164 SECTIONAL ADDRESSES 
nervous system to perform their functions. ‘Their function is to make 
the organism respond rapidly and effectively to changes in its environ- 
ment, and to achieve this they have developed a specialised structure, 
and a complex arrangement in the body. ‘They send out long threads 
of protoplasm which serve for the rapid transmission of signals, and 
they are linked to one another by elaborate branching connections in the 
brain and the spinal cord. 
THE DEVELOPMENT OF THE NERVOUS SYSTEM. 
The mapping of this network of paths was begun many years ago, 
and was the first step in the analysis. No progress could have been 
made without it, and its results are of vital importance to neurology. 
We are now witnessing a fresh period of interest in the geography of 
the central nervous system, but the problem is not how the nerve cells 
and their fibres are arranged, but why they are arranged as they are. 
R. G. Harrison in his recent Croonian Lecture recalled the time when 
he first cultivated living nerve cells outside the body. ‘That experiment 
made twenty-three years ago, marks the new epoch better than any 
other, for, besides introducing the method of tissue culture, it settled a 
long and bitter controversy as to the origin of nerve fibres. Nowadays 
the most elaborate transplantation experiments are carried out by the 
embryologists on amphibian larve. Animals are produced with super- 
numerary limbs, eyes, noses, and even spinal cords. The growing nervous 
system is faced with these unusual bodily arrangements, and by studying 
the changes induced in it we can form some idea of the factors which 
determine its normal structure. A review published this summer by 
Detweiler gives a vivid impression of the plasticity of the developing 
nervous system in the hands of the experimenter. As a rule it accepts 
the extra limb or sense organ, links it by nerve fibres to the rest of the 
organism and may develop more nerve cells to deal with it. ‘The forces 
which mould the nervous system seem to come partly from within the 
central mass of nerve cells and partly from the body outside. These 
forces may be chemical or electrical gradients, and often the nerve fibres 
seem to grow in particular directions because they cling mechanically 
to structures already laid down, e.g. to the main arteries of the limbs. 
It is unlikely that a simple formula will be found for such a complex 
arrangement, but the fact remains that the arrangement can be pro- 
foundly modified at the will of the experimenter. Its detail seems to 
depend not so much on the innate properties of particular cells as on the 
environment provided by the rest of the organism. 
THE REACTIONS OF THE NEURONES. 
This new embryological work supports the older in showing that the 
nervous system is made up of ‘ neurones,’ cells with thread-like extensions, 
and that they are the only active elements in it. ‘These elements are all 
cast in the same mould, but are shaped differently by the forces of 
development. To this we can now add the fact that all neurones seem 
to do their work in much the same way. ‘The activity which they show 
is in some respects remarkably simple. It is essentially rhythmic: a 
