272 SPLANCHNOLOGY. 



the animal. This tissue is found in the mucous membrane^ 

 of the air-passages, where its cilia tend by their motion to expel 

 particles of foreign matter which are inhaled with the breath ; 

 it is found also in the ventricles of the brain, in the canal of the 

 spinal cord, and in the mucous membrane lining the uterus 

 and Fallopian tubes of the female, and part of that lining the 

 vas deferens of the male ; and in the convoluted uriniferous 

 tubes. 



MUCOUS MEMBRANES. 



These structures are widely diffused, lining the canals of the 

 four systems now under consideration, and becoming continuous 

 with the skin at each of their orifices. A mucous membrane con- 

 sists of one or more layers of epithelial cells placed upon a sub- 

 epithelial layer or corium, the latter consisting of a modified 

 form of connective tissue, and serving as a matrix in which are 

 situated the nerves and vessels of the membrane, together with 

 numerous glands. 



Some authorities have described a membrane separating the 

 cellular from the vascular layer, and it has received the name of 

 the basement membrane. By others it is regarded as a mere 

 condensation of the corium. The secretion of the glands proper 

 to the mucous membranes is mucus, a viscid fluid which lubri- 

 cates their free surfaces. The surface of a mucous membrane 

 may be smooth, as in the air-passages, or it may be papillated — 

 i.e., furnished with small projections, papillae, as in the tongue ; 

 it may be rugeous, or thrown into folds or rugas, as in the oeso- 

 phagus and stomach ; in the small intestine it presents finger-like 

 projections termed villi, and is then said to be villous, and in the 

 same situation the membrane presents valve-like folds, and may 

 be termed valvular. 



The mucous membrane which lines the digestive canal becomes 

 continuous in the pharynx Avith that lining the respiratory canal, 

 the pharynx being a cavity common to the two systems ; hence 

 the entire membrane may be termed the gastro-pulmonaoy 

 mucous membrane, which is also produced so as to line the 

 Eustachian tube and tympanic cavity of the ear, and is, in some 

 animals, continuous by means of the lachrymal duct with the 

 conjunctiva of the eye. At the lips and anus it is directly con- 

 tinuous with the skin. In a similar manner, the mucous mem- 

 brane of the urinary canal becomes continuous with that of the 



