226 THE ORGANS 



oral mucosa are small branched tubular glands. Only in those parts 

 of the mucous membrane which are closely attached to underlying 

 bone, as on the gums and hard palate, are mucous glands few or 

 entirely absent. While the deeper portions of the glands are in the 

 submucosa, some of the tubules usually He in the stroma of the mucous 

 membrane. 



The ducts open upon the surface and are lined with a continuation 

 of the surface stratified squamous epithelium as far as the first bifur- 

 cation. Here the epithelium becomes stratified columnar, and this, 

 as the smaller branches are approached, passes over into the simple 

 columnar type. Not infrequently ducts of small secondary glands 

 empty into the main duct during its passage through the mucosa. 



According to the character of their secretions, the oral glands are 

 divided into : 



{a) Mucous glands, which secrete a mucin-containing fluid 

 (mucus) ; 



{h) Serous glands, which secrete a serous (albuminous) fluid; 



(c) Mixed glands, the secretion of which is partly mucous and 

 partly serous. 



Morphologically, also, a similar distinction can be made in regard 

 to the glandular epithelium which lines the terminal tubules, the 

 tubules of mucous glands being hned with "mucous" cells, those of 

 serous glands with "serous cells," while of the mixed glands the cells 

 of some tubules are mucous, of others serous. In certain tubules 

 both mucous and serous cells occur. The appearance which these 

 cells present depends largely upon their secretory condition at the 

 time of death. 



■^, Serous cells when resting have a slightly granular protoplasm, 

 which in the fresh condition is highly refractive, giving the cells a 

 transparent appearance. With the beginning of secretion the gran- 

 ules increase in number and the cells become darker.,* Stained with 

 hsematoxyUn-eosin, serous tubules have a purple or red color. ^, The 

 nuclei are spherical or oval, and are situated between the centre and 

 base of the cell (Fig. 185, p. 284). 



Mucous cells are in the quiescent state rather small cuboidal or 

 pyramidal cells, with cloudy cytoplasm and nuclei situated at the 

 base of the cell. When active the mucous cells are much larger, 

 with clear cytoplasm and with nuclei flattened against the basement 

 membrane. ; The protoplasm of the fresh unstained mucous cell is 

 less highly refractive than that of the serous cell. It consequenth' 



