SECRETION. 157 



character of the secretion and upon the histological appearance of the secreting 

 cells. The classification as originally made was applied only to the salivary 

 glands and to similar glands found in the mucous membranes of the mouth 

 and oesophagus, the air-passages, conjunctiva, etc. The chemical difference 

 in the secretions of the two types consists in the fact that the secretion of the 

 albuminous (or serous) glands is thin and watery, containing in addition to 

 possible enzymes only water, inorganic salts, and small quantities of albumin ; 

 while that of the mucous glands is stringy and viscid owing to the presence 

 of mucin. As examples of the albuminous glands we have the parotid in 

 man and the mammalia generally, the submaxillary in some animals (rabbit), 

 some of the glands of the mucous membrane of the mouth and nasal cavities, 

 and the lachrymal glands. As examples of the mucous glands, the submaxil- 

 lary in man and most mammals, the sublingual, the orbital, and some of the 

 glands of the mucous membrane of the mouth-cavity, oesophagus, and air- 

 passages. The histological appearance of the secretory cells in the albuminous 

 glands is in typical cases markedly different from that of the cells in the 

 mucous glands. In the albuminous glands the cells are small and densely 

 filled with granular material, so that the cell outlines, in preparations from the 

 fresh gland, cannot be distinguished (see Figs. 70 and 72). In the mucous 

 glands, on the contrary, the cells are larger and much clearer (see Fig. 73). 

 In microscopic preparations of the fresh gland the cells, to use Langley's 

 expression, present the appearance of ground glass, and granules are only 

 indistinctly seen. Treatment with proper reagents brings out the granules, 

 which are, however, larger and less densely packed than in the albuminous 

 glands, and are imbedded in a clear homogeneous substance. Histological 

 examination shows, moreover, that in some glands, e. g. the submaxillary 

 gland, cells of both types occur. Such a gland is usually spoken of as a 

 mucous gland, since its secretion contains mucin, but histologically it is a 

 mixed gland. The terms mucous and albuminous or serous, as applied to the 

 entire gland, are not in fact perfectly satisfactory, since not only do the mucous 

 glands usually contain some secretory cells of the albuminous type, but albu- 

 minous glands, such as the parotid, may also contain cells belonging to the 

 mucous type. The distinction is more satisfactory when it is applied to the 

 individual cells, since the formation of mucin within a secreting cell seems to 

 present a definite histological picture, and we can recognize microscopically a 

 mucous cell from an albuminous cell although the two may occur together in 

 a single alveolus. 



Goblet Cells. The goblet cells found in the epithelium of the intestine 

 afford an interesting example of mucous cells. The epithelium of the intes- 

 tine is a simple columnar epithelium. Scattered among the columnar cells are 

 found cells containing mucin. These cells are originally columnar in shape 

 like the neighboring cells, but their protoplasm undergoes a chemical change 

 of such a character that mucin is produced, causing the cell to become swollen 

 at its free extremity, whence the name of goblet cell. It has been shown that 

 the mucin is formed with the substance of the protoplasm as distinct granules 



