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
§17. Stimulation of the Organ of Vision 
The nervous system of the body is acted on by external agents of 
various kinds, which produce changes in the state of the nerves. 
These changes may sometimes be detected by auxiliary apparatus, for 
example, by studying the electrical reactions; but they are also mani¬ 
fested by their actions on other parts of the body with which the nerves 
are connected. The change of state of the so-called motor nerves is 
accompanied by contractions of corresponding muscles. Under the 
same circumstances, other nerves, known as sensory nerves, excite 
sensations in the brain which is the organ of consciousness of the body. 
Now in the case of the motor nerves, no matter how diverse the external 
action may be—tearing, crushing, cutting, burning, eroding, shocking 
by electricity,—the invariable result is the contraction of the corres¬ 
ponding muscle, the only difference being one of degree. Therefore, 
apart from their qualitative differences, these various influences, so far 
as their relation to the motor nerves are concerned, are called stimuli. 
Quantitatively, we speak of a stimulus as being strong or weak accord¬ 
ing to the amount of twitching that is produced. The resulting altera¬ 
tion of the state of the nerve due to a stimulus is called stimulation 
or excitation. Similarly, the ability of the stimulated nerve to make 
the muscle contract is known as its excitability. The latter is affected 
by mortification and by external influences of many kinds. 
The sensory nerves may be analyzed in the same way. External 
agencies, which acting on a motor nerve would cause contraction of 
the muscle, have another peculiar sort of effect on a sensory nerve 
and give rise to a sensation, provided the nerve is alive and not dis¬ 
connected with the brain. But there is undoubtedly an essential 
difference here, because there are qualitative differences in the sensa¬ 
tion corresponding to qualitative differences in the stimulus. But 
although different stimuli cause different sensations, still their effects 
are invariably sensations, that is, invariably actions of a kind that do 
not occur otherwise and are peculiar to the living body. Accordingly, 
the abstract conception of stimuli and stimulation as used first with 
reference to the motor nerves has been transferred likewise to the 
sensory nerves. Thus, the external agencies which acting on the 
sensory nerves excite sensations are also called stimuli, and the change 
itself that takes place in the nerve is said to be a stimulation. 
The state of stimulation that may originate at any part of a nerve 
fibre through the action of stimuli is always conducted to all other 


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