206 MUCOUS MEMBRANES. 



of the tint, as well as its extent, is influenced by circumstances accom- 

 lianymg or immediately preceding death. Thus the state of inflam- 

 mation, or the local application of stimuli to the membrane, such as 

 irritant poisons, or even food in the stomach, is apt to produce increased 

 redness ; and all the mucous membranes are liable to be congested with 

 blood and suflfused with redness when death is immediately preceded 

 by obstruction to the circulation, as in cases of asphyxia, and in many 

 diseases of the heart. 



Structure. — A mucous membrane is composed of coriuni and epithe- 

 Jlum. The epithelium covers the surface ; it may be scaly and stratified 

 as in the mouth and throat, columnar as in the intestine, or ciliated as 

 in the respiratory tract and uterus. The membrane which remains 

 after removal of the epithelium is named the coriwn, as in the analogous 

 instance of the true skin. The corium may be said to consist of a 

 fibro-vascular layer, of variable thickness, bounded superficially or next 

 the epithelium by an extremely fine transparent lamella, named lase- 

 ment-membrane by Bowman, andprimar/j membrane, limitar}j membrane, 

 and membrana jjrop-ia by others who have described it. 



The basement-membrane is best seen in parts where the mucous 

 membrane is raised into villous processes, or where it forms secreting 

 crypts or minute glaudular recesses, such as those which abound in the 

 stomach and intestinal canal (fig. 137, b). On teasing out a portion of 

 the gastric or intestinal mucous membrane under the microscope, some 

 of the tubular glands are here and there discovered which are tolerably 

 Avell cleared from the surrounding tissue, and their parietes are seen to 

 be formed of a thin pellucid film, which is detached from the adjoining 

 fibro-vascular layer, the epithelium perhaps still remaining in the inside 

 of the tube or having escaped, as the case may be. The fine film 

 referred to is the basement-membrane. It may by careful search be seen 

 too on the part of the corium situated between the orifices of the glands, 

 and on the villi, when the epithelium is detached, although it cannot be 

 there so readily separated from the vascular layer. In these parts it 

 manifestly forms a superficial boundary to the corium, passing continu- 

 ously over its eminences and into its recesses, defining its surface, and 

 supporting the epithelium. In other parts Avhere villi and tubular 

 glands are wanting, and especially where the mucous membrane, more 

 simply arranged, presents an even surface, as in the tympanum and 

 nasal sinuses, the basement-membrane is absent, or at least not demon- 

 strated. 



The basement-membrane, as already said, forms the peripheral 

 boundary of the corium ; it is in immediate connection with the epi- 

 thelium. By its under surface it closely adjoins the fibro-vascular 

 layer, with the retiform tissue, d, of Avhich it is in connection. The 

 vessels of the corium advance close up to the basement-membrane, but 

 nowhere penetrate it. In structure the membrane in question seems at 

 first sight perfectly homogeneous, but treatment with nitrate of silver 

 brings to view the outlines of flattened epithelioid connective tissue 

 cells, of which it is in reality composed.* 



^\\Q fibro-vascular layer of the corium is composed of vessels both 

 sanguiferous and lymphatic, with connective tissue — areolar and reti- 



* In the large intestine of the frog, and perhaps also in some other parts, these cells, 

 instead of adhering by their edges, intercommunicate by processes, so as to form a close 

 network instead of a continuous membrane. 



