EPITHELIUM. 245 



veloped according to the method first described (page 120),/ree cell- 

 development; they being formed in a plasma exuded upon the base- 

 ment-membrane. 1 They subsequently multiply by the fissuration 

 of the cells and nuclei in the lower layers. They are constantly 

 growing, and on reaching maturity, they lose their vitality and 

 become detached or desquamate. In the mouth and alimentary 

 canal they are detached also by mechanical causes. 



The reparation of epithelium also takes place by fissuration, unless 

 all the layers of cells have been removed ; in which case there is 

 doubtless a development de novo, as at first. Sometimes, however, 

 a long time is required for the formation of a perfect epithelium ; 

 as is seen especially upon the surface of cicatrices after entire loss 

 of the skin. 



Functions of Epithelium. 



The functions of epithelium vary with the different varieties, and 

 also in different parts of the body. 



I. The scaly epithelium is specially for secretion and protection. 



The simple scaly epithelium of serous membranes, mucous folli- 

 cles and glands, and, in part, of the eye and the internal ear, and 

 the compound scaly epithelium of synovial membranes are for 

 secretion of serous or mucous fluids, as the case may be. 



The simple scaly epithelium of the lymphatics and bloodvessels, 

 of the ocular membranes, not alluded to in the preceding paragraph, 

 and of the Graafian vesicle; the compound scaly epithelium extend- 

 ing from the lips to the cardia, at the commencement of the nostrils, 

 on the lachrymal ducts, conjunctiva, and tympanic cavity; that 

 covering the vulva, vagina, and lower third of the uterine cavity; 

 and that lining the bladder, ureters, pelvis of the kidney and female 

 urethra are for protection, and doubtless also, to some extent, for 

 secretion. 



The compound scaly epithelium of the skin the epidermis is 

 almost exclusively protective. 



II. The conoidal epithelium is for secretion, absorption, or pro- 

 tection. 



The simple conoidal epithelium extending from the cardia, through 



1 It has been suggested that the fusiform slender epithelial cells of the larger 

 arteries and some veins, are related in their development to the striped lamellae 

 which underlie them. 



