MUCOUS MEMBRANES. 317 



ternal parts communicate with the exterior, and by which either 

 matters are eliminated from the body or foreign substances 

 taken into it. They are soft and velvety, and extremely vas- 

 cular. Their general structure resembles that of serous mem- 

 branes. It consists of epithelium, basement-membrane, and 

 fibro-cellular or areolar tissue containing bloodvessels, lym- 

 phatics, and nerves. The structure of mucous membranes is 

 less uniform, especially as regards their epithelium, than that 

 of serous membranes ; but the varieties of structure in different 

 parts are described in connection with the organs in which 

 mucous membranes are present, and need not be here noticed 

 in detail. The external surfaces of mucous membranes are 

 attached to various other tissues ; in the tongue, for example, 

 to muscle ; on cartilaginous parts, to perichondrium ; in the 

 cells of the ethmoid bone, in the frontal and sphenoid sinuses, 

 as well as in the tympanum, to periosteum ; in the intestinal 

 canal, it is connected with a firm submucous membrane, which 

 on its exterior gives attachment to the fibres of the muscular 

 coat. 



The mucous membranes are described as lining certain prin- 

 cipal tracts. 1. The digestive trad commences in the cavity of 

 the mouth, from which prolongations pass into the ducts of the 

 salivary glands. From the mouth it passes through the fauces, 

 pharynx, and oesophagus, to the stomach, and is thence con- 

 tinued along the whole tract of the intestinal canal to the ter- 

 mination of the rectum, being in its course arranged in the 

 various folds and depressions already described, and prolonged 

 into the ducts of the pancreas and liver and into the gall-blad- 

 der. 2. The respiratory tract includes the mucous membrane 

 lining the cavity of the nose, and the various sinuses commu- 

 nicating with it, the lachrymal canal and sac, the conjunctiva 

 of the eye and eyelids, and the prolongation which passes along 

 the Eustachian tubes and lines the tympanum and the inner 

 surface of the membrana tympani. Crossing the pharynx, and 

 lining that part of it which is above the soft palate, the respi- 

 ratory tract leads into the glottis, whence it is continued, through 

 the larynx and trachea, to the bronchi and their divisions, 

 which it lines as far as the branches of about -$ of an inch in 

 diameter, and continuous with it is a layer of delicate epithelial 

 membrane which extends into the pulmonary cells. 3. The 

 genito-urinary tract, which lines the whole of the urinary pas- 

 sages, from their external orifice to the termination of the 

 tubuli uriniferi of the kidneys, extends into and through the 

 organs of generation in both sexes, into the ducts of the glands 

 connected with them ; and in the female becomes continuous 



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