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 projec- 

 tions, and also a number of minute pits (fig. 9). These pro- 



Fig. 9. DIAGRAM KEPRESENTING THE Mucous MEMBRANE OP THE 

 INTESTINAL CANAL. 



a a, absorbent vessels; ft 6, basement membrane; c c, epithelium-cells of level 

 surface of membrane ; d d, 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 mlli, 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. 



