MUCUS. 71 



the layer beneath it, so there is also a continuous desquamation 

 or separation of the epithelium of the mucous surfaces, which 

 sometimes occurs in men, who are in other respects healthy, 

 to such an extent that thick clots of mucus are expectorated in 

 the morning; which, on being examined with the microscope, 

 contain merely epithelium-cells. This, which is formed by a 

 mere act of separation from the uppermost layer of epithelium, 

 is regarded by Henle as normal mucus : he gives it the name, 

 however, of epithelium, and restricts the term mucus to the 

 morbid secretion of the mucous surfaces in which mucus-cor- 

 puscles (of which I shall speak presently) are found. I have 

 always found these corpuscles in the secretion from the nasal 

 and pulmonary mucous membrane of perfectly healthy persons : 

 they are mixed in a small quantity with the epithelium- cells, and 

 become increased when the mucous membrane is irritated. 



Physical character of mucus. 



Normal mucus, when fresh and recently secreted, is denser 

 than water, and when mixed with that fluid it gradually sinks 

 to the bottom of the vessel, unless it should be hindered from 

 doing so by extraneous causes. 



Dried mucus sinks very rapidly : normal mucus from the 

 lungs or nostrils usually floats on water for a considerable 

 period; in fact it was regarded as characteristic of mucus to 

 float on water, in contradistinction to pus, which always sinks. 

 A more careful investigation enables us to trace the floating 

 of the mucus to two causes : first to the number of air-bubbles 

 that are entangled in it, (after the removal of which it sinks) ; 

 and, secondly, to the proportionally small amount of solid con- 

 stituents in the secretion. The insolubility of fluid mucus in 

 water is the cause of the long retention of the air-bubbles. 

 When mucus contains pus, the proportion of solid consti- 

 tuents increases, the fluid portion diminishes, and its place is 

 supplied by albumen. Water rapidly permeates mucus in this 

 state, the air-bubbles escape, and it speedily falls to the bottom 

 in consequence of its specific gravity. Mucus from the bladder 

 or from the intestines does not swim on water in consequence 

 of the absence of air-bubbles. 



When some fresh, fluid, transparent, nasal, or bronchial mucus 



