104 



THE ESSENTIALS OF HISTOLOGY. 



with alcohol, are clear and swollen. But if examined rapidly in serum,, 

 or in solutions of salt of from 2 to 5 per cent., they are seen to be 

 occupied by large and distinct granules, formed of a substance which is- 

 known as mucigen (fig. 195, a). This mucigen is dissolved out of the 

 cell and discharged into the lumen of the alveolus, and into the ducts- 

 when the gland is stimulated to activity. But in each alveolus there 

 are some smaller cells which do not contain mucigen, and these gener- 



FIG. 194. ALVEOLI OF A SEROUS GLAND. A, AT REST. B, AFTER A SHORT PERIOD OK 



ACTIVITY. 0, AFTER A PROLONGED PERIOD OF ACTIVITY. (Lailgley. ) 

 In A and B the nuclei are obscured by the granules of zyniogen. 



ally form crescentic groups which lie next to the basement-membrane 

 (fig. 192, c). These are the so-called crescents of Gianuzzi ; their con- 

 stituent cells are known also as marginal cells. In alveoli, on the other 



FIG. 195. Mucous CELLS FROM FRESH SUBMAXILLARY GLANDS OF THE DOG. (Langley.) 



a, from a resting or loaded gland ; b, from a gland which has been secreting for some time ; 

 a', b', similar cells which have been treated with dilute acid. 



hand, which do not secrete mucus, but watery or serous saliva, such as 

 the parotid in all animals, and some of the alveoli of the human sub- 

 maxillary, the cells are filled with small granules when the gland is at 

 rest, which do not swell with water nor form mucin ; they appear to be 



