DIGESTION 



171 



During activity the cells get rid of their granular material, 

 which gradually passes towards the centre of the acinus or 

 lumen, leaving each cell with a clear outer edge, while that 

 edge next the lumen is still granular (Fig. 60, B). In an ex- 

 hausted condition the cells are smaller and remarkably clear, 

 only a few granules being left in them on the inner edge, while 

 the lumen is now distinct and large, and the nuclei are clearly 

 seen occupying a central position (Fig. 60, C). 



If a mucous gland, such as the submaxillary, be examined at 

 rest under like conditions, the cells are found rilled with granules 

 much larger than those of a serous gland, and a nucleus is seen 

 occupying one edge of the cell (Fig. 61, a). During activity the 

 granules are passed into the lumen of the gland, but they do 

 not leave behind them in the cells the same clear space seen in 

 the serous cell (Fig. 61, b). If the cells, while in an active con- 

 dition, be acted upon by water 

 or dilute acetic acid, the granules 

 swell up and become transparent 

 owing to the mucin they contain, 

 and a delicate network is seen 

 to pervade the cell (Fig. 61, a^. 

 A similar appearance is produced 

 in the exhausted cell (Fig. 61, b'), 

 excepting that less transparent 

 mucin is seen and more granular 

 substance, while the nucleus of 

 the exhausted irrigated gland is 

 seen passing towards the centre 

 of the cell instead of remaining 

 close to the outer wall. Though 

 we have spoken of these granules 

 as mucin, in the gland they are 

 not really mucin, but the mother- 

 substance of it — viz., mucigen — 

 which during the act of secretion is converted into mucin. The 

 same holds good for the serous type ; the granules in the resting 

 gland are the precursors of the ferment or the zymogen of the 

 secretion, from which the secretion is actually formed at the 

 moment it is poured out. 



The outcome of the changes above described proves that the 

 organic elements found in the salivary secretion are manufactured 

 by the cells in the glands ; the inorganic constituents are either 

 the result of filtration or secretion. Experiments made by 

 Langley and Fletcher go to prove that even water and salts are 

 the result of an act of cell-secretion, and not of mere transudation. 



Fig. 61. — Cells from Mucous Gland 

 (Submaxillary Gland of the 

 Dog) (Foster). 



a, From loaded gland ; b, from dis- 

 charged gland; a', b', treated 

 with dilute acetic acid ; a', from 

 loaded ; b', from discharged gland. 



