I.—PHYSIOLOGY 167 
kind of rhythmic breakdown and repair of the surface takes place in this 
part of the neurone as in the nerve fibre, with the important difference 
that the changes develop and subside much less abruptly. The surface 
is not specialised for rapid conduction ; the forces which restore the 
resting equilibrium are less powerful and there is more tendency to 
spontaneous breakdown and to long periods of uninterrupted activity. 
We know that the activity of the grey matter is far more readily influenced 
by chemical changes than is that of the nerve fibre with its elaborate 
fatty sheath and wrappings of connective tissue, and it seems probable 
that both chemical and electric changes may be concerned in the spread 
of activity from one neurone to another. How this spread takes place 
is still uncertain, and it is admittedly the most important problem we 
have to face. In spite of this we can claim to have some of the main 
outlines of neurone activity. Our nervous system is built up of cells 
with a specialised structure and reactions, but the reactions are of a 
type to be found in many other cells. The rhythmic beat of the heart 
is probably due to surface reactions not far different from those in 
the group of nerve cells which produces the rhythmic movements of 
breathing ; and the factors, nervous and chemical, which regulate the 
heart beat are probably much the same as the factors which control the 
discharge of the neurone. We have a store of energy, replenished con- 
stantly by cell metabolism and liberated periodically by surface breakdown. 
The electrical gradients at the active point cause a spread of the breakdown 
to other regions, but sooner or later restoring forces come into play, the 
membranes are healed and the cycle is ready to be repeated. It is a long. 
step from the mechanical precision of an impulse discharge in a nerve 
fibre to the irregularities of a record from the cerebral cortex, but there 
are many intermediate cases which will bridge the gap. 
THe Nervous SYSTEM AS A WHOLE. 
As far as the units are concerned the prospect is encouraging. The 
difficulties begin when we come to the work of the nervous system as 
a whole. Many of its reactions are mechanical enough and can be 
explained in terms of the activity of groups of neurones, but there is 
much that resists this kind of treatment. It is perhaps encouraging 
that the difficulties are greatest when the reactions depend on the cerebral 
cortex, when they involve learning and memory, or, if you prefer it, habit 
formation and conditioning. They have been clearly stated by Lashley, 
and most of them can be reduced in the end to a simple formula, the 
failure of anatomical models of the nervous system. ‘The revolt from the 
anatomical model has been growing for many years, though it may be 
doubted whether its sponsors ever believed in it as much as their critics 
_ suppose. It gave us diagrams of nerve centres and pathways which 
were valuable enough when they referred to known anatomical structure, 
but not when they referred, as they often did, to hypothetical centres 
and to pathways canalised by use. ‘These too may exist, but they are 
not the whole explanation of cortical activity. 
Clinical neurology is partly to blame for the emphasis laid on exact 
localisation. The neurologist must locate brain tumours by analysing 
