34 ELEMENTARY TISSUES. 



moving continually backwards and forwards, and alter- 

 nately rising and falling with a lashing or fanning 

 movement. The appearance is not unlike that of the 

 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, discharges 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 what- 

 ever 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 mem- 

 brane 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 



