CILIA 733 



lation with the constant current or induction shocks causes the 

 whole of the pseudopodia to be drawn in. This illustrates a 

 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 surface, and covered with extremely 

 fine processes, or cilia, about 8 fj, in length, which are planted on 

 a clear band. It lines the respiratory 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 a 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 direction 

 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 muscular con- 

 traction. The work which cilia are capable of performing can be 

 calculated by removing the membrane, fixing it on a plate of glass, 

 cilia outwards, putting weights on the glass plate, and allowing the 

 cilia, like an immense number of feet, to carry it up an inclined plane. 

 Bowditeh found in this way that the cilia on a square centimetre of 

 mucous membrane did nearly 7 gramme-millimetres of work per 

 minute (equal to the raising of 7 grammes to a height of a milli- 

 metre). 



