THE STRUCTURE OF THE MOTOR ORGANS 87 



itive simplicity we find that the cells lining some of the tubes and 

 cavities in its interior undergo a very remarkable change, by which 

 each cell differentiates itself into a nutritive and a highly motile 

 portion. Such cells are found for example lin- 

 ing the windpipe, and are represented in Fig. 46. 

 Each has a conical form, the base of the cone 

 being turned to the cavity of the air-tube, and 

 contains an oval nucleus. On the broader free 

 end are a number (about thirty on the aver- 

 age) 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 



FIG. 45. Cardiac 



muscular tissue, magni- the cells are very closely packed, a bit of the 

 ters. a The ceii-boirnd- mn er surf ace of the windpipe, examined with 



Ce oniy u in ei thl a micros cope, looks like a field of wheat or 

 right-hand portion of barley when the wind blows over it. Each 

 cilium strikes with more force in one direction 

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

 vision of physiological employments. Each 

 proceeds from a cell which was primitively 

 equally motile 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 contractile prop- 

 erties have been condensed, so to speak, in FIG. 46. ciliated 

 that modified portion of the primitive proto- cells> 

 plasmic mass which forms the cilia. These, being supplied with 

 elaborated food by the rest of the cell, are raised above the vulgar 

 cares of life and have the opportunity to devote their whole at- 

 tention to the performance of automatic movements; which are 

 accordingly far more rapid and precise than those executed 



