STRUCTURE AND OFFICES OF MUCOUS MEMBRANE. 



149 



in any important particular, from the same tissue elsewhere ; and the white 

 and the yellow fibrous elements may be detected in it, in varying propor- 

 tions, in different parts, the latter being especially abundant in the Skin and 

 the Lungs, which owe to it their peculiar elasticity. Hence the Mucous 

 Membranes for the most part yield Gelatine, on being boiled. There is some 

 reason to believe, that the Skin also contains non-striated muscular fibres 

 scattered through it. The regeneration of all the forms of Mucous Mem- 

 brane, after loss of substance by disease or injury, is very complete, and takes 

 place with considerable rapidity. 



178. The essential character of the Mucous Membranes, in regard alike to 

 their offices and their arrangement, is altogether different from that of the 

 Serous and Synovial membranes. For, whilst the latter form shut sacs, 

 whose contents are destined to undergo little change, the former either cover 

 the external surface of the body, or line tubes and cavities in its interior, 

 which have free outward communications; and they thus constitute the me- 

 dium, through which all the changes are effected, that take place between the 

 living organism and the external world. Thus, in the gastro-intestinal mucous 

 membrane, we find a provision for reducing the food, by means of a solvent 

 fluid poured out from its follicles; whilst the villi, or root-like filaments, 

 which are closely set upon the surface of that same membrane, are specially 

 adapted to absorb the nutrient materials thus reduced to the liquid state. This 

 same membrane, at its lower part, constitutes an outlet through which are cast 

 out, not merely the indigestible residuum of the food, but also the excretions 

 from numerous minute glandule in the intestinal wall, which result from the 



Fig. 45. 



Diagram of the structure of an involuted Mucous Membrane, showing the continuation of its elements 

 in the follicles and villi; F, F, two follicles; b, basement membrane ; c, submucous tissue; e, epithelium ; 

 v. vascular layer ; n, nerve; v, villus, covered with epithelium; v', villus whose epithelium has been shed. 



decomposition of the tissues, and which must be separated and cast forth from 

 them to prevent further decay. Again, the bronchio-pulmonary mucous mem- 

 brane serves for the introduction of oxygen from the air, and for the exhala- 

 tion of water and carbonic acid. The mucous membranes prolonged into the 



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