126 



NORMAL HISTOLOGY. 



general appearance of the cells depends upon the number and size of the 

 particles or droplets of secretion stored within their cytoplasm and changes 

 markedly with the variations of functional activity of the gland. When a 

 serous gland is at rest, its cells are loaded with secretion and appear, therefore, 

 larger and coarsely granular. After active function, on the other hand, the 

 cells are exhausted and appear smaller and almost free from granules, often 

 exhibiting a differentiation into a clear outer zone, devoid of granules, and a 

 darker inner zone, next the lumen, in which secretion-granules still remain. 

 The mucous glands elaborate a clear viscid homogeneous secretion, 

 which when present in quantity, as during rest, distends the cells, crowding 

 the nuclei against the basement membrane and giving the cells a clear 



and transparent character. 

 When loaded and distended 

 with secretion, the transpar- 

 ent cells have well-defined 

 outlines and a narrow 

 peripheral zone containing 

 the displaced nucleus and 

 granular cytoplasm. After 

 prolonged activity, the ex- 

 hausted cells contain rela- 

 tively little secretion, hence 

 the threads of spongioplasm 

 are no longer separated 

 but near together. In con- 

 sequence, the cells lose their 

 transparency and become 

 smaller, darker and more 

 granular than when the 

 gland is resting. 



The alveoli of mixed 

 mucous glands often contain 

 crescentic groups of small 

 cells lying between the usual 

 large clear elements and the 

 basement membrane ( Fig. 

 165). These are the demi- 

 lunes. Although opinions 

 differ as to their nature, it is 

 probable that ordinarily they 

 are aggregations of serous cells. In order to afford means of escape for their 

 secretion, since the serous cells are excluded from the lumen of the alveolus by 

 the mucous elements, minute intercellular channels, the secretion-canaliculi, 

 pass from the main lumen to the demilunes (Fig. 166). Such secretion- 

 canaliculi are not limited to mixed mucous glands, but are found in serous 

 alveoli and in other glands containing isolated secreting cells, as in the peptic 

 glands of the stomach (Fig. 198). Probably not all demilunes are composed 

 of serous cells, since small groups of mucous cells, when containing little 

 secretion, become peripherally displaced by the distended cells and then 

 appear as crescents. These, however, are not provided with secretion- 

 canaliculi. Minute secretion-channels have been described within the cyto- 

 plasm of glandular epithelium, as the liver cells ; it is undecided, however, 

 whether such intracellular secretion-canaliculi are preformed or not. 



Demilune of 

 serous cells 



Duct 



. Mucous cells 



Demilune 



FIG. 165. Section of sublingual gland, showing serous cells 

 grouped as demilunes. X 245. 



