Manchester Memoirs, Vol xlii. (1898), No. \%. 39 



ration between the material of the two, general con- 

 siderations would lead us to conclude that a line of 

 demarcation does exist on the one side of which the 

 molecules belong to the one unit and obey the one nucleus, 

 and on the other side belong to the other unit and obey 

 the other nucleus. In no tissue, whether animal or vege- 

 table, is an instance known of a common border-land 

 between two cells, belonging to both cells and governed 

 by the nuclei of the two. Hence we may infer that apparent 

 structural fusion does nob imply functional fusion, that the 

 molecules on one side of the functional line of demar- 

 cation behave or may behave differently from those on the 

 other side. We may still infer, however, that a palpable 

 structural gap, though not essential to the mere occurrence 

 of differentiation, must in some way determine the extent 

 or character of the differentiation. Knowing, as we do, 

 so little of the actual nature of the molecular changes 

 which underlie neural events, our ideas as to how such an 

 obvious gap affects the transference of events from one 

 unit to another must necessarily be vague and obscure ; 

 but we cannot do otherwise than suppose that the trans- 

 ference is different when it proceeds along a tract of 

 nervous material, continuous as mere nervous material 

 though changing in character where the tract leaves one 

 unit to join the other, from what it is when it has in some 

 way or other to pass across a bridge composed of material 

 not nervous at all. As we have seen, the one objective 

 token of a nervous impulse is an electric change, and though 

 nervous events cannot be identified with ordinary electric 

 events as at present known, yet the similarity of nervous 

 events to electric events is greater than their similarity to 

 any other physical events ; and we may perhaps, leaning 

 on this similarity, illustrate the possible influence of such 

 a bridge of non-nervous material in the course of a nervous 

 tract by saying, that the difference which it introduces is 



