52 MUCOUS MEMBRANES :—EPITHELIUM. 
and they are as continually replaced by newly-formed cells, 
which are produced on the surface of the basement-mem- 
brane, at the expense of the fluid that transudes through 
it from the blood-vessels copiously distributed to its under 
surface. 
41. Mucous membrane may either exist in the condition of 
a simple expanded surface, or may have a much more complex 
arrangement, by which its surface is greatly increased. The 
simple mucous membrane, such as that which lines the nose 
and air-passages, is found, for the most part, where no ab- 
sorption has to be performed, and where only a moderate 
amount of secretion is necessary. But where it is to absorb 
as well as to secrete, it is usually involuted or folded upon 
itself, in such a manner as to form a series of little projéec- 
tions, and also a number of minute pits (fig. 9). These pro- 
Fig. 9.—DIAGRAM REPRESENTING THE Mucous MEMBRANE OF THE 
INTESTINAL CANAL. 
aa, absorbent vessels; 6b, basement membrane; ec, epithelium-cells of level 
surface of membrane; dd, cylindrical epithelium-cells of villus; e e, secreting 
cells of follicle. 
jections sometimes have the form of long folds; in other 
instances they are narrow filaments, crowded together so as 
almost to resemble the pile of velvet. In either case, the 
absorbent surface is vastly increased ; but chiefly so by these 
filaments, which are termed villi, and act as so many little 
rootlets. On the other hand, it is in the pits or follicles, that 
the production of the fluid which is to be separated or) 
secreted from the blood, chiefly takes place.—Not only are 
the flat expanded surfaces of the mucous membrane covered 
with epithelium cells, but the villi also are sheathed by) 
them; and the secreting follicles are lined by the same, || 
