SECRETING GLANDS. 405 



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 trans- 

 parent 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 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, 

 rather than epithelium. Indeed, no certain distinction 

 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 pro- 

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

 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. 



The structure of these elementary portions of a 



