IRRITABILITY. 325 



ning, and entirely confined to it, but we everywhere see 

 that a certain excitation is necessary for its production. 

 Every vital action presupposes an excitation, or if you 

 like an irritation. The irritability of a part, therefore 

 appears to us the criterion, by which we can judge 

 whether it is alive or not. Whether, for example, a 

 nerve be alive or dead, we cannot immediately deter- 

 mine by an anatomical examination of it, conducted 

 either microscopicaUy or macroscopically. In the out- 

 ward appearance, in the more obvious structural ar- 

 rangements, which we are able to decipher by the aid 

 of our auxiliaries, we rarely find sufficient to enable us 

 to come to a decision upon a point such as this. 

 Whether a muscle is alive or dead, we are but little 

 able to judge, inasmuch as we find its structure still 

 preserved in parts which perished years ago. I found 

 in a foetus, which, in a case of extra-uterine pregnancy, 

 had lain thirty years in the body of its mother, the 

 structure of the muscles as intact as if it had just been 

 born at its full time. Czermak examined parts of 

 mummies, and found in them a number of tissues which 

 were in a state of such perfect preservation, that the 

 conclusion might very well have been come to, that the 

 parts had been taken from a living body. Our notion 

 of the death, decease, or necrosis of a part, is based 

 upon nothing more or less than this, that whilst its form 

 is preserved, and indeed in spite of it, we can no longer 

 detect any irritability in it. This has been most clearly 

 shown quite recently in the course of some investigations 

 into the more hidden properties of nerves. Now that, 

 by the investigations of Dubois-Reymond, activity has 

 been shown to exist in nerves even when in a so-called 

 state of repose, and that it has been discovered, that in 

 a nerve, even when seemingly at rest, electrical pro- 

 cesses are continually going on, and that it constantly 



