THE INNER TISSUES OF ANIMALS. 357 



others. Let us ask next what will determine the 



differences of distance travelled in different directions. Ob- 

 viously any molecular agitation spreading from a centre, will 

 go furthest along routes that offer least resistance. What 

 routes will these be ? Those along which there lie most mole- 

 cules that are easily changed by the diffused molecular motion, 

 and which yet do not take up much molecular motion in as- 

 suming their new states. Molecules which are tolerably stable 

 will not readily propagate the agitation ; for they will absorb 

 it in the increase of their own oscillations, instead of passing 

 it on. Molecules which are unstable but which, in assuming 

 isomeric forms, absorb motion, will not readily propagate it ; 

 since it will disappear in working the changes in them. But 

 unstable molecules which, in being isomerically transformed, 

 do not absorb motion, and still more those which, in being 

 so transformed, give out motion, will readily propagate any 

 molecular agitation ; since they will pass on the impulse either 

 undiminished, or increased, to adjacent molecules. If 



then we assume, as we are not only warranted in doing but 

 are obliged to do, that protoplasm contains two or more 

 colloids, either mingled or feebly combined (since it cannot 

 consist of simple albumen or fibrin or casein, or any allied 

 proximate principle) ; it may be concluded that any mole- 

 cular agitation set up by what we call a stimulus, will diffuse 

 itself further along some lines than along others, if the com- 

 ponents of the protoplasm are not quite homogeneously dis- 

 persed, and if some of them are isomerically transformed 

 more easity, or with less expenditure of motion, than 

 others; and it will especially travel along spaces occupied 

 chiefly by those molecules which give out molecular mo- 

 tion during their metamorphoses, if there should be any 

 such. But now let us ask what structural effects 



will be wrought along a tract traversed by this wave of 

 molecular disturbance. As is shown by those transforma- 

 tions which so rapidly propagate themselves through colloids, 

 molecules that have undergone a certain change of form, 



