Sensation depends upon certain nerve-currents, which affect the brain,
but are usually localized or “felt” in a particular spot or area of
the body. If a finger is burnt, for example, the pain is apparently
felt _in_ the _finger_; nevertheless the actual sensation takes place
_in_ the _brain_, and if the sensory (or afferent) nerves were cut, no
pain would be experienced. Normally, we react to a stimulus of this
kind by immediately withdrawing the hand; this is due to the motor (or
efferent) nerve-currents, which are sent out, commanding the muscles
to move the hand and arm in question. A definite “reaction” has then
taken place. Reactions of this sort are constantly going on, but most
of them never rise into consciousness. Only when the sensation is
powerful enough to rise above the so-called “threshold” does it rise
into consciousness, and the reaction to the given stimulus is then
consciously directed.
The outer world acts upon the periphery (surface) of the body, giving
rise to all sorts of sensations. Again, various internal organs,
if they are not functioning normally, will give rise to internal
sensations. All such sensations are conveyed by sensory nerves; and,
when these flow from any specific sense-organ, only the characteristic
sense impression is thereby conveyed. Thus, the duty of the optic nerve
is to convey sensations of sight; if the eye be struck violently, the
optic nerve, in such case, does not transmit pain--only sight--and we
“see stars” instead. This phenomenon is due to the sudden and violent
transmission of a nerve current flowing along these nerve-tracts.
Similarly with all the other senses.
This fact has given rise to the theory of “specific nervous
energies,”--to the theory, that is, that each sensory nerve conveys the
particular sort of current appropriate to it, and no other. Certain
internal organs, however, appear to have no sensation. For example, the
brain can be cut up, without any sensation on the part of the subject.
(Of course, before reaching the brain, pain would be experienced on the
surface of the head, in the skull, etc.)
Sensations are our most primitive mental constituents. They are
distinguished from Perceptions by the fact that the latter are more
complex; they are sensations, _plus_ ideas about the sensation; hence
“pure sensations” are impossible to us after the first few days of life.
Sensations differ greatly in the degree of their intensity; a
feeble sensation, however, if continued, will ultimately rise into
consciousness. On the other hand, sensations which intruded into
consciousness at first may sink below its threshold after a time, and
are no longer noticed. Thus, a workman may sleep in a boiler-factory;
soldiers have learned to sleep amidst the constant booming of cannon,
etc. Certain sensations also give rise to a sort of reverse of
themselves. Thus, if one gazes at a red spot for a time, and then looks
at the white ceiling, a _green_ spot will be seen to form there. This
is a so-called “after image,” and is due to fatigue. This can readily
be demonstrated in the case of vision.