The conduction of such stimulation to parts removed some distance from the sense organ suggests paths of transmission comparable to those which transmit nervous impulses in animals.
The action of atropine in dilating the pupil is also aided by a stimulation of the fibres from the sympathetic nervous system, which innervate the remaining muscle of the iris - the dilator pupillae.
Irritability, again, although a notable quality of living substance, is not peculiar to it, for many inorganic substances respond to external stimulation by definite changes.
Here he stated the principle, not before recognized, that the kind of sensation following stimulation of a sensory nerve does not depend on the mode of stimulation but upon the nature of the sense-organ.
Thus light, pressure, or mechanical stimulation acting on the retina and optic nerve invariably produces luminous impressions.