318 SECRETION. 



structure being situated in the submucous areolar tissue. The columnar epi- 

 thelium covering these membranes rests upon an amorphous structure called 

 basement-membrane. The epithelium generally presents but few layers, and 

 sometimes, as in the intestinal canal, there is only a single layer. The cells 

 are prismoidal, with a large, free extremity, and a pointed end which is at- 

 tached. The cells of the lower strata are shorter and more rounded than 

 those in the superficial layer. The cells are pale and very closely adherent 

 to each other by their sides, each with a moderate-sized, oval nucleus and one 

 or two nucleoli. The length of the cells is -g-^ to -g-J-g: of an inch (30 to 40 /*), 

 and their diameter, 7 oW to YF<FO f an i ncn (8 to 10 /x). When villosities 

 exist on the surface of the membranes, the cells follow the elevations and do 

 not fill up the spaces between them, as in most of the membranes covered 

 with squamous epithelium. 



The mucous membrane of the urinary bladder, of the ureters and of the 

 pelvis of the kidneys can not be classed in either of the above divisions. In 

 these situations the membrane is covered with mixed epithelium, presenting 

 all varieties of form between the squamous and the columnar, some of the 

 cells being caudate and quite irregular in shape. 



Mechanism of the Secretion of Mucus. Nearly every one of the many 

 fluids known under the name of mucus is composed of the products of sev- 

 eral different glandular structures. Certain membranes which do not possess 

 glands, as the mucous lining of the ureters and of a great portion of the 

 urinary bladder, are capable of secreting mucus. The mucous membrane of 

 the stomach produces an alkaline, viscid secretion, during the intervals of di- 

 gestion, when the gastric glands do not act ; and the gastric glands, during 

 digestion, secrete a fluid of an entirely different character. The fluid pro- 

 duced by the follicles of the small intestine likewise has peculiar digestive 

 properties. These considerations and the fact that the entire extent of the 

 mucous membranes is covered with more or less secretion show that the gen- 

 eral epithelial covering of these membranes is capable of secreting a fluid 

 which forms one of the constituents of what is ordinarily recognized as 

 mucus. It is impossible, 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 regard as mucus, the secretion which is 

 found upon mucous membranes, except when, as in the case of the gastric or 

 the intestinal juice, a special fluid can be recognized by certain distinctive 

 physiological properties. 



In the membranes covered with columnar epithelium, which are usually 

 provided with simple follicles, the secretion is produced mainly by these fol- 

 licles, but in part by the epithelium covering the general surface. The 

 membranes covered with squamous epithelium usually contain but few folli- 

 cles and are provided with simple racemose glands situated in the submucous 

 structure, which are to be regarded as appendages to the membrane. The 

 secretion 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 secre- 



