The Mind and the Brain | Page 7

implies that of a
body in which it is lodged. This body must have complicated organs;
its limbs presuppose the soil on which the animal rests, its lungs the
existence of oxygen vivifying its blood, its digestive tube, aliments
which it digests and assimilates to its substance, and so on. We may
indeed admit that this outer world is not, in itself, exactly as we
perceive it; but we are compelled to recognise that it exists by the same
right as the nervous system, in order to put it in its proper place.
The second fact of observation is that the sensations we feel do not give
us the true image of the material X which produces them. The
modification made in our substance by this force X does not necessarily
resemble in its nature the nature of that force. This is an assertion
opposed to our natural opinions, and must consequently be
demonstrated. It is generally proved by the experiments which reveal
what is called "the law of the specific energy of the nerves." This is an
important law in physiology discovered by Müller two centuries ago,
and consequences of a philosophical order are attached to it. The facts
on which this law is based are these. It is observed that, if the sensory
nerves are agitated by an excitant which remains constant, the
sensations received by the patient differ according to the nerve affected.
Thus, the terminals of an electric current applied to the ball of the eye
give the sensation of a small luminous spark; to the auditory apparatus,
the current causes a crackling sound; to the hand, the sensation of a

shock; to the tongue, a metallic flavour. Conversely, excitants wholly
different, but affecting the same nerve, give similar sensations; whether
a ray of light is projected into the eye, or the eyeball be excited by the
pressure of a finger; whether an electric current is directed into the eye,
or, by a surgical operation, the optic nerve is severed by a bistoury, the
effect is always the same, in the sense that the patient always receives a
sensation of light. To sum up, in addition to the natural excitant of our
sensory nerves, there are two which can produce the same sensory
effects, that is to say, the mechanical and the electrical excitants.
Whence it has been concluded that the peculiar nature of the sensation
felt depends much less on the nature of the excitant producing it than
on that of the sensory organ which collects it, the nerve which
propagates it, or the centre which receives it. It would perhaps be going
a little too far to affirm that the external object has no kind of
resemblance to the sensations it gives us. It is safer to say that we are
ignorant of the degree in which the two resemble or differ from each
other.
On thinking it over, it will be found that this contains a very great
mystery, for this power of distinction (specificité) of our nerves is not
connected with any detail observable in their structure. It is very
probably the receiving centres which are specific. It is owing to them
and to their mechanism that we ought to feel, from the same excitant, a
sensation of sound or one of colour, that is to say, impressions which
appear, when compared, as the most different in the world. Now, so far
as we can make out, the histological structure of our auditory centre is
the same as that of our visual centre. Both are a collection of cells
diverse in form, multipolar, and maintained by a conjunctive pellicule
(stroma). The structure of the fibres and cells varies slightly in the
motor and sensory regions, but no means have yet been discovered of
perceiving a settled difference between the nerve-cells of the optic
centre and those of the auditory centre. There should be a difference, as
our mind demands it; but our eye fails to note it.
Let us suppose, however, that to-morrow, or several centuries hence, an
improved technique should show us a material difference between the
visual and the auditory neurone. There is no absurdity in this

supposition; it is a possible discovery, since it is of the order of material
facts. Such a discovery, however, would lead us very far, for what
terribly complicates this problem is that we cannot directly know the
structure of our nervous system. Though close to us, though, so to
speak, inside us, it is not known to us otherwise than is the object we
hold in our hands, the ground we tread, or the landscape which forms
our horizon.
For us it is but a sensation, a real sensation when we observe it in the
dissection of an animal, or the autopsy of one of