THE STRUCTURE AND FUNCTIONS OF NERVE-TISSUE. 31 



tile. But soon after protoplasm begins to assume definite 

 shapes, recognized by us as specific forms of life, some of its 

 parts are habitually exposed to the action of forces different 

 from those to which other of its parts are exposed. Conse- 

 quently, as protoplasm continues to assume more and more 

 varied forms, in some cases it must happen that parts thus 

 peculiarly situated with reference to external forces will be 

 more frequently stimulated to contract than are other parts 

 of the mass. Now in such cases the relative frequency with 

 which waves of stimulation radiate from the more exposed 

 parts, will probably have the effect of ' creating a sort of polar 

 arrangement of the protoplasmic molecules lying in the line 

 through which these waves pass, and for other reasons also 

 will tend ever more and more to convert these lines into 

 passages offering less and less resistance to the flow of such 

 molecular waves, — i.e., waves of stimulation as distinguished 

 from waves of contraction. And lastly, when lines offering 

 a comparatively low resistance to the passage of molecular 

 impulses have thus been organically established, they must 

 then continue to grow more and more definite by constant 

 use, until eventually they become the habitual channels of 

 communication between the parts of the contractile mass 

 through which they pass. Thus, for instance, if such a line 

 has been established between the points A and B of a con- 

 tractile mass of protoplasm, when a stimulus falls upon A, a 

 molecular wave of stimulation will course through that line 

 to B, so causing the tissue at B to contract — and this even 

 though no wave of contraction has passed through the tissue 

 from A to B. Such is a very meagre epitome of Mr. Spencer's 

 theory, the most vivid conception of which may perhaps be 

 conveyed in a few words by employing his own illustration, 

 viz., that just as water continually widens and deepens the 

 channel through which it flows, so molecular waves of the 

 kind we are considering, by always flowing in the same 

 ne tracts, tend ever more and more to excavate for them- 

 selves functionally differentiated lines of passage. When 

 su<h a line of passage becomes fully developed, it is a nerve- 

 fibre, distinguishable as such by the histologist; but before it 

 arrives at this its completed stage, i.e., before it is observahh' 

 as a distinct structure, Mr. Spencer calls it a "line of dis- 

 charge."* 



• A certain amount of experimental verifaalion has becu lent to this 



