THE LUNGS. 



163 



Most usually they are placed close together, over the whole of 

 the terminal surface of the cells, according to Valentin, on the 



Fig. 233. 



average, to the number of 10 to 22, which appears to be rather 

 under the mark; more rarely they occur in smaller number, 

 or even, as it is said, singly upon a cell. But in this case, care 

 must be taken not to regard cohering cilia as single ones, as 

 might happen, particularly in the embryo. In their chemical 

 relations the cells of the ciliated epithelium correspond pre- 

 cisely with those of the cylinder-epithelium, and especially, 

 the separation of the cell membrane on the addition of water, 

 may also be remarked in them. The cilia are of much more 

 delicate consistence than the cell membrane and are very rea- 

 dily detached upon any maceration of the epithelium ; more or 

 less altered by almost all reagents, they are, by many, at once 

 destroyed ; in chromic acid, however, they may be preserved 

 pretty well. In man, the ciliary motion is directed, in the 

 trachea, from below upwards and may often be perceived 

 fifty-two or even fifty-six and seventy-eight hours after death 

 (Biermer, Gosselin). There is nothing to show the occurrence, 

 normally, of a desquamation of the ciliated epithelium of the 

 larynx and air passages. Occasionally, it is true, isolated, 



Fig. 233. Ciliated epithelium from the human trachea, x 350 diam. A, epithelium 

 in situ : a, most external portion of the elastic longitudinal fibres ; b, homogeneous, 

 most external layer of the mucous membrane ; c, deepest, rounded cells ; d, middle, 

 elongated cells ; e, most superficial cells, supporting cilia. B, isolated cells from the 

 various layers. 



