280 SECRETION 



the submucous structure. Many of them are provided, also, with 

 simple racemose glands, the ducts passing through the membrane, and 

 the glandular structure being situated in the submucous areolar tissue. 

 The columnar epithelium covering these membranes rests on an amor- 

 phous structure called basement-membrane. The epithelium usually 

 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 attached end. The cells of the lower strata 

 are shorter and more rounded than those in the superficial layer. The 

 cells are pale and 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 ^ 7 to -Q^-Q of an inch (30 to 40 /-t), and their diameter, 

 3 Wo to srVo f an i nc h (8 to IO A 1 )- When villosities exist on the sur- 

 face 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 

 stratified 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 secretions known under the general name of mucus is made up of 

 the products of several different glandular structures. Certain mem- 

 branes 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 digestion, when the gastric 

 glands do not act ; and the gastric glands, during digestion, secrete a 

 liquid of an entirely different character. The secretion produced by 

 the follicles of the small intestine likewise has peculiar digestive prop- 

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

 mucous membranes is covered with more or less secretion show that 

 the general epithelial covering of these membranes is capable of secret- 

 ing a liquid which forms one of the constituents of what is ordinarily 

 recognized as mucus. It is impossible, however, to separate the secre- 

 tion of the superficial layer of cells from the other secretions found on 

 the mucous membranes ; and it will be more convenient to regard as 

 mucus the secretion found upon mucous membranes, except when, as 

 in the case of the gastric or the intestinal juice, a special secretion can 

 be recognized by certain distinctive physiological properties. 



