CHAP. II.] CILIARY MOTION. 61 



organic and inorganic elements are incessant during the ceaseless 

 round of organizing and disorganizing actions of which every tissue 

 is the seat, as long as it continues living. The currents alluded to 

 in the preceding paragraph are visible indications of the presence of 

 these organic movements. 



3. The molecular movements of nerve and muscle under stimula- 

 tion have been already mentioned in the preceding chapter. The 

 capacity of exhibiting these movements exists only while the nutri- 

 tive process continues to be carried on in the respective tissues 

 it ceases with life. It would appear that the precise chemical con- 

 stitution, which is essential to its existence, is of so unstable a 

 nature as to be constantly prone to change, and to require incessant 

 renewal ; or, it may be, that this capacity is one which is only 

 developed during the active operation of certain chemical forces, as 

 if it depended necessarily on certain peculiarities of the organic 

 elements when in a nascent or changing state. 



In muscular movement there is a visible approximation of the 

 ultimate particles of the tissue in a determinate direction, as will be 

 further explained in the proper place; and in this consists the 

 whole value of muscular tissue as a part of the mechanism of the 

 body. All those motions in the living body which are visible to 

 the naked eye, and many of those which cannot be seen without 

 the aid of lenses, are effected by muscular action. By it canals or 

 tubes adapt themselves to their contents ; the heart propels the 

 vital fluid ; the digestive canal transmits the ingesta from one part 

 to another ; the excretory reservoirs, or ducts, expel their contents ; 

 and lastly, by it the attitudes are maintained, and the locomotive 

 function is performed. 



Ciliary Motion. In the same category with the molecular mo- 

 tions of the living body we would place that singular phenomenon, 

 now well ascertained by multiplied observations, which is called 

 Ciliary motion. 



Certain surfaces, which are, in their natural and healthy state, 

 lubricated by fluid, are covered with a multitude of hair-like pro- 

 cesses, of extreme delicacy of structure and minuteness of size. 

 These are called cilia, from cilium, an eye-lash. They are generally 

 conical in shape, being attached by their bases to the epithelium 

 that covers the surface on which they play, and tapering gradually 

 to a point ; or, as Purkinje and Valentin state, they are more or 

 less flattened processes, of which the free extremities are rounded ; 

 and this latter form prevails in the human subject. They vary 

 in length from the ToW to the T2 Ju f an inch. They are dis- 



