CILIA 



707 



universal property of protoplasm, excitability, or the power of re- 

 sponding to certain influences, or stimuli, by manifestations of the 

 peculiar kind which we distinguish as vital or physiological. Many 

 unicellular organisms and the chief varieties of the white blood- 

 corpuscles possess the power of amoeboid movement; and we have 

 already dwelt upon some of the important functions fulfilled by 

 such movement in the higher animals and in man. A great dis- 

 tinction between this kind of contraction and that of a muscular 

 fibre is that it takes place in any direction. 



Cilia. Cilia possess a higher and more specialized grade of 

 contractility. They are very widely distributed in the animal 

 kingdom; and analogous structures are also found in many low 

 plants, such as the motile bacteria. 



In the human subject ciliated 

 epithelium usually consists of 

 several layers of cells, the most 

 superficial of which are pear-shaped, 

 the broad end being next the sur- 

 face, and covered with extremely 

 fine processes, or cilia, about 8 /* 

 in length, which are planted on 

 a clear band. It lines the respi- 

 ratory passages, the middle ear and 

 Eustachian tube, the Fallopian tubes, 

 the uterus above the middle of the 

 cervix, the epididymis, where the 

 cilia are extremely long, and the 

 central cavity of the brain and 

 spinal cord. 



Ciliary motion can be readily 

 studied by placing a scraping from 

 the palate of a frog or a small 

 portion of the gill of a fresh-water 

 mussel under the microscope in a drop of physiological salt solution. 

 The motion of the cilia is at first so rapid that it is impossible to 

 make out much, except that a stream of liquid, recognized by the 

 solid particles in it, is seen to be driven by them in a constant direc- 

 tion along the ciliated edge. When the motion has become less 

 quick, which it soon does if the tissue is deprived of oxygen, it is 

 seen to consist in a swift bending of the cilia in the direction of the 

 stream, followed by a slower recoil to the original position, which 

 is not at right angles to the surface, but sloping streamwards. All 

 the cilia on a tract of cells do not move at the same time ; the motion 

 spreads from cell to cell in a regular wave. The energy of ciliary 

 motion may be considerable, although far inferior to that of mus- 

 cular contraction. The work which cilia are capable of performing 



Fig. 233. Time-Marker. Arrange- 

 ment for marking 2 -second inter- 

 vals. D, seconds pendulum, with 

 platinum point E soldered on; A, 

 mercury trough, into which E dips 

 at end of its swing ; B, Daniell cell ; 

 C, electro - magnet which draws 

 down writing - lever F when the 

 current is closed by E dipping into 

 A; G, spring (or piece of india- 

 rubber), which raises F as soon as 

 current is broken. 



