 28, d). Other gland cells 

 are the granular cells, swollen bodies completely filled with secre- 

 tory granules (fig. 26, Ko). Naturally all grades of transition 

 between pavement and glandular epithelium'occur. Commonly 

 the latter name is only emjiloyed when the gland cells are especially 

 numerous, thereby giving to the epithelial area a pre-eminently 

 secretory character. This is especially the case with the structures 

 which have the name of glands, among which we distinguish 

 unicellular and multicellular glands. 



Unicellular Glands.— Unicellular and multicellular glands 

 increase the secretory surface by invagination. Invagination of a 

 single cell produces the unicellular glands which are chiefly found 

 among the invetebrate animals (fig. 

 29); a gland cell here becomes so 

 enormous that there is no room for it '^ ^ 

 in the epithelium, but it is pushed a 

 into the deeper, the subepithelial 

 layers, the nucleated cell body, dis- 

 tended by secretion, sending up a 

 slender process, the duct, to the 

 eiDithelial surface. 



Multicellular Glands. — In the for- 

 mation of multicellular glands a con- 

 siderable area of glandular epithelium 

 grows as a c^dindrical cord or tube 

 from the surface down into the deeper 



tissues; this cord of cells seldom Fig. 29— Unicellular glands from 

 -. . , edge of the mantle of Hrlijc po- 



remams simple; it usually branches matia. c, epithelium; d, unioeiiu- 



T « , -, -, , -, lar glands; rj, pigment cells. 



and lorms the compound glands, 



which may consist of hundreds or thousands of glandular sacs, all 

 emptying into a common duct. Among the multicellular glands 

 are to be distinguished tubular and acinous (racemose) forms. In 

 tubular glands (fig. 30) the simple or branched glandular pouches 

 preserve the same tubular diameter from beginning to end; in the 

 acinous glands (fig. 31), on the contrary, the blind end of the 

 glandular pouch widens into a sac (acinus), largely composed of 

 secretory cells, and related to the outer part of the glandular 

 pouch, the duct, as grapes are to their stem. To the tubular 

 glands belong the liver, kidney and sweat glands of man; to the 



