23S PHYSIOLOGY OF MUSCLES AND NERVES. 



the similarity in tin- arrmirem -at of their small^t 

 particles. 



But together with many points of resemblance. 

 iK-rve aad 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- 

 tl IK 'iice of continuous electric currents, exhibits tho- 

 elianges in excitability which we observed under the 

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

 spond with elianges 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 partido, 

 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 wat.-r. 

 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 t\\o atoms of oxygen ; 

 a molecule of common salt consists of an atom of 

 natron and an atom of chlorine, and so on. 1 A piece 

 of salt contains a very large number of such atoms 

 of ehlovinc and natron, but each of these 



1 Details of tin mid molecular theory will IT found in 



'Tlic ]S'(j\v Chi -niistry.' Coukc (lull-mat i-!i:il ScicuLi::- Si 

 vul. ix.). 



