MUCOUS MEMBRANES. 



233 



that we meet with the peculiar secretion termed Mucus ; which appears to be ex- 

 pressly formed to shield these surfaces from the irritation they would suffer, 



Fig. 23. 





Diagram of the structure of an involuted Mucous Membrane, showing the continuation of its elements in 

 the follicles and villi: F, F, two follicles; 6, basement membrane; c, submucous tissue; e, epithelium; v, vascu- 

 lar layer; n, nerve; v, villus, covered with epithelium; v', villus whose epithelium has been shed. 



through the contact of air, or of solids or liquids. This secretion is also found 

 on the lining membrane of the larger excretory ducts of most of the glands ; 

 and it is mixed in greater or less amount, with most of the secretions discharged 

 by them. It is found also upon the lining membrane of the gall-bladder, and 

 of the urinary bladder. When these membranes are in a state of unusual irri- 

 tation, the amount of mucus which they discharge is very considerable; but it 

 ordinarily forms an extremely thin layer. The characters of Mucus, obtained 

 from various sources, are by no means invariable. In general, however, it may 

 be described as a fluid of peculiar viscidity, either colorless or slightly yellow, 

 transparent or nearly so, incapable of mixing with water, and sinking in it, 

 except when buoyed up by bubbles entangled in its mass, which is commonly 

 the case with the bronchial and nasal mucus. This fluid contains from 4? to 

 6? per cent, of solid matter, of which a small part consists of salts resembling 

 those of the blood ; whilst the chief organic constituent is a substance termed 

 Mucin, to which the characteristic properties of the secretion are due. This 

 appears to be an albuminous compound, altered by the action of an alkali; for, 

 as Dr. Babington has shown, any albuminous fluid may be made to present the 

 peculiar viscidity of mucus, by treating it with liquor potassaa. That the mucin 

 of mucus is held in solution by an alkali, appears from this, that it is readily 

 precipitated by acids, which neutralize the base : and that a sort of faint coagu- 

 lation may be induced even by water, which withdraws the base from it. When 

 Mucus is examined with the Microscope, it is found to contain numerous epi- 

 thelium-scales (or flattened cells); together with round granular corpuscles, con- 

 siderably larger than those of the blood, and closely resembling the nuclei of the 

 epithelium-cells, which are commonly termed mucus-corpuscles. In the more 



