THE EPIDERMIS. 35 



mined. They have, at all events, been frequently admitted. 

 Mucous metamorphosis, as in the cylinder cells, is also fre- 

 quently met with here. 



Lymphoid cells may also occur between the cylinder and 

 the ciliated cells ; indeed, they may even penetrate to the in- 

 terior of the cell body (comp. Fig. 16). 



Let us here devote a few words to the remarkable ciliary 

 motion. 



Discovered in olden times, subsequently, especially of late 

 days, frequently investigated, it has not yet been satisfactorily 

 explained. Its occurrence in the animal kingdom is quite 

 variable. Sometimes this part vibrates, at others that, 

 sometimes nearly all the surfaces ; at others, as in the arthro- 

 poda, not at all. What purpose does this work of the cilia? 

 serve ? 



If we fold a suitable detached piece of mucous membrane and 

 examine the edge of the fold, we have the appearance of an 

 undulating border, of a flickering candle-flame. If we look 

 from above downwards on to it, it suggests the comparison to 

 a field of grain agitated by the wind. 



Small bodies suspended in the fluid medium, color granules, 

 blood cells, sweep past the folded border, when we examine 

 with a high magnifying power. When we use very weak 

 lenses, the excursion proceeds more slowly; a cell will require 

 several minutes to make its course. 



When the ciliary motion is still in full vital energy, and 

 several vibrations then take place in a second, the human eye 

 is powerless. It is only when the action becomes retarded 

 that we perceive the separate motions — the regular, syn- 

 chronous, and homogeneous vibrations of the ciliae. At- 

 tempts have been made to distinguish several varieties of 

 these vibrations ; as for example, a hook-like and an oscilla- 

 tory. 



We pass over this ; but we must here mention an apparently 

 strange condition. In the ciliary movement, we see the cilia? 

 vibrate towards one side, and the small passing bodies take 

 the opposite direction ! 



