404 SECRETION. 



the intestinal canal to the termination 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-bladder. 2. The 

 respiratory tract includes the mucous membrane lining the 

 cavity of the nose, and the various sinuses communicating 

 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 respiratory 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 con- 

 tinuous with it is a layer of delicate epithelial mem- 

 brane which extends into the pulmonary cells. 3. The 

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

 passages, 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 with the serous membrane of the abdo- 

 men at the fimbrise of the Fallopian tubes. 



Along each of the above tracts, and in different portions of 

 each of them, the mucous membrane presents certain struc- 

 tural peculiarities adapted to the functions which each part 

 has to discharge; yet in some essential characters mucous 

 membrane is the same, from whatever part it is obtained. 

 In all the principal and larger parts of the several tracts, it 

 presents, as just remarked, an external layer of epithelium, 

 situated upon basement-membrane, and beneath this, a 

 stratum of vascular tissue of variable thickness, which in 

 different cases presents either out-growths in the form of 

 papillae and villi, or depressions or involutions in the form 

 of glands. But in the prolongations of the tracts, where 



