28 



EPITHELIUM 



[CH. IIL 



a 



FIG. 42. Ciliated epithelium of the human 

 trachea, a, layer of longitudinally arranged 

 elastic fibres; b, basement membrane; c, 

 deepest cells, circular in form ; d, inter- 

 mediate elongated cells; e, outermost layer 

 of cells fully developed and bearing cilia, 

 x 350. (Kolliker.) 



Each cell is surmounted by a bunch of fine tapering filaments. 



They were originally called cilia because of their resemblance in shape 



to eyelashes. They differ from 

 eyelashes in being extremely 

 small, and in not being stiff; 

 they are in fact composed of pro- 

 toplasm. During life these move 

 to and fro, and so produce a cur- 

 rent of fluid over the surface they 

 cover. Like columnar cells, they 

 may form goblet cells and dis- 

 charge mucin. 



In the larger 

 ciliated cells, it 

 will be seen that 

 the border on 

 which the cilia 

 are set is bright, 



and composed of little knobs, to each of which a 



cilium is attached ; in some cases the knobs are 



prolonged into the cell protoplasm as filaments 



or rootlets (fig. 43). According to some observers 



these rootlets are outgrowths from the multiplied 



centrosome of the cell. 



The bunch of cilia is homologous with the 



striated border of columnar cells. 



Ciliated epithelium is found in the human 



body, (1) lining the air passages, but not in the 



alveoli of the lungs ; these are lined by pavement 



epithelium ; (2) in the Fallopian tubes and upper 



part of the uterus ; (3) in the ducts of the testis 



known as the vasa efferentia and coni vasculosi ; 



here the cilia are the longest found in the body ; 



(4) in the ventricles of the brain and central 



canal of the spinal cord ; (5) the tail of a sperma- 

 tozoon may also be regarded as a long cilium. 

 In other animals cilia are found in other 



parts; for instance, in the frog the mouth and 



gullet are lined by ciliated cells ; in the tadpole, 



the whole surface of the body and especially the 



gills are covered with cilia. Among the inverte- Fl - 43. aiiated ceii from 



~ . o , , the intestine of a mol- 



brates one finds many protozoa completely covered iusc. (Engeimann.) 

 with cilia ; in many embryos the cilia are arranged 

 in definite bands round the body ; in the rotifers or wheel animal- 

 cules, a ring of cilia round the mouth gives the name to this par- 



