MUCUS. 355 



glandular structures. According to Robin, mucus proper is produced 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 mem- 

 branes which do not possess glands, as the mucous lining of the ureters and a great por- 

 tion 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 

 fluid of an entirely different character. The fluid produced by the follicles of the small 

 intestine likewise has peculiar digestive properties. These circumstances, 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 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, we can recognize a special fluid by cer- 

 tain distinctive physiological properties. 



In the membranes covered with cylinder-epithelium, which are usually provided with 

 numerous simple follicles, the secretion is produced mainly by these follicles, but in part 

 by the epithelium covering the general surface. The membranes 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 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 secretion of mucus 

 beyond what has already been stated in connection with the general mechanism of secre- 

 tion. All the raucous membranes are quite vascular, and the cells covering the mem- 

 brane and lining the follicles and glands attached to it have the property of taking from 

 the blood the materials necessary for the formation of the secretion. These principles 

 pass out of the cells upon the surface of the membrane in connection with water and 

 inorganic salts in variable proportion. Many of the cells themselves are desquamated 

 and are found in the secretion, together with a few leucocytes, which are produced upon 

 mucous surfaces with great facility. 



Composition and Varieties of Mucus. In comparing the secretions of the different 

 mucous membranes, each one will be found to possess certain distinctive peculiarities, 

 more or less marked ; but there are certain general characters which belong to all varie- 

 ties of mucus. The fluid is usually a mixture of the secretion from the simple membrane 

 and the product of its follicles or glandular appendages and always contains a certain 

 amount of desquamated epithelium ; and it is frequently possible, from the microscopical 

 characters of the epithelium, to indicate the part from which any given specimen of mucus 

 has been taken. This desquamation of epithelium must not be regarded as a necessary 

 condition of the secretion of mucus, any more than the desquamation of the epidermic 

 scales is to be regarded as a condition necessary to the secretion of perspiration or seba- 

 ceous matter. It is a property of the epidermis and the epithelial covering of mucous 

 membranes to be regenerated by the formation of new cells from below, the effete struct- 

 ures being thrown off, and the admixture of these with mucus is simply accidental. 

 The leucocytes, formerly called mucus-corpuscles, are the result of irritation of the mu- 

 cous membrane and are not constant constituents of normal mucus. 



All the varieties of mucus are more or less viscid ; but this character is very variable 

 in the secretions from different membranes, in some of them the secretion being quite 

 fluid, and in others, almost semisolid. The different kinds of mucus vary considerably in 



