SECRETION. 329 



3. The Genito-urinary tract, which lines the whole of the urinary 

 passages, from their external orifice to the termination of the tuhuli 



Jr O * 



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 ab- 

 domen at the fimbrise of the Fallopian tubes. 



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

 them, present certain structural peculiarities of the mucous membrane, 

 adapted to the functions which 'each part has to discharge; yet in some 

 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 vas- 

 cular tissue of variable thickness, containing lymphatic vessels and 

 nerves. The vascular stratum or corium, together with the basement 

 membrane and epithelium, in different 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 basement-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 be- 

 tween the epithelium-cells of mucous membranes and gland-cells. 



Mucous Fluid : Mucus. From all mucous membranes there is se- 

 creted either from the surface or from certain special glands, or from 

 both, a more or less viscid, grayish, or semi-transparent fluid, of alka- 

 line reaction and high specific gravity, named mucus. It mixes imper- 

 fectly with water, but, rapidly absorbing liquid, it swells considerably 

 when water is added. Under the microscope it is found to contain epi- 

 thelium and leucocytes. It is found to be made up, chemically, of a 

 nitrogenous principle called mucin, which forms its chief bulk, of a 

 little albumen, of salts chiefly chlorides and phosphates, and water with 

 traces of fats and extractives. 



SECRETING GLAXDS. 



The structure of the elementary portions of a secreting apparatus, 

 namely epithelium, simple membrane, and blood-vessels having been al- 



