COMPOSITION AND VARIETIES OF MUCUS 281 



In the membranes covered with columnar epithelium, which usually 

 are provided with simple follicles, the secretion is produced mainly by 

 these follicles but in part by the epithelium covering the general sur- 

 face. In certain parts, particularly the intestinal mucous membrane, 

 mixed with the ordinary columnar epithelium, are the so-called goblet- 

 cells that are supposed to produce and discharge a viscid secretion 

 containing mucin or mucinogen. These cells have already been 

 described. (See page 212.) The membranes covered with squamous 

 epithelium usually contain but few follicles 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 pro- 

 duced by the epithelium on the free surface and is mixed with secre- 

 tions of the mucous glands. 



There is nothing to be said in regard to the mechanism of the secre- 

 tion of mucus in addition to what has already been stated in connection 

 with the general mechanism of secretion. Mucous membranes are 

 quite vascular ; and the cells covering the membrane 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 

 matters pass out of the cells upon the surface of the membrane in con- 

 nection with water and inorganic salts in varying proportions. Many 

 of the cells themselves are thrown off and are found in the secretion, 

 together with a few leucocytes, which latter are produced on mucous 

 surfaces with great facility. 



Composition and Varieties of Mucus. All the varieties of mucus 

 are more or less viscid ; but this character is variable in secretions from 

 different membranes, in some of them the secretion being quite liquid, 

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

 considerably in general appearance. Some are clear and colorless ; but 

 the secretion usually is grayish and semitransparent. Examined with 

 the microscope, in addition to the mixture of epithelium and occasional 

 leucocytes, which give to the liquid its semiopaque character, the mass 

 of the secretion presents a finely-striated appearance, as if it were com- 

 posed of thin layers of nearly transparent substance with many folds. 

 These delicate striae usually do not interlace with each other, and they 

 are rendered more distinct by the action of acetic acid. This appear- 

 ance, with the peculiar effect of the acid, is characteristic of mucus. 

 Some varieties of mucus present very fine pale granules and a few 

 small globules of oil. 



On the addition of water, mucus is somewhat swollen but is not 

 dissolved. An exception to this is the secretion of the conjunctival 

 mucous membrane, which is coagulated by water. As a rule the re- 



