EPITHELIUM 



109 



intestine into the smallest possible particles, it will be 

 found that the process has detached numerous, minute, 

 conical bodies, about ^ mm. (y^ in.) in length, polygonal 

 in transverse section, and having 

 one end flat and the other 

 more 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 



FIG. 28. Columnar epithelial cells 



from the frog's intestine. 

 m. droplet of mucus exuding 

 from cell ; nu. nucleus. 



their narrower 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 (nu\ 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). 

 FIG. 2 9 .-ciiiated Ciliated Epithelium. By the same method 



epithelial cells /- i , . 



from the mu- the^mucous membrane of the mouth is also 



cous membrane , .. .. , ,-, 



of the frog's seen to be lined by an epithelium, but the 

 (From Parker's cells comprising it (Fig. 29) are shorter in 



Biology, after . . .1*1 ., i i i 



Howes.) 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 



