The threshold-value of the stimuli adequate for the various senses may be extraordinarily lowered.
This theory of complementary colours as due to the polarity in the qualitative action of the retina is followed by some criticism of Newton and the seven colours, by an attempt to explain some facts noted by Goethe, and by some reference to the external stimuli which cause colour.
By means of vibrations or shocks transmitted through the - Sub water, or by displacements in the balance or position of the animal, the otoliths are caused to impinge against the bristles of the sensory cells, now on one side, now on the other, causing shocks or stimuli which are transmitted by the basal nerve-fibre to the central nervous system.
In them areas are found whence stimuli excite movements of this or that finger alone, of the upper lip without the lower, of the tip only of the tongue, or of one upper eyelid by itself.
Of the response of the nerve, it is found that unmistakable signs of fatigue appear even very soon after commencement of the excitation of the nerve, and the muscle ceases to give any contraction in response to stimuli applied indirectly to it through its nerve.
The increments of contraction become, however, less and less, until the succeeding stimuli serve merely to maintain, not to augment, the existing degree of contraction.
One end of the body, through contact, during locomotion, with fresh tracts of medium and other forms of stimuli, has become more specialized than the rest, and here the nervous system and sense-organs are more densely aggregated than elsewhere, forming a means of controlling locomotion and of correlating the activities of the inner organs with the varying stimuli that impinge upon the body.
Certain it is that in the course of the waking day a great number of stimuli play on the sense organs, and through these produce disintegration of the living molecules of the central nervous system.
A further stage in evolution is that the muscle-cells lose their connexion with the epithelium and come to lie entirely beneath it, forming a sub-epithelial contractile layer, developed chiefly in the tentacles of the polyp. The of the evolution of the ganglioncells is probably similar; an epithelial cell develops processes of nervous nature from the base, which come into connexion with the bases of the sensory cells, with the muscular cells, and with the similar processes of other nerve-cells; next the nerve-cell loses its connexion with the outer epithelium and becomes a sub-epithelial ganglion-cell which is closely connected with the muscular layer, conveying stimuli from the sensory cells to the contractile elements.
But he had also to endure countless objections to his mathematical statement of Weber's law, to his unnecessary assumption of units of sensation, and to his unjustifiable transfer of the law from physical to physiological stimuli of sensations, involving in his opinion his parallelistic view of body and mind.