THE ALIMENTARY CANAL 



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epithelium or mucous membrane which calls for a careful 

 description. It is a single layer of cells which are indi- 

 vidually prismatic in form with their long dimension 

 vertical to the free surface. All that enters the body 

 from the alimentary system must pass through or be- 

 tween these cells. The term mucous is used because this 



FIG. 40. The principle of glandular structure. In the upper figure 

 a simple microscopic gland is supposed to be laid open by a section along 

 its vertical axis. The cells are seen to surround a recess into which they 

 discharge their secretion. Below, the same structure is shown in its 

 entirety, and in addition the encircling blood-vessels which contribute 

 to make good the losses suffered by the secreting cells. 



epithelium is overlaid with a slimy film more or less 

 protective in function, the product of certain specialized 

 cells. 



The lining epithelium is not to be thought of as a 

 smooth expanse; it is raised into many prominences and 

 depressed into many pockets. The pockets of the 

 epithelium are the minute glands to which reference has 



