CILIATED CELLS. 



115 



can hardly be called motor tissues, and so are placed in a 

 group by themselves under the name of undifferentiated 

 tissues. 



Ciliated Cells. As the growing Body develops from its 

 primitive simplicity we find that the cells lining some oi 

 the tubes and cavities in its interior undergo a very re- 

 markable change, by which each cell differentiates itself into 

 a nutritive, and a highly motile and spontaneous portion. 

 Such cells are found for example lining the windpipe, and 

 a number are represented in Fig. 47. Each has a conical 

 form, the base of the cone being turned to the cavity of 

 the air-tube, and contains an oval nucleus, with a nucleolus. 

 On the broader free end are a number (about thirty on the 

 average) of extremely fine processes called cilia. During 

 life these are in constant rapid move- 

 ment, lashing to and fro in the liquid 

 which moistens the interior of the 

 passage; and as the cells are very 

 closely packed, a bit of the inner sur- 

 face of the windpipe examined with 

 a microscope, looks like a field of wheat 

 or barley when the wind blows over 

 it. Each cilium strikes with more 

 force in one direction than in the opposite, and as this di- 

 rection of more powerful stroke is the same for all the cilia 

 on any one surface, the resultant effect is that the liquid in 

 which they move is driven one way. In the case of tho 

 windpipe for example it is driven up towards the throat, 

 and the tenacious liquid or mucus which is thus swept 

 along is finally coughed or "hawked" up and got rid of, 

 instead of accumulating in the deeper air-passages away 

 down in the chest. 



These cells afford an extremely interesting example of the 

 division of physiological employments. Each proceeds from 

 a cell which was primitively equally motile, automatic, and, 

 nutritive in all its parts. But in the fully developed state 

 the nutritive duties have been especially assumed by the 

 conical cell-body, while the automatic and contractile prop- 

 erties have been condensed, so to speak, in that modified 



FIG. 47.-CUiated cells. 



