LESSON 28.] EPITHELIUM. 95 



open into this canal, the duct (ductus choledochus) which conveys 

 the bile from the liver to the first of the small intestines (Duodenum), 

 the salivary, and other glands. 



475. Both the forms of epithelium already described, are fre- 

 quently ciliated; the object of this arrangement appears to be to 

 propel fluids over the particular surface upon which they are placed, 

 and admirably they are adapted to this purpose ! When these organs 

 are moving in full activity, nothing can be seen but the incessant, 

 rapid whirl of particles of extraneous matter contained in the water ; 

 when their activity begins to cease, the exact form of the motion 

 then becomes visible. On such occasions it will be perceived that 

 the down stroke is given with great energy and activity, the cilia 

 recovering their position by a slower motion ; this can be successfully 

 imitated by striking the arm down very quickly, and lifting it back 

 in slower time. 



476. It has been already remarked in the Introduction, that the 

 motion of vibratile cilia is alike independent of the will, and of vi- 

 tality, as its action can be distinctly seen in epithelium removed from 

 the body, long after death. 



477. This phenomenon is not restricted to the human subject, or 

 to the higher mammalia, but is much better seen, and for a greater 

 length of time, in the lower forms of animal life. Thus : detached 

 epithelial scales from the mucous membrane of the human nose, have 

 been seen actively swimming through water by the agency of their 

 vibratile organs, some hours after their removal ; from the mucous 

 membrane lining the air- tubes of man (bronchi), for sixty hours after 

 death, and so vigorously, as to leave no doubt they would continue 

 in motion for a much longer period of time ; but the cilia of a Tor- 

 toise has been seen in active motion fifteen days after death, notwith- 

 standing the body was putrid ! 



A view of ciliated cylinder-epi- FlQ - lf 



thelium, of the truncated- cone form, 

 is given (Fig. 163), in which the 

 cilia (b) will be found occupying 

 the upper enlarged portion ; the 



nuclei (a) are clearly seen in the ciliated epithelium. 



centre of each cell. 



478. If the tongue of a Frog be slightly scraped with a knife, 

 and the product placed in a drop of water, and examined by the mi- 

 croscope, masses of ciliated epithelium of the pavement kind will be 

 found (Fig. 164-1). These will be seen swimming through the water 



