We may surmise that the simplest auditory organs with their otoliths have a homogeneous sensational system, analogous perhaps to our systems of sensations of pressure. The special development of the organ as seen in the cochlea of higher animals explains the evolution of an extraordinarily complex sensational system from this originally homogeneous system. Still, the structure remains similar in this respect, that it seems adapted, in the latter case as in the former, to the best possible transfer of the physical stimulus to the sensory nerve rather than to any transformation of the stimulus. This view agrees with the observed fact that, just as sensations of pressure may be perceived, on regions of the skin not supplied with special receiving organs, so, in the case of certain animals, such as birds, where the conditions are specially favorable for their transmission, sound-vibrations are transferred to the auditory nerve and sensed even after the removal of the whole auditory organ with its special receiving structure.
With smell, taste, and sight the case is essentially different.
Organs are present which render direct action of the stimuli on the sensory nerves impossible. The external stimuli are here received through special organs and modified before they excite the nerves. These organs are specially metamorphosed epithelial cells with one end exposed to the stimulus and the other passing into a nerve fibre. Everything goes to show that the receiving organs here are not merely for the transfer of the stimuli, but rather for their transformation . In the three cases under discussion it is probable that the transformation is a chemical process. In smell and taste we have external chemical agencies, in sight we have light as the causes of chemical disintegrations in the sensory cells; these processes in the cells then serve as the real stimuli.
These three senses may, as chemical senses, be distinguished from the mechanical senses of pressure and sound. It is impossible to say with any degree of certainty, to which of these two classes sensations of cold and hot belong. One indication of the direct relation between stimuli and sensation in mechanical senses, as contrasted with the indirect relation in chemical senses, is that in the first case the sensation lasts only a very little longer than the external stimulus, while in the latter case it persists very much longer. Thus, in a quick succession of pressures and more especially of sounds, it is possible to distinguish clearly the single stimuli from one another; lights, tastes, and smells, on the other hand, run together at a very moderate rate of succession.
4. Since peripheral and central stimuli are regular physical concomitants of elementary sensational processes, the attempt to determine the relation between stimuli and sensations is very natural. In attempting to solve this problem, physiology generally considers sensations as the result of physiological stimuli, but assumes at the same time that in this case any proper explanation of the effect from its cause is impossible, and that all that can be undertaken is to determine the constancy of the relations between particular stimuli, and the resulting sensations. Now, it is found in many cases that different stimuli acting on the same end-organ produce the same sensations; thus, for example, mechanical and electrical stimulations of the eye produce light sensations. This result was generalized in the principle, that every receiving element of a sense-organ and every simple sensory nerve-fibre together with its central terminus, is capable of only a single sensation of fixed quality; that the various qualities of sensation are, therefore, due to the various physiological elements with different specific energies.
This principle, generally called the "law of specific energy of nerves", is untenable for three reasons, even if we neglect for the moment the fact that it simply refers the causes of the various differences in sensations to a qualtalitas occutlta of sensory and nervous elements.
1) It is contradictory to the physiological doctrine of the development of the senses. If, as we must assume according to this doctrine, the complex sensational systems are derived from systems originally simpler and more homogeneous, the physiological sensory elements must have undergone a change also. This, however, is possible only under the condition that organs may be modified by the stimuli which act upon them. That is to say, the sensory elements determine the qualities of sensations only secondarily, as a result of the properties which they acquire through the processes of stimulation aroused in them. If, then, these sensory elements have undergone, in the course of time, radical changes due to the nature of the stimuli acting upon them, such changes could have been possible only under the condition that the physiological stimulations in the sensory elements varied to some extent with the quality of the stimulus.
