THE NERVES. 239 



(in 2^^^^'^' cooking salt) is like every other. But a 

 muscle, a nerve, or any other organic tissue, is much 

 more complex in structure. Molecules of albumen, 

 of fats, of various salts, of water, and so on, are 

 mingled in it. A very small piece of such a tissue 

 must be regarded from a chemical point of view as a 

 compound of very many different substances. To avoid 

 confusion, the name ' muscle-element ' or * nerve- 

 element ' has been given to these particles, in which 

 we assume the existence of all the qualities of muscle 

 or nerve, but this name expresses nothing further than 

 a fragment of a muscle or nerve. Even such a frag- 

 ment must be regarded as of very complex structure. 

 Very complex physical and chemical processes may 

 take place within it ; and the processes of muscle and 

 nerve activity, the actual nature of which is as yet 

 quite unknown to us, are certainly connected with such 

 chemical and physical processes. If electric forces also 

 occur in such a nerve- or muscle-element, it is not sur- 

 prising that these also undergo various changes. Of 

 this sort must be the changes which occur during ac- 

 tivity and during electrotonus. 



In speaking, as we have occasionally done, of nerve- 

 and muscle-molecules, we have, therefore, not used the 

 term molecule quite in the clear and fixed sense in 

 which the term is used in chemistry. Our conception 

 was rather of something which, itself composed of va- 

 rious chemical substances, forms a unit of another 

 order. For the sake of brevity we shall still sometimes 

 use the expression in this sense, as, after the explana- 

 tion which has now been given, we may do this without 

 fear of being misunderstood. A muscle- or a nerve- 

 molecule accordingly means a group of chemical mo- 



