EPITHELIAL CELLS. 355 



lated epithelium is gradually supplanted by tlie cylindrical, as it passes 

 from the sesophagus to line the interior of the stomach. This is termed 

 transition epithelium. 



CiMated (^helium, No. 2, b, Plate XII. — 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 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 j it must at the same time be remem- 

 bered, that though the epithelium is in close connection with the cutis, 

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

 formation and nourishment from it. Cytoblastema is given out from the 

 blood J the nucleus is first formed by granules, and around the nucleus 

 ia established the cell-membrane, taking, at the same time, the parti- 

 cular form of the epithelium to be developed ; and this development 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 delicate 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 exfoli- 

 ation takes place regularly, as may be exampled in reptiles and the 

 batrachia, who throw off their skin : the moulting of birds is analo- 

 gous. 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. 



CUia. — The most remarkable circumstance in connection with cells 

 is the movement of the cilia : these are delicate processes, microscopi- 

 cally thin, and square at the end in man generally, but tapering in ani- 

 mals. These cilia are in constant motion, by them fluids and particles 

 suspended in fluids are carried along, and in this manner reach the sur- 

 face. There are three ways in which the cilia ordinarily move : the 

 rotatory, the undulatory, and the waving, like a field of wheat set in 

 motion by a steady breeze. No satisfactory explanation has been 

 given of the cause of this vibratile motion. The current produced by 

 them is from within outwards, in most places ; in the respiratory pas- 

 sages, on the contrary, it is from without inwards. In the frog's mouth 



