NORMAL HISTOLOGY. 



FIG. 22. 



FIG. 23. 



variety, the outermost cells alone are distinctly columnar ; these are 

 usually modified at their outer ends, becoming pointed, forked, or 

 club-shaped, in order to fit between the irregularly polyhedral and 

 pyriform elements of the deeper strata. The nucleus is situated about 

 the middle of the columnar surface cells, and somewhat eccentrically 

 nearer the basement-membrane in the deeper cells. The protoplasm 

 of columnar epithelium often contains particles of mucous secretion, 

 indicating the beginning of those changes which result in the produc- 

 tion of the goblet-cells. _^ 



Modified Epithelium. The free surfaces of the epithelium in 

 particular localities, as noted in detail in the foregoing summary, 

 are armed with minute hair-like processes, or cilia ; these, by their 



constant active vibration, create 

 a current, which serves to free 

 the mucous membranes from 

 accumulation of mucus and of- 

 fending foreign or irritating sub- 

 stances. Cilia are specializations 

 of the protoplasm, with which 

 they are probably directly and 

 intimately connected ; widely dis- 

 tributed and attached to the 

 various forms of epithelium in 

 the lower animals, in man and the 

 higher mammals cilia are limited 

 to columnar cells. 



The exact number of individual 

 cilia attached to the free surface 



of a single cell varies, but there are, probably, between one and two 

 dozen such appendages usually present. Their length likewise 

 differs with locality, those lining the human epididymis being about 

 ten times longer than those of the trachea. When analyzed, by 

 careful observation of favorable cells in not too rapid vibration, the 

 motion will be seen to consist of two parts a rapid primary move- 

 ment, directed to correspond with the general current, and a slower 

 secondary return to the original position, the free end of the cilium 

 describing a course resembling that of a whip-lash. The vibrations, 

 whose rate has been estimated at about ten per second, do not occur 

 simultaneously in all the cells, but exhibit a progression, one cell 

 after the other taking part in the motions, whereby a series of distinct 

 waves of ciliary motion is produced ; in addition, a certain periodicity 

 or rhythm often characterizes the vibrations. 



When favorable conditions obtain, including a sufficient supply of 

 moisture, oxygen, and heat, ciliary motion may be maintained for 



Ciliated epithelium 

 from trachea : g, a cell 

 filled with mucus about 

 to be discharged. 



Isolated elements of 

 ciliated columnar epi- 

 thelium from trachea : 

 a, m, i, cells from sur- 

 face, middle, and deep- 

 est strata. 



