FUNCTIONAL IRRITATION. 331 



If we take a single ciliated cell, and, after entirely 

 isolating it from the body, allow it to swim about, and 

 wait until a state of complete repose has declared itself, 

 we can again call forth the peculiar movements of its 

 cilia by adding a small quantity of potash or soda to 

 the fluid, a quantity not large enough to produce corro- 

 sive effects upon the cell, but sufficient, upon penetra- 

 tion into it, to induce a certain change in its contents. 

 A peculiarly interesting fact, however, is that the num- 

 ber of substances which will act, as stimuli, upon ciliated 

 epithelium, is limited to these two. This explains how 

 it happened that Purkinje and Valentin (who, it is well 

 known, first made experiments, and those upon a very 

 extensive scale, upon ciliary movement), although they 

 experimented with a very large number of substances, 

 at last, after they had tried all sorts of things mechan- 

 ical, chemical and electrical stimuli came to the con- 

 clusion that there was no stimulus whatever, which 

 could provoke the ciliary movement. ' I had the good 

 fortune incidentally to stumble upon the peculiar fact, 

 that potash and soda are such stimuli. Here we cer- 

 tainly cannot call in any nervous influence to our aid, 

 and such influence appears to be the less admissible for 

 the reason that, in accordance with the well-known ex- 

 periments, the ciliary movement is maintained in the 

 dead body at a time when other parts have already be- 

 gun to putrefy. The ciliated epithelium of the frontal 

 sinuses and the trachea is found in human corpses in a 

 state of perfect excitability thirty-six to forty-eight 

 hours after death, when every trace of irritability has 

 long vanished from the remainder of the body. 



Much the same is the case with all other excitable 

 parts. We see nearly everywhere that certain excitants 

 act more readily than others, and that many are totally 

 incapable of producing any particular effect. Nearly 



