ond 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. This movement through the neuron is called the _nervous impulse_.

*Purpose of the Impulse. *--Though the nature of the nervous impulse is not
understood, (103) its purpose is quite apparent. It is the means employed
by the nervous system for controlling and cooerdinating the different parts
of the body. The arrangement of the neurons enables impulses to be started
in certain parts of the nervous system, and the property of conductivity
causes them to be passed _as stimuli _to other parts. This enables
excitation at one place to bring about action at another place.

Acting as stimuli, the impulses seem able to produce two distinct effects:
first, to throw resting organs into action and to increase the activity of
organs already at work; and second, to diminish the rate, or check
entirely, the activity of organs. Impulses producing the first effect are
called _excitant_ impulses; those producing the second effect,
_inhibitory_ impulses.

*Functions of the Parts of Neurons.*--The _cell-body_ serves as a nutritive
center from which the other parts derive nourishment. Proof of this is
found in the fact that when any part of the neuron is separated from the
cell-body, it dies, while the cell-body and the parts attached to the
cell-body may continue to live. In addition to this the cell-body probably
reenforces the nervous impulse.

The _dendrites_ serve two purposes: first, they extend the surface of the
cell-body, thereby enabling it to absorb a greater amount of nourishment
from the surrounding lymph; second, they act as _receivers of stimuli_
from other neurons. The same impulse does not pass from one neuron to
another. An impulse in one neuron, however, is able to excite the