240 THE SOLIDS. 



sometimes cease altogether for a time, and then suddenly 

 commence again. It is also sufficiently powerful to effect 

 the entire displacement of the cell or corpuscle to whicli 

 the ciliae are attached, and many of which may frequently 

 be seen moving freely and quickly about, usually in circles, 

 in the field of the microscope. This curious spectacle is 

 most readily witnessed in the ciliary cells of the trachea 

 of the frog, which are of a different form from those of the 

 mammalia, being rounded in place of elongated and conoid al. 

 (See Plate XXL fig. 1.) 



The combined motion of the ciliae is also capable of 

 putting in movement either fluids or solid particles which 

 may come into contact with them. This fact those who 

 are given to microscopic investigation will have had many 

 opportunities of verifying, and one may easily at any time 

 acquire the proof of it by mixing with the fluid in which 

 the ciliae are acting some fine powder, as, for example, of 

 carbon. 



The motion of the ciliae, when acting in combination, is 

 stated to take place always in a determined direction from 

 within outwards. It would appear, however, from the ob- 

 servations of Purkinje and Valentin * that the direction is 

 capable of reversion : thus, these observers saw the accessory 

 branchiae of the anodon vibrate during from six to seven 

 minutes in one direction, and afterwards, during the same 

 lapse of time, in an opposite. 



The influence of physical and chemical reagents upon the 

 vibratile movement has been carefully examined. Thus, if 

 a portion of vibratile epithelium be touched or scraped, the 

 motions of the ciliae will become more active, and sometimes 

 commence again even after they had become extinct. They 

 cease at a temperature below the freezing point, and at a 

 degree of heat sufficient to occasion the coagulation of the 

 animal fluids. Galvanism destroys their action, but in a 

 local manner, a fact which a reference to the constitution 

 of epithelium by the union of separate cells may serve to 

 explain. Amongst chemical reagents, narcotics are without 



* Motus Vibrat, p, 67. 



