DIGESTION IN THE MOUTH. 305 



based upon the character of the secretion, is farther supported by actual 

 morphological differences in the character of the gland-cells. In the 

 serous glands, which are exemplified by the parotid of man and other 

 mammals, the acini are lined by a layer of granular cells, which, in the 

 quiescent condition, completely fill the acinus (Figs. 124 and 125). The 

 nucleus under such conditions is barely distinguishable, its presence 

 being obscured by the large number of granules present. As secretion 

 takes place, these granules disappear, seemingly being broken up and 

 used to form the secretion. During activity, therefore, the outer portion 

 of each cell of a serous gland becomes clear and transparent, and this 

 condition gradually spreads toward the centre of the cell. These 

 changes have been most studied in the parotid of the rabbit. When at 

 rest the nucleus is small, irregular, and devoid of nucleoli. When caused 

 to secrete by stimulation of the sympathetic nerve the cells become 



FIG. 125. PAROTID OF RABBIT AFTER IRRI- 



FIG. 124. PAROTID OF RABBIT IN THE TATION OF THE SYMPATHETIC NERVE. 



RESTING CONDITION. (Heidenhain.) (Heidenhain.) 



smaller, the nuclei become large and round, while the nucleoli may even 

 be detected, and the whole cell stains more deeply with carmine. It thus 

 appears that -during rest granules are manufactured, which disappear 

 during the activity of the cell. 



In the mucous glands, of which the submaxillary or orbital glands of 

 the dog may be taken as a type, the appearances are more complex. When 

 a microscopic preparation is prepared of the resting salivary gland, the 

 cells only stain with difficulty with carmine, this apparently being due to 

 the presence of a large amount of mucin-like substance which occupies 

 the entire cell with the exception of a small amount of unchanged proto- 

 plasm, readily staining with carmine, which remains around the nucleus. 

 In such a section, prepared of the resting gland, in each acinus will usually 

 be found one or more half-moon shaped cells lying outside the muciparous 

 cells, which readily stain with carmine, which possess two or more nuclei, 



20 



