CILIATED EPITHELIUM. 671 



cilia attached to the broad end. Examples of the situations 

 in which these are found are, investing membrane of the 

 respiratory passages, u-pper part of the pharynx, larynx, 

 bronchi, and 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 contact with the cutis, 

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

 its materials of formation and nourishment from it. 



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 connexion 

 with cells is the movement of their cilia. There are three 

 ways in which the cilia ordinarily move the rotatory, the 

 undulatory, and the wavy, 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 passages, on the contrary, it is 

 from without inwards. In the Frog's mouth, it takes the 

 same course. The ciliary motion may be seen in the kidney 

 of the Erog or Newt ; the cilia in the latter continue in 

 active motion for some minutes after the animal is dead. 

 Make a very thin section of the kidney with a sharp knife, 

 and take care to disturb the structure as little as possible ; 

 then moisten it with a little of the serum of the animal, 

 place it in a glass cell, and cover with thin glass and a 

 magnifying power of 250 diameters. 



Pigment. Pigment granules are found in greater 01 

 ^ss quantities in the skin and bodies of white and dark 



