SECRETION". 301 



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 tym- 

 panum 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 (^ mm.) 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 also into the organs of generation in both sexes, 

 and into the ducts of the glands connected with them : and in the female 

 becomes continuous with the serous membrane of the abdomen at the 

 fimbriae of the Fallopian tubes. 



Structure. These mucous tracts, and different portions of each of 

 them, present certain structural peculiarities, adapted to the functions 

 which each part has to discharge ; yet in sonie essential characters the 

 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 a basement 

 membrane, and beneath this, a stratum of vascular tissue of variable 

 thickness, containing lymphatic vessels and nerves. The vascular 

 stratum, together with the basement membrane and epithelium, in differ- 

 ent cases, is elevated into minute papillae and villi, or depressed into 

 involutions in the form of glands. But in the prolongations of the 

 tracts, where they pass into gland-ducts, these constituents are reduced 

 in the finest branches of the ducts to the epithelium, the primary or base- 

 ment-membrane, and the capillary blood-vessels spread over the outer 

 surface of the latter in a single layer. 



The primary or basement membrane is a thin transparent layer, sim- 

 ple, homogeneous, or composed of endothelial cells. In the minuter 

 divisions of the mucous membranes, and in the ducts of glands, it is the 

 layer continuous and correspondent with this basement-membrane that 

 forms the proper walls of the tubes. The cells also, which, lining the 

 larger and coarser mucous membranes, constitute their epithelium, are 

 continuous with and often similar to those which, lining the gland-ducts, 

 are called gland-cells. No certain distinction can be drawn between the 

 epithelium-cells of mucous membranes and gland-cells. 



Mucous Fluid: Mucus. From all mucous membranes there is secreted 

 either from the surface or from certain special glands, or from both, a 



