EPITHELIUM. 43 



waves in a field of corn, or swiftly running and rippling 

 water. The general result of their movements is to pro- 

 duce a continuous current in a determinate direction, and 

 this direction is invariably the same on the same surface, 

 being usually in the case of a cavity towards its external 

 orifice. 



Uses of Epithelium. The various kinds of epithelium 

 serve one general purpose, namely, that of protecting, 

 and at the same time rendering smooth, the surfaces on 

 which they are placed. But each also, probably, dis- 

 charges a special office in relation to the particular function 

 of the membrane on which it is placed. 



In mucous and synovial membranes it is highly probable 

 that the epithelium-cells, whatever be their forms and 

 whatever their other functions, are the organs in which by 

 a regular process of elaboration and secretion, such as will 

 be afterwards described, mucus and synovial fluid are 

 formed and discharged. (See chapter on Secretion.) 



Ciliated epithelium has another superadded function. 

 By means of the current set up by its cilia in the 

 air or fluid in contact with them, it is enabled to 

 propel the fluids or minute particles of solid matter, 

 which come within the range of its influence, and aid 

 in their expulsion from the body. In the respiratory 

 tract of mucous membrane the current set up in the air 

 may also assist in the diffusion and change of gases, on 

 which the due aeration of the blood depends. In the 

 Fallopian tube the direction of the current excited by the 

 cilia is towards the cavity of the uterus, and may thus be 

 of service in aiding the progress of the ovum. Of the 

 purposes served by the cilia which line the ventricles of the 

 brain nothing is known. 



The nature of ciliary motion and the circumstances by 

 which it is influenced will be considered hereafter. (See 

 chapter on Motion.) 



Epithelium is devoid of blood-vessels, nerves, and 



