32 EPITHELIUM. [CH. in. 



in a drop of o'6 per cent, solution of common salt (normal saline 

 solution), the cilia are seen to be in constant rapid motion, each 

 cilium being fixed at one end,- and swinging or lashing to and 

 fro. The general impression given to the eye of the observer 

 is very similar to that produced by waves in a field of corn, or 

 swiftly running and rippling water, and the result of their move- 

 ment is to produce a continuous current in a definite direction, 

 and this direction is the same on the same surface, being usually 

 in the case of a cavity towards the external orifice. 



There is not only rhythmicality in the movement of a single 

 cilium, but each acts in harmony with its fellows in the same 

 cell, and on neighbouring cells. 



The uses of cilia can from the above be almost guessed ; in 

 the respiratory passages they create a current of mucus with 

 entangled dust towards the throat ; in the Fallopian tube or 

 oviduct they assist the ovum on its way to the uterus ; in the 

 gullet of the frog they act downwards and assist swallowing ; in 

 the ciliated protozoa they are locomotive organs. Over the 

 gills of marine animals they keep up a fresh supply of water, 

 and in the case of the rotifers, which are fixed animals, the current 

 of water brings food to the mouth. 



Ciliary motion is independent of the will, of the direct influence 

 of the nervous system, and of muscular contraction. It may 

 continue for several hours after death or removal from the body, 

 provided the portion of tissue under examination be kept moist. 

 Its independence of the nervous system is shown also in its 

 occurrence in the lowest invertebrate animals which are un- 

 provided with anything analogous to a nervous system, in its 

 persistence in animals killed by prussic acid, by narcotic or 

 other poisons, and after the direct application of narcotics, such 

 as morphia, opium, and belladonna, to the ciliary surface, or of 

 electricity through it. The vapour of chloroform arrests the 

 motion, but it is renewed on the discontinuance of the applica- 

 tion. The movement ceases when the cilia are deprived of 

 oxygen, although it may continue for a time in the absence of 

 free oxygen, but is revived on the admission of this gas. Car- 

 bonic acid stops the movement. The contact of various sub- 

 stances, e.g., bile, strong acids, and alkalis, will stop the motion 

 altogether ; but this seems to depend chiefly on destruction of 

 the delicate substance of which the cilia are composed. Tem- 

 peratures above 45 C. and near o C. stop the movement, 

 whereas moderate heat and dilute alkalis are favourable to the 

 action and revive the movement after temporary cessation. The 

 exact explanation of ciliary movement is not known ; whatever 



