We are now to study the means whereby the neurons are made to
control and cooerdinate the different parts of the body and bring about the
necessary adjustment of the body to its surroundings. This work of the
neurons naturally has some relation to their properties.
*Properties of Neurons.*--The work of the neurons seems to depend mainly
upon two properties--the property of irritability and the property of
conductivity. _Irritability_ was explained, in the study of the muscles
(page 243), as the ability to respond to a stimulus. It has the same
meaning here. The neurons, however, respond more readily to stimuli than
do the muscles and are therefore more irritable. Moreover, they are
stimulated by all the forces that induce muscular contraction and by many
others besides. They are by far the most irritable portions of the body.
_Conductivity_ is the property which enables the effect of a stimulus to
be transferred from one part of a neuron to another. On account of this
property, an excitation, or disturbance, in any part of a neuron is
conducted or carried to all the other parts. Thus a disturbance at the
distant ends of the dendrites causes a movement toward the cell-body and,
reaching the cell-body, the disturbance is passed through it into the
axon.
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