454 MOTION. 



CHAPTER XVII. 



CAUSES AND PHENOMENA OF MOTION. 



THE most evident vital motions observable in the bodies of 

 animals, are performed in one or other of the following ways : 

 first, by means of the oscillatory motion or vibration of micro- 

 scopic cilia, with which the surfaces of certain membranes are 

 beset ; and secondly, by the contraction of fibres which either 

 have a longitudinal direction and are fixed at both extremities, 

 or form circular bands ; the contraction or shortening of the 

 fibres bringing the parts to which they are fixed nearer to each 

 other. There are, besides, various molecular movements allied 

 to those which need not here be considered. 



CILIARY MOTION. 



Ciliary motion consists in the incessant vibration of fine, 

 pellucid, blunt processes, about ^^^ of an inch long, termed 

 cilia (Figs. 153, 154), situated on the free extremities of the 

 cells of epithelium covering certain surfaces of the body. 



The distribution and structure of ciliary epithelium and the 

 microscopic appearances of cilia in motion have been already 

 described (p. 37). 



Ciliary motion seems to be alike independent of the will, of 

 the direct influence of the nervous system, and of muscular 

 contraction, for it is involuntary ; there is no nervous or mus- 

 cular tissue in the immediate neighborhood of the cilia, and 

 it continues 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 appar- 

 ently unprovided with anything analogous to a nervous sys- 

 tem, in its persistence in animals killed by prussic acid, by 

 narcotic or other poisons, and after the direct application of 

 narcotics to the ciliary surface, or the discharge of a Leyden 

 jar, or of a galvanic shock through it. The vapor of chloro- 

 form arrests the motion ; but it is renewed on the discontinu- 

 ance of the application (Lister). According to Kuhne, the 

 movement ceases in an atmosphere deprived of oxygen, but is 

 revived on the admission of this gas. Carbonic acid stops the 



