156 MANUAL OF HISTOLOGY. 



The destruction of epithelial cells is brought about, first, by solution, 

 nextly, by mechanical attrition. This naturally deprives the system 

 daily of a certain quantity of albuminoid matters, though in an altered 

 condition. 



REMAEKS. The question as to a connection between epithelial cells and the ele- 

 ments of connective substances and of nerve-tissue, must be discussed in a future 

 section. 1. I have convinced myself of the presence of granules of cinnabar in the 

 cylinder cells of a frog's intestine, three days after their injection into the circula- 

 tion. 



96. 



Owing to their composition decaying epithelia are of the greatest im- 

 portance in the formation of mucus. The consideration of these tissues 

 must, therefore, extend itself over fluids likewise. 



We understand by mucus a coating of a rather thick semifluid sub- 

 stance, more or less stringy and tenacious, which covers the surfaces of 

 all mucous membranes in varying quantity, and endows the latter with 

 their usual moistness and smoothness. It must also be regarded, owing 

 to its consistence, as well fitted to form a protecting medium against 

 chemical action, and it is probably not indifferent, besides, to the inter- 

 change of gases. 



Mucus is without odour and tasteless, and variable in its reactions. It 

 is found sometimes transparent, sometimes more opaque, white or yellowish. 

 Microscopical examination discloses to us in it the cast off epithelial and 

 gland-cells of the locality in which it is formed, but in variable number ; 

 and besides these a small cell, the so-called mucus-corpuscle, whose appear- 

 ance, size, and bearing repeats the colourless blood-cell completely, as well 

 as the elements of chyle and lymph, and whose origin, as far as is at pre- 

 sent known, may be very various. It may spring not only from epithelial 

 cells, but also from those of connective tissue and lymphatic organs. To 

 these are added the cells of the glandular formation of the part from 

 which the liquid is obtained. Again, owing to its viscidity, mucus 

 entangles usually a number of very minute air-bubbles. From all this it 

 would appear that mucus is a most variable substance. From an ana- 

 tomical point of view it is only a mixture of many dissimilar matters, and 

 amongst others of various gland juices, which endow it further with 

 chemical differences, as an expression of which we recognise the multi- 

 farious fermenting properties of the several kinds of mucus. 



Chemical anaylsis discloses as a solid constituent a peculiar substance 

 already mentioned ( 14) known as mucin. Besides this we find extrac- 

 tive matters, fats, and mineral constituents. 



Among the latter chlorine, phosphoric, sulphuric, and carbonic acid, 

 silicates, lime, and soda are said to exist in mucus. The following 

 table from Nasse may be taken as an example of its quantitative composi- 

 tion. By subjecting human mucus, which had been coughed up, to 

 analysis, he obtained the following results : 



Water, 955 '52 



Solid constituents, . . . . . 44*48 



Mucin (and a trace of albumen), . . 23'75 



Extractive matters, 9 '8 2 



Fats, . 2-89 



Mineral constituents, . . . . 8 -02 



