I—PHYSIOLOGY 165 
series of rapid alternations between the resting and the active state, 
due probably to rapid breakdown and repair of the surface. ‘This at 
least is a fair description of the way in which the nerve fibres carry out 
their function of conducting messages, and we can detect the same 
kind of pulsating activity in the nerve cells of the brain. 
The evidence comes from the analysis of minute electric changes, for 
cell activity sets up electrical eddies in the surrounding fluid, and these 
can be measured with a minimum of interference. ‘The clearest results 
are given by the peripheral nerve fibres which connect the central nervous 
system to the sense organs and the muscles. The nerve fibres are 
conveniently arranged in bundles to form the nerve trunks: each fibre 
is an independent conducting path and there may be a thousand such 
paths in a fair-sized nerve, but it is not a difficult matter to study what 
takes place in the single fibre when it conducts a message. We may 
begin with an external stimulus acting on a sense organ, a structure 
which includes the sensitive ending of a nerve fibre as an essential part. 
The ending is excited by the stimulus, the delicate equilibrium of its 
surface is upset and the disturbance tends to spread along the fibre. 
The spreading is an active process: it takes place because the fibre has 
a store of energy ready to be liberated at a moment’s notice, and because 
the changes which attend its liberation at one point upset the balance at 
the next point and cause the same activity there. ‘The spread of a flame 
along a fuse is a well-worn analogy. But the nerve fibre is so constituted 
that a disturbance at any point is almost immediately cut short. The 
change spreads along it as a momentary wave—a brief impulse followed 
inevitably by a brief interval of rest and recovery. If the sense organ 
remains excited a second impulse passes up the fibre, and then another 
and another as long as the stimulus is effective. ‘The impulses in a given 
nerve fibre are all alike in magnitude, rate of travel, etc., but the frequency at 
which they recur depends on the intensity of the stimulus, rising sometimes 
as high as 300 a second in each fibre, or falling as low as 10. All the 
' nervous messages take this form ; the central nervous system is continually 
bombarded by trains of such impulses passing along the slender threads of 
protoplasm from the sense organs, and is continually sending out trains 
of impulses to the muscles. 
The conducting threads or nerve fibres are exceedingly insensitive to 
changes in their environment: their endings in the sense organs are 
exceedingly sensitive. ‘The sole function of the ending is to act as the 
trigger mechanism for firing off the impulses, and the sole function of the 
nerve fibre is to carry the message without distortion. Both are specialised 
parts of the neurone with specialised reactions, but it is important to note 
that these reactions are not peculiar to the nervous system. Muscle 
fibres, developed from the mesoderm and specialised for contraction, 
conduct impulses which seem to differ merely in their time relations 
from those in nerve fibres, and they can also be made to behave like the 
sensory endings by treatment with various salt solutions. In sodium 
chloride, for instance, a series of impulses will be set up in a muscle 
fibre when it is stretched, as they would be in one of the sense organs 
whose sole duty is to act as ‘ stretch receptors.’ The muscle fibre makes 
