EPITHELIUM 



109 



Fig. 28. — Columnar epithelial cells 

 from the frog's intestine. 

 ;k. droplet of mucus exuding 

 from cell ; nu. nucleus. 



intestine into the smallest possible particles it will be 

 found that the process has detached numerous, minute, 

 conical bodies, about -}^ mm. (yi^ in.) in length, polygonal 

 in transverse section, and having 

 one end flat and the other 

 pointed (Fig. 28). These 

 bodies are called epithelial cells : 

 in the natural position they lie 

 closely applied to one another, 

 like the blocks of a wood pave- 

 ment, their flattened ends facing 

 the cavity of the intestine, while 

 their pointed ends abut against 



the submucosa (p. 70). Thus the epithelial cells together 

 form an epithelium or epithelial layer of the mucous mem- 

 brane directly bounding the cavity of the enteric canal. 



Each cell consists of protoplasm and contains a rounded, 

 granular nucleus (tiu), which is made very conspicuous by 

 staining, and in which are one or more small bodies or nucleoli. 

 Certain of these cells have a space towards 

 their free ends containing slime or mucus, 

 and thus have the form of little cups or gob- 

 lets : they are known as goblet-cells (see right 

 hand cell in Fig. 28). 



Ciliated Epithelium. — By the same method 

 the mucous membrane of the mouth is also 

 seen to be lined by an epithelium, but the 

 cells comprising it (Fig. 29) are shorter in 

 proportion to their length, and each is pro- 

 duced on its free surface into a number of 

 delicate, transparent threads of protoplasm called cilia, which, 

 in the living condition are in constant movement, lashing 

 backwards and forwards like minute whip-lashes, or, more 



Fig. 29.— Ciliated 

 epithelial cells 

 from the mu- 

 cous membrane 

 of the frog's 

 mouth. 



(From Parker's 

 Biology, after 

 Howes.) 



