322 HAND-BOOK OF PHYSIOLOGY. 



glands connected with them; and in the female becomes continuous with 

 he serous membrane of the abdomen at the fimbrias of the Fallopian tubes. 



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

 of each of them, the mucous membrane presents certain structural pecu- 

 liarities adapted to the functions which each part has to discharge; } T et 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, sit- 

 uated upon basement-membrane, and beneath this, a stratum of vascular 

 tissue of variable thickness, containing lymphatic vessels and nerves 

 which in different cases presents either outgrowths in the form of papillae 

 and villi, or depressions or involutions in the form of glands. But in the 

 prolongations of the tracts, where they pass into gland-ducts, these con- 

 stituents 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, 

 simple, 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. It thus 

 appears, that the tissues essential to the production of a secretion are, in 

 their simplest form, a membrane, having on one surface blood-vessels, and 

 on the other a layer of cells, which may be called either epithelium-cells 

 or 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 more or less viscid, greyish, or semi-transparent fluid, of alkaline 

 reaction and high specific gravity, named mucus. It mixes imperfectly 

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

 water is added. Under the microscope it is found to contain epithelium 

 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 Glands. The structure of the elementary portions of a 

 secreting apparatus, namely epithelium, simple membrane, and blood- 

 vessels having been already described in this and previous chapters, we 

 may proceed to consider the manner in which they are arranged to form 

 the varieties of secreting glands. 



