318 SECRETION. 



with the serous membrane of the abdomen at the fimbrise of 

 the Fallopian tubes. 



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

 each of them, the mucous membrane pres'ents 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 pre- 

 sents, 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 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 constituents are reduced in the finest branches of the 

 ducts to the epithelium, the primary or basement-membrane, 

 and the capillary bloodvessels spread over the outer surface 

 of the latter in a single layer. 



The primary or basement-membrane is a thin transparent 

 layer, simple, homogeneous, and with no discernible structure, 

 which on the larger mucous membranes that have a layer of 

 vascular fibro-cellular tissue, may appear to be only the 

 blastema or formative substance, out of which successive 

 layers of epithelium-cells are formed. But in the minuter di- 

 visions 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, rather than epithelium. Indeed, no certain dis- 

 tinction can be drawn between the epithelium-cells of mucous 

 membranes and gland-cells. In reference to their position, as 

 covering surfaces, they might all be called epithelium-cells, 

 whether they lie on open mucous membranes, or in gland- 

 ducts ; and in reference to the process of secretion, they might 

 all be called gland-cells, or at least secreting-cells, since they 

 probably all fulfil a secretory office by separating certain 

 definite materials from the blood and from the part on which 

 they are seated. It is only an artificial distinction which 

 makes them epithelial cells in one place, and gland-cells in 

 another. 



It thus appears, that the tissues essential to the production 

 of a secretion are, in their simplest form, a simple membrane, 

 having on one surface bloodvessels, and on the other a layer of 

 cells, which may be called either epithelium-cells or gland- 



