MTCUS. 49 



The columnar epithelium covering these membranes rests 

 upon an amorphous structure, called basement-membrane. 

 It 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 attached. The lower strata of cells are shorter 

 and more rounded than those in the superficial layer. The 

 cells are pale, very closely adherent to each other by 'their 

 sides, and provided with a moderate-sized, oval nucleus with 

 one or two nucleoli. The length of the cells is from -g^-g- to 

 -g-J-g- of an inch, and their diameter from ao 1 00 to 2 do of an 

 inch. When villosities exist on the surface of the mem- 

 branes, the cells follow the elevations and do not fill up the 

 spaces between them, as in most of the membranes covered 

 with pavement-epithelium. 



The mucous membrane of the urinary bladder, the 

 ureters, and the pelvis of the kidneys, cannot be classed in 

 either of the above divisions. They are covered with mixed 

 epithelium, presenting all varieties of form between the 

 pavement and the columnar, some of the cells being caudate 

 and quite irregular. 



Mechanism of the Secretion of Mucus. Nearly every 

 one of the great variety of fluids known under the name of 

 mucus is composed of the products of several different glan- 

 dular structures. According to Robin, mucus proper is pro- 

 duced by the epithelial cells of that portion of the membrane 

 situated on the surface, between the opening of the so-called 

 mucous follicles or glands ; * while the secretion of these 

 special glandular organs always possesses peculiar properties. 

 It is undoubtedly true that certain membranes which do not 

 possess glands, as the mucous lining of the ureters and a 

 great portion of the urinary bladder, are capable of secreting 



of the air-passages ; and vol. ii., Digestion, pp. 212, 313, and 389,. for a descrip. 

 tion of the glands of the stomach and intestines. 



1 ROBIX, Lefon* stir les humeurs, Paris, 1867, p. 438. 

 4 



