38 Foster, Physical Basis of Psychical Events. 



other but at the linkage of unit with unit, each unit acting 

 as an individual differentiated whole, the circumstances 

 attending linkage deserve attention. As we have seen, 

 the mode of linkage, which, if not universal, is at least 

 usual, is that the terminals of an axon (of the stem or of a 

 collateral) link with the dendrites or perikaryon of another 

 unit, so that the axon of one unit works on the axon of 

 another unit, not directly, but through the means of the 

 dendrites or perikaryon, the direction of activity being 

 centrifugal away from the nucleus along the axon. We 

 have no evidence of the terminals of one axon linking on 

 to the terminals of another axon so as to suggest that an 

 axon can work directly on an axon, or start an action 

 which proceeds centripetally towards the nucleus. This 

 however is perhaps not a matter of vital importance. We 

 came to the conclusion some time back that the unit acts 

 as a whole, that the axon does not act apart from the rest 

 of the unit, and that dendrite and perikaryon act for and 

 through the axon. Hence we may further conclude that, 

 to whatever part of one unit another be linked, the effect, 

 which is an activity of the axon, is in the main the same. 



We have further seen that a linkage is in the vast 

 majority of cases, if not universally, a thing of contact not 

 of fusion ; that is to say there exists an obvious gap 

 between the nervous material of the one unit and 

 that of the other. And we may conclude that the 

 existence of this gap is one of the conditions deter- 

 mining the differentiation of action which takes place 

 or begins at the linkage. Such a palpable gap is not 

 essential to the occurrence of differentiation. That 

 is the result of the individuality of each of the two linked 

 units, is the outcome of the fact that the powers of the one 

 unit are not identical with those of the other. Even in 

 the case where one unit seems to fuse wholly with the 

 other, where the best optical analysis can detect no sepa- 



