Bauhaus-Universität Weimar

Titel:
Dictionary of philosophy and psychology including many of the principal conceptions of ethics, logics, aesthetics ... and giving a terminology in English, French, German and Italian, vol. 2 [lead-zwing]
Person:
Baldwin, James Mark
PURL:
https://digitalesammlungen.uni-weimar.de/viewer/image/lit29448/794/
VISION 
degeneration of the optical cortex, with every¬ 
thing below the cortex in a perfectly healthy 
condition (Subj. Lichtempf. bei totalem Ver¬ 
luste des Sehvermögens, m. Zerstörung d. 
Rinde beider Hinterhauptslappen, Diss., Mar¬ 
burg, 1895, P. Schirmer). This proves to 
perfection that chemical changes in the cortex, 
although not brought about by excitation 
coming in from below, suffice to affect con¬ 
sciousness (and with spatial attribute as well 
as simple sensation quality). On the other 
hand, there are cases on record of most dis¬ 
turbing visual sensations (rings and balls of 
colour) due to irritation of the cortex caused 
by a diseased retina which was entirely blind 
to light—as was proved by the fact that these 
disturbances ceased when the eye in question 
was enucleated. (It is therefore impossible 
to say that what is usually called Idio-ketinal 
Light (q. v.), or the self-light of the retina, 
is or is not in reality cerebral light ; most 
probably it is of both cerebral and retinal 
origin, since it is beyond question that 
internal irritations from either source do, 
upon occasion, enter consciousness.) 
There are, then, aside from the conducting 
fibres, four separate stations, in general, in 
the affection of consciousness by external 
light—the retina, which is, indeed, not only 
a neuro-epithelial surface, but also a true 
nervous centre shifted to the periphery 
(cephalopods, which have, taken together, all 
the different nervous layers of the human eye, 
have some of them in the brain and not in the 
retina)1 ; the basal ganglia ; the primary visual 
centres in the occipital lobe; and the final 
association-centres. Each of these may 
apparently be excited to its characteristic 
activity by internal sources of activity, in the 
absence of incoming stimulation from below. 
In the lower animals the visual process is 
certainly of much less complexity than in the 
human visual organ; all that is essential to 
such a process is that there should be some 
form of reaction to the transverse vibrations 
of the ether. Any animal in which a portion 
of the ectoderm is so differentiated as to be 
a receptive organ for this form of excitation 
may be said to possess an eye, whether the 
reaction to the excitation is conscious or 
unconscious ; in certain of the lower forms of 
animal life the whole surface of the body is 
obscurely sensitive to light. Sensations of 
colour (as well as of form), as they exist in 
the perfected eye, are modalities of the funda¬ 
mental luminous sensations, which are without 
question of rather recent phylogenetic develop¬ 
ment. Wherever there is an eye with two 
distinct forms of visual elements, rods and 
cones, it is probable that there is a sense of 
colour. Below that, there is no evidence 
of this aspect of the luminous sensation : 
many observers have declared that the lower 
animals have a colour sense, and that they 
have strong colour-preferences (Gräber) ; but 
this conclusion is not warranted, for a pre¬ 
ference for one region of the spectrum over 
another may perfectly well be a preference 
for a particular degree of brightness. Since 
we have found out that the relative brightness 
of the different spectral regions is, for our¬ 
selves, totally different according as the illu¬ 
mination is faint or bright (the Pukkinje 
Phenomenon, q. v.), there is no reason to infer 
that animals have any sense for actual colour 
from the fact that they go from one coloured 
apartment into another, even though these 
have been made equally bright for the normal 
human eye. It has lately been found, in fact, 
that even bees are attracted by odours rather 
than by any sense for colour : flowers when 
covered by paper are visited by them just as 
frequently as if they were exposed to view. 
The eye is considered to be the most highly 
developed of the sense organs, not only be¬ 
cause of its comparative perfection as an optical 
apparatus (the lens is a piece of living matter 
which approaches the regularity of a solid with 
mechanically perfected curved surfaces), but 
also because of the number of different forms 
in which it effects sensible discrimination. 
The pressure sense, the heat sense, the cold 
sense are senses with good local discrimina¬ 
tion, but with variation within a single ter¬ 
minal organ for intensity only, without dis¬ 
crimination of quality—we cannot tell whether 
a given amount of heat comes to us from the 
infra-red or the red or the yellow rays of the 
spectrum. In the ear we have discrimination for 
different objective vibration-rates (the sound¬ 
waves) in the form of the different subjective 
quality attached to notes of different pitch, 
and to this discrimination is given up the 
physiological space-discrimination in the 
auditory organ—namely, the succession of 
fibres of the basilar membrane ; there is left 
no means of acute local discrimination, and, 
in fact, in the auditory sense we have no 
space-discrimination other than by the greater 
loudness of a sound heard by one ear than by 
1 The optic nerve is not a nerve, properly speaking, 
but a portion of cerebral white matter pushed 
forward (Greef, Arch.f. Ophthal., xxix. 85, 1900). 
the other, or by a rough difference in quality, 
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