540 THE MICROSCOPE. 



Ciliated epithelium, Fig. 274, d. The cells of this do 

 not differ materially from those of the cylindrical ; the 

 great distinction between the two is, that in the former 

 there are no cilia attached to the broad end. Examples of 

 the situations in which these are found are, investing 

 membrane of the respiratory passages, upper part of the 

 pharynx, larynx, and bronchi, the lateral ventricles of the 

 brain, &c. 



Epithelium is found to grow from the surface of the 

 cutis outwards in most places it is constantly growing 

 outwards, and as continually being thrown off from the 

 surface : it must at the same time be remembered, that 

 though the epithelium is in close connexion with the cutis, 

 or true skin, it is not a deposit from it, but derives only 

 its materials of formation and nourishment from it. Cyto- 

 blastema is given out from the blood, the nucleus is first 

 formed by granules, and around the nucleus is established 

 the cell membrane, taking, at the same time, the particular 

 form of the epithelium to be developed ; and this develop- 

 ment is dependent upon the individual energies of the cell 

 itself, and not the cutis, whence it derives its nourishing 

 materials. Its uses would seem to be to protect the deli- 

 cate cutis from friction and external agents. 



The epidermis is destitute of sensibility, yet it invests 

 very sensitive parts : it is not vascular, but invests very 

 vascular parts. Its exfoliation takes place regularly, as 

 may be exampled in reptiles and the Batrachia, who throw 

 off their skin ; the moulting of birds is analogous. In the 

 early periods of life in the human subject, exfoliation takes 

 place from the surface of the skin ; from the mouth the 

 morsel of food is always mixed with detached cells. In the 

 process of digestion the same thing occurs in fact, it is 

 only when the epithelium cells are thrown off that the 

 gastric juice is secreted by the tubes of the stomach. 



Cilia. The most remarkable circumstance in con- 

 nexion with cells is the movement of their cilia : these are 

 delicate processes, microscopically thin, and square at the 

 end in man generally, but tapering in animals. These 

 cilia are in constant motion by them fluids and particles 

 suspended in fluids are carried along, and in this manner 

 reach the surface. There are three ways in which the cilia 



