THE STRUCTURE OF THE MOTOR ORGANS, 111 



tains 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 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, ex- 

 amined with a microscope, looks like a 

 field of wheat or barley when the wind 



_ . .f. FIG. 50. Ciliated cells. 



blows over it. .bach cilmm 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 wind- 

 pipe 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 accumu- 

 lating 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 arid contractile prop- 

 erties 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 per- 

 formance 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 com- 

 position 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 constitution. 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 



