THE STRUCTURE OF THE MOTOR ORGANS 75 



called cilia. During life these are in constant rapid movement, 

 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 

 surface of the windpipe, examined with a mi- 

 croscope, 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 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 off, 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 

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

 portion of the primitive protoplasmic 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 attention to the performance of automatic 

 movements; which are accordingly far more rapid and precise 

 than those executed by the whole cell before any division of labor 

 had occurred in it. 



That the movements depend upon the structure and composi- 

 tion of the cells themselves, and not upon influences reaching them 

 from the nervous or other tissues, is proved by the fact that they 

 continue for a long time in isolated cells, removed and placed in a 

 liquid, as blood-serum, which does not alter their physical consti- 

 tution. In cold-blooded animals, as turtles, whose constituent 

 tissues frequently retain their individual vitality long after that 

 bond of union has been destroyed which constitutes the life of the 

 whole animal as distinct from the lives of its different tissues, the 

 ciliated cells in the windpipe have been found still at work three 

 weeks after the general death of the animal. 



