CHAP, ii.] THE CONTRACTILE TISSUES. 165 



and of the respiratory and generative passages, are lined with 

 ciliated epithelium, and by the action of cilia, fluid containing 

 various particles and generally more or less viscid is driven 

 outwards along the passages towards the exterior of the body. 



A typical epithelium cell, such as may be found in the trachea, 

 is generally somewhat wedge-shaped with its broad end circular 

 or, rather, polygonal in outline, forming part of the free surface 

 of the epithelium, and with its narrow end, which may be a blunt 

 point or may be somewhat branched and irregular, plunged among 

 smaller subjacent cells of the epithelium, or reaching to the con- 

 nective tissue below. 



The cell-body is, over the greater part of its extent, composed 

 of protoplasmic substance with the usual granular appearance. 

 At about the lower third of the cell is placed, with its long axis 

 vertical, an oval nucleus, having the ordinary characters of a 

 nucleus. So far the ciliated cell resembles an ordinary epithelium 

 cell ; but the free surface of the cell is formed by a layer of 

 hyaline transparent somewhat refractive substance which when 

 the cell is seen, as usual, in profile appears as a hyaline refractive 

 band or border. From this border there project outward a 

 variable number, 10 to 30, delicate tapering hair-like filaments, 

 varying in length, but generally about a quarter or a third as 

 long as the cell itself; these are the cilia. Immediately below 

 this hyaline border the cell-substance often exhibits more or less 

 distinctly a longitudinal striation, fine lines passing down from 

 the hyaline border towards the lower part of the cell-substance 

 round the nucleus. The hyaline border itself usually exhibits a 

 striation as if it were split up into blocks, each block correspond- 

 ing to one of the cilia, and careful examination leads to the 

 conclusion that the hyaline border is really composed of the fused 

 thicker basal parts of the cilia. 



The cell-body has no distinct external membrane or envelope 

 and its substance is in close contact with that of its neighbours, 

 being united to them either by a thin layer of some cement 

 substance, or by the simple cohesion of their respective surfaces. 

 At all events the cells do cohere largely together, and it is difficult 

 to obtain an isolated living cell, though the cells may be easily 

 separated from each other when dead by the help of dissociating 

 fluids. When a cell is obtained isolated in a living state, it is 

 very frequently found to have lost its wedge shape and to have 

 become more or less hemispherical or even spherical ; under the 

 unusual conditions, and freed from the support of its neighbours, 

 the cell-body changes its form. 



The general characters just described are common to all 

 ciliated epithelium cells, but the cells in different situations vary 

 in certain particulars, such as the exact form of the cell -body, the 

 number and length of the cilia, &c. 



94. Ciliary action, in the form in which it is most common 



