50 SECRETION. 



mucus. The mucous membrane of the stomach produces an 

 alkaline, viscid secretion, during the intervals of digestion, 

 when the gastric tubules do not act ; and the gastric tubules, 

 during digestion, secrete a fluid of an entirely different char- 

 acter. The fluid produced by the follicles of the small in- 

 testine likewise has peculiar digestive properties. These 

 circumstances, and the fact that the entire extent of the mu- 

 cous membranes is covered with more or less secretion, show 

 that the general epithelial covering of these membranes is 

 capable of secreting a fluid which forms one of the constitu- 

 ents of what is ordinarily recognized as mucus. It is im- 

 possible, however, to separate the secretion of the superficial 

 layer of cells from the other fluids that are found on the 

 mucous membranes ; and it will be more convenient to re- 

 gard as mucus, the secretion which is found upon mucous 

 membranes, except when, as in the case of the gastric or the 

 intestinal juice, we can recognize a special fluid by certain 

 distinctive physiological properties. 



In the membranes covered with cylinder-epithelium, which 

 are usually provided with numerous simple follicles, the se- 

 cretion is produced mainly by these follicles, but in part by 

 the epithelium covering the general surface. The mem- 

 branes covered with pavement-epithelium usually contain 

 but few follicles, and are provided with simple racemose 

 glands situated in the submucous structure, which are to be 

 regarded rather as appendages to the membrane. The secre- 

 tion is here produced by the epithelium on the free surface, 

 and is always mixed with fluids resulting from the action of 

 the mucous glands. 



There is nothing to be said with regard to the mechanism 

 of the secretion of mucus beyond what has already been 

 stated in connection with the general mechanism of secretion. 

 All the mucous membranes are quite vascular, and the cells 

 covering the membrane and lining the follicles and glands at- 

 tached to it have the property of taking from the blood the 

 materials necessary for the formation of the secretion 



