TISSUES, ORGANS, AND SYSTEMS. 



55 



Fig. 13. 



2. Epithelium. — Soft nucleated cells, nowhere densely 

 corneous; rounded, polygonal, fusiform, cylindrical or conical 

 in shape ; sometimes possessing cilia, sometimes not, and 

 occurring in one or many layers. Hence we have the fol- 

 lowing forms : 



a. Epithelium in a single stratum. 



(1) With rounded, polygonal cells (pave- 

 ment epithelium in a single layer). 



This exists as an investment of the true 

 serous membranes, of most synovial mem- 

 branes, of the cerebral ventricles (ependyma) 

 of the membrane of Demouvs, of the back 

 of the iris, and of the inner surface of the 

 choroid (pigment layer), of the capsule 

 of the lens and of the retina, of the internal ear, of the 

 endocardium, of the veins, of many glandular vesicles and 

 canals (racemose glands, kidneys, sudoriparous and cerumi- 

 nous glands, lungs), and of the ductus interlobular es of the 

 liver. 



(2) With fusiform, superficially united cells 

 (fusiform epithelium). 



Epithelium of the arteries, and of many 

 veins. 



(3) With cylindrical cells (cylinder-epi- 

 thelium). 



In the intestine from the cardia to the 

 anus, in Lieberkuhn's glands, in the ex- 

 cretory ducts of the gastric glands, as well 



Fig. 14. 





\ ; 



as of all the other glands which 

 open into the intestine ; also 

 of the lacteal and lachrymal 

 glands ; in the male urethra, 

 the vas deferens, the vesicidce 

 seminales, the excretory ducts 

 of the prostate, of Cowper's, 



Fig. 15. 



^ 



V 



Fig. 13. Epidermis of a two months' human embryo, still soft, like epithe- 

 lium, x 350. 



Fig. 14. Epithelial cells of the vessels, the longer ones from the arteries, the 

 shorter from the veins. 



Fig. 15. Epithelium of the intestinal villi of the rabbit, x 300. 



