Bauhaus-Universität Weimar

708 
OF THE SENSES GENERALLY. 
adding any new element to their nature. The sensations of the nerves 
of touch are therefore states or qualities proper to themselves, and 
merely rendered manifest by exciting causes external or internal. The 
sensation of smell also may be perceived independently of the appli¬ 
cation of any odorous substance from without, the nerve of smell being 
thrown by an internal cause into the condition requisite for the produc¬ 
tion of the sensation. This perception of the sensation of odours without 
an external exciting cause, though not of frequent occurrence, has been 
many times observed in persons of an irritable nervous system; and the 
sense of taste is probably subject to the same affection, although it would 
always be difficult to determine whether the taste might not be owing to 
a change in the qualities of the saliva or mucus of the mouth; the sen¬ 
sation of nausea, however, which belongs to the sensations of taste, is 
certainly very often perceived as the result of a merely internal affection 
of the nerves. The sensations of the sense of vision, namely colour, 
light, and darkness, are also perceived independently of all external 
exciting cause. In the state of the most perfect freedom from excite¬ 
ment, the optic nerve has no other sensation than that of darkness. 
The excited condition of the nerve is manifested, even while the eyes 
are closed, by the appearance of light, or luminous flashes, which are 
mere sensations of the nerve, and not owing to the presence of any 
matter of light, and consequently are not capable of illuminating any 
surrounding objects. Every one is aware how common it is to see 
bright colours while the eyes are closed, particularly in the morning 
when the irritability of the nerves is still considerable. These phe¬ 
nomena are very frequent in children after waking from sleep. Through 
the sense of vision, therefore, we receive from external nature no im¬ 
pressions which we may not also experience from internal excitement 
of our nerves; and it is evident that a person blind from infancy in 
consequence of opacity of the transparent media of the eye, must have 
a perfect internal conception of light and colours, provided the retina 
and optic nerve be free from lesion. The prevalent notions with regard 
to the wonderful sensations supposed to be experienced by persons blind 
from birth when their sight is restored by operation, are exaggerated 
and incorrect. The elements of the sensation of vision, namely the 
sensations of light, colour, and darkness, must have been previously as 
well known to such persons as to those of whom the sight has always 
been perfect. If, moreover, we imagine a man to be from his birth 
surrounded merely by external objects destitute of all variety of colours, 
so that he could never receive the impressions of colours from without, 
it is evident that the sense of vision might nevertheless have been no 
less perfect in him than in other men; for light and colours are innate 
endowments of his nature, and require merely a stimulus to render 
them manifest. 
The sensations of hearing also are excited as well by internal as by 
external causes; for, whenever the auditory nerve is in a state of excite¬ 
ment, the sensations peculiar to it, as the sounds of ringing, humming, 
&c. are perceived. It is by such sensations that the diseases of the 
auditory nerve manifest themselves; and, even in less grave transient 
affections of the nervous system, the sensations of humming and ringing
        

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