244 THE TISSUES. 



a frog. It lias been asserted that they all strike towards the outlet of 

 the passage on which they are found; an assertion needing confir- 

 mation however. The motion seems to depend much upon the 

 state of the cells in respect to fluidity ; since it will continue many 

 hours after death (even 78), in case of man (Gosselin), if the cells 

 are kept moist. 1 The cilia are among the most minute objects oc- 

 curring to the histologist; being ^^ to 5 ^^ of an inch long, and 

 not exceeding ^J^ to ^-Q^O f an inch in diameter. They were 

 discovered by Purkinje and Valentin in 1834. Figs. 151 and 152 

 show them on both kinds of conoidal epithelium. (Also Fig. 91.) 



Distribution. It has been seen that the ciliated epithelium (either 

 simple or compound conoidal), lines the whole extent of the air- 

 passages from just within the nostrils to the termination of the 

 finest bronchial tubes, and the communicating cavities also ; as the 

 sinuses and cells of the face, Bustachian tube, and membrana tym- 

 pani, and the ductus ad nasum and lachrymal sac. 



Further than this, the ciliated epithelium lines the upper two- 

 thirds of the cavity of the uterus, and the Fallopian tubes through- 

 out. The canals in the Wolfnau bodies of the foetus must also be 

 added. 



Peculiarities. It is an interesting fact that the epithelium of the 

 upper part of the uterus and of the Fallopian tubes is not ciliated 

 previously to puberty. 



Disease respects the distinctions made in regard to the different 

 varieties of epithelium. In croup, the nasal passages are almost 

 invariably first affected, and the disease follows the course of the 

 ciliated epithelium over the posterior surface of the velum, and 

 thence into the larynx and trachea, and not along the oesophagus 

 into the stomach. Again, a disease commencing .in the lower half 

 of the pharynx, or the tonsils, does not soon extend to the larynx 

 and trachea, as a general rule. Besides, the uterine glands may be 

 diseased for an indefinite period without the disease extending either 

 to the uterine cavity or to the vagina; its conoidal epithelium being 

 bounded both above and below by the scaly variety. 



Development of Epithelium. 

 The first cells laid down to form an epithelium are probably de- 



1 The motion of the cilia is destroyed by many chemical and mechanical agents ; 

 and Virchow has recently found that a solution of potassa or soda re-excites it. 

 He infers from his experiments that the substance of the cilia nearly approximates 

 musculine. 



