STRUCTURE. cxcvii 



signed to a scries of oblique folds of a similar permanent kind, though 

 on a smaller scale, which exist withiu the cystic duct. 



Physical properties. In most situations the mucous membranes are 

 nearly opaque or but slightly translucent. They possess no great degree 

 of tenacity and but little elasticity, and hence are readily t->rn by a mo- 

 derate force. As to colour, they cannot be said intrinsically to have any, 

 and when perfectly deprived of blood they accordingly appear white or at 

 most somewhat grey. The redness which they commonly exhibit during 

 life, and retain in greater or less degree in various parts after death, is 

 due to the blood contained in their vessels, although it is true that, after 

 decomposition has set in, the red matter of the blood, becoming dissolved, 

 transudes through the coats of the vessels, and gives a general red tinge to 

 the rest of the tissue. The degroe of redness exhibited by the mucous mem- 

 branes after death is greater in the foetus and infant than in the adult. It 

 is greater too in certain situations ; thus, of the different parts of the 

 alimentary canal, it is most marked in the stomach, pharynx, and rectum. 

 Again, the intensity of the tint, as well as its extent, is influenced by cir- 

 cumstances accompanying or immediately preceding death. Thus the state 

 of inflammation, 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 suffused 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 corium and epithe- 

 lium. The epithelium covers the surface, and has already been described 

 (p. iii., et sqq.). The membrane which remains after removal of the epi- 

 thelium is named the corium, as in the analogous instance of the true 

 skin. The corium may be said to consist of a fibro-vascular layer , of vari- 

 able thickness, bounded superficially or next the epithelium by an ex- 

 tremely fine transparent lamella, named basement-membrane by Bowman, 

 and primary membrane, limitary membrane, and membrana propria by 

 others who have described it. It must be explained, however, that these 

 two constituents of the corium cannot in all situations be separated from 

 each other, nor indeed can the presence of both be proved by actual demon- 

 stration in all parts of the mucous membranes. 



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 

 glandular recesses, such as those which abound in the stomach and intes- 

 tinal canal. 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 well 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 separated from the vascular layer. In these 

 parts it manifestly forms a superficial boundary to the corium, passing 

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

 and supporting the epithelium. In other parts where villi and tubular 

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



