Bauhaus-Universität Weimar

Helmholtz's treatise on physiological optics. Volume 2. Edited by James P. C. Southall. Translated from the 3rd German edition
Helmholtz, Hermann von
11, 12.1 
§17. Stimulation of the Organ of Vision 
of pain referred to the region of distribution of the nerve in the fourth 
and fifth finger, as well as another localized at the place struck, which 
is more unpleasant than that resulting when the skin alone is stimu¬ 
lated. This must be referred to the nerves of the nerve trunk. In the 
same way when the eyeball is pressed at the outer angle, the pain of 
the pressure is felt locally by means of the sensory nerves of this 
region, and the light that is seen is supposed to be in the region of the 
bridge of the nose. Something of a similar nature may happen when 
the optic nerve trunk is stimulated. 
That the optic nerve and the retina, both capable of being stimu¬ 
lated by so delicate an agency as light, are tolerably insensitive to the 
roughest mechanical maltreatment, that is, have no sensation of pain, 
has seemed a remarkable paradox. The explanation, however, is 
simple, because the quality of all sensations of the optic nerve belongs 
to the group of light sensations. The sensibility is not lacking, but the 
form of the sensation is different from that usually associated with this 
particular kind of stimulus. 
Light sensations due to internal conditions are very varied. There 
are a number of luminous phenomena, occurring in all diseased condi¬ 
tions of the eye or of the entire body, that may take up the whole 
field or may be localized in it. In the latter case they take sometimes 
the form of irregular spots and sometimes fantastic figures of men or 
animals, etc. Mechanical causes often participate in these effects, as, 
for example, increased blood-pressure in the vessels or humors of the 
eye. Thus, on releasing the eye from uniform pressure, parts of the 
vascular figure often flash out; and sometimes, after violent exertion 
separate pulsating parts, maybe larger portions, of the vascular figure 
are visible.1 In other cases there may be a sort of chemical stimulation 
due to altered condition of the blood, for example, by narcotic poison¬ 
ing. Finally, many of these phenomena also may be explained as due 
to a spread of a state of excitation within the central part from other 
parts of the nervous system to the origin of the optic nerve. When 
the state of excitation in a stimulated sensory nerve is imparted to 
another that is not acted on by the stimulus at all, we call it an asso¬ 
ciated sensation. For example, looking at large bright surfaces, such 
as sunlit snow, causes many persons to feel a simultaneous tickling in 
the nose. The sound of certain scraping or squeaking noises makes a 
cold chill run down the back. Apparently, such associated sensations 
may occur also in the visual apparatus when other sensory nerves are 
1 Purkinje, Zur Physiologie der Sinne. I. 134. II. 115. 118. —Subjektive Erschei¬ 
nungen nach Wirkung der Digitalis II. 120.


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