DESCRIPTION OF PLATES. 155 
which covers the exterior of the body is made up 
of similar cells, 
The epithelium of mucous membranes and of 
the external surface of the body is constantly being 
shed. In hospital wards and other places where 
human beings are crowded together, the dust 
suspended in the atmosphere or deposited upon 
floors, window-ledges, etc., is to a considerable 
extent composed of this shed epithelium, wzless 
cleanliness 1s rigidly enforced. 
It would be out of place, in the present work, 
to go into details as to the differences presented 
by epithelium from various localities; but atten- 
tion is called to the very curious form of cell seen 
in Plate IV. Fig. 2.1. This is what is known as a cili- 
ated epithelium cell; and the peculiarity consists 
in the presence upon one side — that which faces 
outward when the cell is in position upon the sur- 
face of a mucous membrane — of a quantity of 
vibratile filaments called cilia. 
This ciliated epithelium lines the air-passages of 
vertebrate animals; and the office of the vibrating 
filaments, which are of uniform length, and which 
move in unison, seems to be to prevent the ac- 
cumulation of the natural secretions of the mucous 
membrane by propelling them in the direction of 
the orifice of the respiratory apparatus. 
This ciliary action may continue for some hours 
after a cell is detached from its normal position ; 
and such a cell, immersed in water of a proper 
1 An accident to the negative has made it necessary to substitute 
another figure for the one here described. 
