§17. Stimulation of the Organ of Vision
The nervous system of the body is acted on by external agents of
various kinds, which produce changes in the state of the nerves.
These changes may sometimes be detected by auxiliary apparatus, for
example, by studying the electrical reactions; but they are also mani¬
fested by their actions on other parts of the body with which the nerves
are connected. The change of state of the so-called motor nerves is
accompanied by contractions of corresponding muscles. Under the
same circumstances, other nerves, known as sensory nerves, excite
sensations in the brain which is the organ of consciousness of the body.
Now in the case of the motor nerves, no matter how diverse the external
action may be—tearing, crushing, cutting, burning, eroding, shocking
by electricity,—the invariable result is the contraction of the corres¬
ponding muscle, the only difference being one of degree. Therefore,
apart from their qualitative differences, these various influences, so far
as their relation to the motor nerves are concerned, are called stimuli.
Quantitatively, we speak of a stimulus as being strong or weak accord¬
ing to the amount of twitching that is produced. The resulting altera¬
tion of the state of the nerve due to a stimulus is called stimulation
or excitation. Similarly, the ability of the stimulated nerve to make
the muscle contract is known as its excitability. The latter is affected
by mortification and by external influences of many kinds.
The sensory nerves may be analyzed in the same way. External
agencies, which acting on a motor nerve would cause contraction of
the muscle, have another peculiar sort of effect on a sensory nerve
and give rise to a sensation, provided the nerve is alive and not dis¬
connected with the brain. But there is undoubtedly an essential
difference here, because there are qualitative differences in the sensa¬
tion corresponding to qualitative differences in the stimulus. But
although different stimuli cause different sensations, still their effects
are invariably sensations, that is, invariably actions of a kind that do
not occur otherwise and are peculiar to the living body. Accordingly,
the abstract conception of stimuli and stimulation as used first with
reference to the motor nerves has been transferred likewise to the
sensory nerves. Thus, the external agencies which acting on the
sensory nerves excite sensations are also called stimuli, and the change
itself that takes place in the nerve is said to be a stimulation.
The state of stimulation that may originate at any part of a nerve
fibre through the action of stimuli is always conducted to all other
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