244 THE SOLIDS. 



In the open cavities, the destruction of epithelium is doubt- 

 less more considerable and more rapid ; it is also more easy 

 to determine in these : thus during mastication, deglutition, 

 and digestion, a considerable amount of epithelium becomes 

 disturbed, removed from the surface to which it was at- 

 tached, and mixed up with tlie saliva, mucus, gastric fluid, 

 food, &c., and finally is discharged from the system with the 

 foeces, in which by microscopic examination it may readily be 

 detected. 



The fact of the gradual and continual destruction of epi- 

 thelium may likewise be ascertained by an examination of 

 the several fluids discharged from the system, as the saliva, 

 the mucus, from either mouth, nose, lungs, urine, seminal or 

 menstrual fluids, in all of which the microscope will reveal 

 an abundance of epithelial cells. 



The same fact may also be determined by a microscopic 

 examination of the scum which collects during the night on 

 the lips and around the base of the teeth of many persons. 



In certain situations the epithelium undergoes not merely a 

 gradual, but also a periodical destruction, as in the uterus at 

 the monthly periods and after parturition. 



The continual destruction of epithelium having been thus 

 rendered manifest, its renewal follows as a matter of ne- 

 cessity. 



In irritation of the mucous membrane of the bronchi, nose, 

 and alimentary canal, it is possible that the epithelium, during 

 the period of the continuance of the irritation, is entirely 

 destroyed. 



USES OF EPITHELIUM. 



The first use of the epithelium is a passive one, it serving 

 like the epidermis as a protection to the more delicate parts 

 which lie immediately beneath it. 



The second use is active, the epithelium doubtless being 

 an important agent in secretion. ** 



Each epithelial cell may be regarded as a gland reduced to 

 its most simple type or condition, it embodying all that k 



