THE THIRD HUXLEY LECTURE 527 



Thus the pigment-ceUs afforded ocular demonstration of the truth, to which 

 I was otherwise led by inference, that an irritant, when producing inflammatory 

 congestion, prostrates for the time being the vital energies of the tissues on 

 which it acts. 



It is to be observed that mere paralysis of the nervous apparatus of the 

 irritated area would have been followed by diffusion of the pigment, as occurred 

 after section of the nerves in the thigh, so that the suspension of diffusion as 

 well as concentration shows that the special pigmentary functions had been 

 arrested. 



It was of course a familiar fact that nerves may be temporarily paralysed 

 by the direct action of pressure, cold, and other agents upon them. But, so 

 far as I am aware, it had not been known that the tissues general^ are liable 

 to be thrown into a state of suspended vital energy by injurious influences. 



An experiment upon another form of tissue seems so illustrative of this 

 subject that I am induced to relate it in detail. It was an attempt to study 

 the effect of warmth upon the ciliated epithelium of the frog's tongue. It was 

 easy to obtain the material for examination by gently scraping the surface of 

 the organ and diffusing the product in a drop of water. Individual isolated 

 cells were then to be seen with their cilia in motion, which might continue for 

 a considerable period. But special arrangements were necessary in order to 

 avoid killing them with the warmth, to which, as we have seen, the tissues of 

 the frog are peculiarly sensitive. I succeeded by arranging them in a film of 

 water between two delicate cover-glasses, the whole mass being so thin that 

 it could be very quickly heated and as rapidly cooled. The object being placed 

 under the microscope, I interposed a small cautery at a low black heat between 

 the reflector and the stage and watched the result. The ciliary motion, which 

 had been somewhat languid, became at once increased in rapidity, but resumed 

 its former rate if the cautery was at once withdrawn. If, however, the appli- 

 cation was somewhat longer continued, the active motion soon gave place to 

 a state of complete rest, in which the cilia stood straight like the hairs of a brush. 

 The hot iron being removed the instant that this effect was observed, slight 

 indefinite movement soon began to show itself in individual cilia ; and before 

 long all were again in action as before the heat was applied. If the cautery 

 was made somewhat hotter the motionless condition was produced almost 

 immediately, preceded by a momentary period of excessively active motion. 

 But if the warm application was immediately suspended, recovery occurred, 

 as in the former case. And the same experiment might be repeated again 



the pigincnt-cclls never recover ; the collections of pij^nient gradually lose their sharjiness of outline 

 and are ultimately absorbed, leaving a permanently white spot in the web. 



