354 SECRETION. 



ing fills up the spaces between these papillaB, so that the membrane presents a smooth 

 surface. Between the chorion and the epithelium, is an amorphous basement-membrane. 

 The mucous glands open upon the surface of the membrane by their ducts, but the glan- 

 dular structure is situated in the submucous areolar tissue. These glands have many of 

 them been described in connection with the mucous membrane of the mouth, pharynx, 

 and O3sophagus. They are generally simple racemose glands, presenting a collection of 

 follicles arranged around the extremity of a single excretory duct, lined or filled with 

 rounded, nucleated epithelium. The pavement-epithelium covering these membranes 

 exists generally in several layers, and presents great variety, both in form and size. The 

 most superficial layers are of large size, flattened, and irregularly polygonal. The deeper 

 layers are smaller and more rounded. The size of these cells is from -g^Vo" to -^ of an 

 inch. The cells are pale, slightly granular, and possess a small, ovoid nucleus, with one 

 or two nucleoli. 



The second variety of mucous membranes, covered with columnar epithelium, is found 

 lining the alimentary canal below the cardiac orifice of the stomach, the biliary passages, 

 the excretory ducts of all the glands, the nasal passages, the upper part of the pharynx, 

 the uterus and Fallopian tubes, the bronchi, the Eustachian tubes, and the male urethra. 

 In certain situations, this variety of epithelium is provided on its free surface with little 

 hair-like processes called cilia. During life the cilia are in constant motion, producing a 

 current generally in the direction of the mucous orifices. Ciliated epithelium is found 

 throughout the nasal passages, commencing about three-quarters of an inch within the 

 nose ; in the upper part of the pharynx ; the posterior surface of the soft palate ; the Eu- 

 stachian tube ; the tympanic cavity ; the larynx, trachea, and bronchial tubes, until they 

 become less than -5^ of an inch in diameter ; the neck and body of the uterus ; the Fal- 

 lopian tubes ; the internal surface of the eyelids ; and the ventricles of the brain. This 

 variety of mucous membrane is formed of a chorion, a basement-membrane, and epithe- 

 lium. The chorion is composed of inelastic and elastic fibres, with fibro-plastic ele- 

 ments, a few unstriped muscular fibres, amorphous matter, blood-vessels, nerves, and lym- 

 phatics. It is less dense and less elastic than the chorion of the first variety and is gen- 

 erally more closely united to the subjacent tissue. The surface of these membranes is 

 generally smooth, the only exception being the mucous membrane of the pyloric portion 

 of the stomach and the small intestines. These membranes are provided with follicular 

 glands, extending through their entire thickness and terminating in rounded extremities, 

 sometimes single and sometimes double, which rest upon the submucous structure. Many 

 of them are provided also with simple racemose glands, the ducts passing through the 

 membrane, the glandular structure being situated in the submucous areolar tissue. 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 -^ to ^ of an 

 inch, and their diameter, from ^^ to -g^ of an inch. 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 pavement-epithelium. 



The mucous membrane of the urinary bladder, the ureters, and the pelvis of the kid- 

 neys, 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 in shape. 



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 



