192 GENERAL ANATOMY. 



anus; the remainder constitutes prolongations or appendages 

 prolonged in a cul-de-sac, more or less extended and rami- 

 fied in the mass of the body, and their orifice terminating 

 either on the external or internal skin. It thus forms an im- 

 mense internal tegument, of much greater extent than the 

 skin. 



262. The mucous membrane, like the skin, presents an 

 adhering and a free surface; the adhering or external surface 

 is generally covered with a particular layer of fibrous cellular 

 tissue, which has been named by Ruysch and other anatomists, 

 the nervous membrane, which Albinus and Haller have de- 

 monstrated to consist of cellular tissue, and which Bichat has 

 called the sub-mucous cellular tissue. This tissue is close, 

 fibrous, white, never contains fat, and rarely any infiltrated 

 serosity; it is traversed by a great number of small branches 

 of vessels and nerves. Several anatomists have assimilated 

 it to the dermis of the skin. However this may be, it is to 

 it, that the hollow organs in a great measure owe their solidity. 

 The mucous membrane is moreover lined throughout the ex- 

 tent of its principal canal and of several of its divisions, by a 

 muscular plane, a kind of internal muscular coat analogous to 

 those muscles we have denominated sub-cutaneous; in some 

 places it is an elastic tissue that covers the mucous membranes, 

 visible in the trachea and the excretory ducts; in others, a 

 true ligamentous tissue, as the periosteum of the nasal fossae, 

 of the sinuses, of the palate, of the alveolar processes, lines 

 this membrane, making of it a fibro-mucous membrane. 



263. The free surface of the mucous membrane presents 

 valvuli, folds and wrinkles, formed by the doubling of the 

 whole thickness of the membrane. The valvuli are formed 

 by folds of the mucous membrane, by the sub-mucous tissue, 

 and by muscular fibres contained in the fold; this is the case 

 in the pylorus, the mouth of the jejunum, in the colon, the ve- 

 lum palati, the orifice of the larynx, &c. The folds contain, 

 in their thickness, sub-mucous tissue only, but they always 

 remain, like the valvuli, and are never effaced : such are the 

 numerous folds of the small intestine which are called valvuli 

 conniventes; the wrinkles on the contrary are accidental or 



