146 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



cylinder is formed from more than one cell, as is shown by the nuclei it 

 contains ; although its cavity seems to be continuous from end to end. 

 And occasionally the cylinders arise by stalk-like prolongations, from 



Fig. 26. 



Vibratile or ciliated Epithelium; a, nucleated cells, resting on their smaller extremities; b, cilia. 



a tessellated epithelium beneath. The two forms of Epithelium pass into 

 one another at various points ; and various transitional forms are then 

 seen, the tessellated scales appearing to rise more and more from the 

 surface, until they project as long-stalked cells, truncated cones, or cylin- 

 ders. 



234. Both these principal forms of Epithelial cells are frequently 

 observed to be fringed at their free margins with delicate filaments, 

 which are termed cilia ; and these, although of extreme minuteness, are 

 organs of great importance in the animal economy, through the extra- 

 ordinary motor powers with which they are endowed. The form of the 

 ciliary filaments is usually a little flattened, arid tapering gradually from 

 the base to the point. Their size is extremely variable ; the largest that 

 have been observed being about l-500th of an inch in length, and the 

 smallest about l-13000th. When in motion, each filament appears to 

 bend from its root to its point, returning again to its original state, like 

 the stalks of wheat when depressed by the wind ; and when a number are 

 affected in succession with this motion, the appearance of progressive 

 waves following one another is produced, as when a wheatfield is agitated 

 by frequent gusts. When the ciliary motion is taking place in full 

 activity, however, nothing whatever can be distinguished, but the whirl 

 of particles in the surrounding fluid; and it is only when the rate of 

 movement slackens, that the shape and size of the cilia, and the manner 

 in which their stroke is made, can be clearly seen. The motion of the 

 cilia is not only quite independent (in all the higher animals at least) 

 of the will of the animal, but is also independent even of the life of the 

 rest of the body ; being seen after the death of the animal, and pro- 

 ceeding with perfect regularity in parts separated from the body. Thus 

 isolated epithelium cells have been seen to swim about actively in water, 

 by the agency of their cilia, for some hours after they have been detached, 

 from the mucous surface of the nose ; and the ciliary movement has 

 been seen fifteen days after death in the body of a Tortoise, in which 

 putrefaction was far advanced. In the gills of the River Mussel, which 

 are among the best objects for the study of it, the movement endures 

 with similar pertinacity. 



235. The purpose of this ciliary movement is obviously to propel 

 fluids over the surface on which it takes place ; and it is consequently 

 limited in the higher animals to the internal surfaces of the body, and 

 always takes place in the direction of the outlets, towards which it aids 



