424 



MUCOUS MEMBRANES. 



Let us now examine a vertical section, in order to arrive at 

 some idea concerning the structure and origin of the false 

 membrane. The section (fig. 122) embraces the entire thickness 

 of a false membrane, together with the mucous surface on which 

 it lies.* 



We see at a glance that the entire false membrane is really 

 made up of the elements described above. Originally spherical, 

 they have come into contact with one another at various points 

 and have become welded together into a plump network, con- 

 sisting, in a manner, of "connecting pieces" only, without any 



Fig 122 



-^^ 



ri-4^~^ ^ 



Vertical section through a croupous plaque on the isthmus 

 faucium, with the mucous fold on which it rests, a — h. 

 False membrane ; c. Normal mucous membrane. -3^. 



trabeculae. All the more elaborate is the system of crescentic 

 and branching fissures which permeate the false membrane, 

 taking the place of meshes. In certain lights they appear dark, 



* Deceptive appearances can only be avoided by making the sections 

 exceedingly thin and absolutely vertical. The following is the best 

 method of preparation. The specimen is immersed in a mixture of 

 glycerine and gum-arabic, which must be clear and viscid. After the 

 fluid has had time thoroughly to permeate the specimen, the latter must 

 be taken out and thrown into alcohol. The gum now becomes quite 

 hard, owing to the extraction of the glycerine; and the specimen is 

 quite ready for being cut into sections of any required thinness. On 

 soaking the sections in a watery fluid (say the carmine solution) the 

 gum is dissolved out, and the tissue alone is left. 



