No. 402.] THE NEURONE THEORY. 465 
tation of the neuropile. If the nucleus of each neurone is a 
center from which the life of the whole neurone is controlled, 
then this contro] must extend, in the case of a given neurone, 
over that portion of the neuropile associated with it; in other 
words, the neuropile cannot be considered a diffuse network, 
as claimed by Gerlach, but must be regarded as divided into. 
provinces, each one of which is under the rule of the neurone 
with which that particular part is associated (see Fig. 1). The 
boundaries between these provinces must, then, mark the real 
limits of the neurones, and that such boundaries actually exist, 
at least in a physiological sense, is to be inferred from two 
classes of observations. First, in the degeneration of the cen- 
tral fibres of one of two intimately associated sets of neurones, 
the degeneration proceeds into the neuropile but not across it, 
showing that the separation of the neurones lies somewhere in 
that material. Secondly, since the embryonic cells from which 
neurones finally develop are in the beginning often widely sep- 
arated from each other, it follows that their association must 
be brought about by their gradual growing towards each other, 
and, as their first step in union would be that of the simple 
contact of their neuropile masses, it is possible that this rela- 
tion is all that ever exists and that the points of separation in 
the neuropile are the original contact points of the developing 
neurones. For this ingenious suggestion we are indebted to 
the Swiss neurologist Forel, who, with a keen appreciation of 
the significance of the embryological investigations made by 
His, has given us what has been called the Contact Theory 
of the relation of neurones. This theory, which, when 
coupled with du Bois-Reymond’s hypothesis of a neuropile 
capable of movement, has been eagerly followed up by many 
psychologists, is generally supposed to be an essential part of 
the neurone theory itself; but, though it was undoubtedly so 
regarded by its earlier advocates, it must be borne in mind 
that it is in reality nota necessary part of this idea, which has 
to do with the conception of the neurone unit asa cell rather 
than with the mutual relations of such units. 
The discussion has now led us to a position where a fairly 
full statement of the grounds for the neurone theory may be 
