238 PHYSIOLOGY OF MUSCLES AND NERVES. 



the similarity in the arrangement of their smallest 

 particles. 



But together with many points of resemblance, 

 nerve and muscle exhibit some points of difference. 

 The muscle during activity changes its form and is 

 able to accomplish work ; the nerve is incapable of 

 this. The nerve, on the other hand, under the in- 

 fluence of continuous electric currents, exhibits those 

 changes in excitability which we observed under the 

 name electrotonus, and which, as we have seen, corre- 

 spond with changes in the distribution of the tensions 

 on the outer surface of the nerve. No correspond- 

 ing phenomena have been shown in 'muscle. Other 

 changes which effect these changes in tension must, 

 therefore, occur within the nerve-element. 



It is a well-known fact that all substances occupy- 

 ing space are regarded as composed of small particles, 

 to which the name molecules is given. In a simple 

 chemical body, such as hydrogen, oxygen, sulphur, iron, 

 and so on, all these molecules consist of homogeneous 

 atoms ; in a chemically compound body, such as water, 

 carbonic acid, and so on, each molecule is composed of 

 several atoms of different kinds. A molecule of water, 

 for instance, consists of an atom of oxygen and two 

 atoms of hydrogen ; a molecule of carbonic acid con- 

 sists of an atom of carbon and two atoms of oxygen ; 

 a molecule of common salt consists of an atom of 

 natron and an atom of chlorine, and so on.' A piece 

 of salt contains a very large number of such atoms 

 composed of chlorine and natron, but each of these 



* Dotails of tlie atomic and molecular theory will be found in 

 * The New Chemistry.' Cooke (Litcrnational Scientific Series, 

 voL ix.). 



