THE SALIVARY GLANDS. 163 



except in that part of the duct which immediately opens into the 

 alveoli (functional part) in this it is flattened (d'). The columnar 

 epithelium of the ducts is peculiar, in that the cells show a distinction 

 into two unequal zones, an outer, larger, striated zone, and an inner, 

 smaller, granular one (fig. 191, d). 



FIG. 192. SECTION OF DOG'S SUBMAXILLABY, STAINED. (Kolliker. 

 a, duct ; b, alveolus ; c. crescent. 



FIG. 193. SECTION OF PART OF THE HUMAN SUBM AXILLARY GLAND. (Heidenhain.) 



To the right of the figure is a group of mucous alveoli, to the left a group of serous alveoli. 



The cells of the alveoli differ according to the substance they 

 secrete. In alveoli which secrete mucus, such as all the alveoli of the 

 dog's submaxillary, and some of the alveoli of the same gland in man 

 (fig. 193), the cells, if examined in watery solution or after hardening 



