61. In proposing the term Neurility, I not only wished to get rid of the ambiguities which hovered round “nerve-force” and “nerve-current,” but to recall the physiological principle that properties are dependent on structures; and therefore that the special property of nerve-tissue is conditioned by its structure. Neurility is, of course, an abstraction; but so is _the_ nerve an abstraction. The concrete manifestations are the several nerve-actions. These we classify and specify. One class we call sensory, another class motor; not because the nerve-action itself is different, but because it is in each class in a different functional relation to other parts. In classing men as governors and governed, employers and employed, we do not suppose anthropological distinctions, but only differences in their social functions.
62. This is the modification of the Law of Bell to which reference was made in § 26. It replaces the idea of two different kinds of nerve, sensory and motor, by that of two different anatomical connections. I need not reproduce here the argument with which I formerly criticised the supposed distinction between sensory and motor nerves; because the old idea is rapidly falling into discredit, and physiologists so eminent as Vulpian and Wundt have explicitly announced their adhesion to the principle of identity,--a principle which, as Vulpian truly remarks, dominates the whole physiology of the nervous system.[106]
63. One development of the theory of Bell, respecting the different kinds of nerve, has been the still accredited hypothesis that each nerve has a “specific energy,” or quality, in virtue of which it acts and reacts only in one way. The optic nerve, no matter how stimulated, only responds by a sensation of color, the auditory nerve only by a sensation of sound; and so on. This hypothesis, which (as I learn from a correspondent)[107] was originally propounded by Bell himself, was developed and made an European doctrine by Johannes Müller, first in his remarkable treatise, _Über die phantastischen Gesichtserscheinungen_ (1826), and afterwards in his _Physiology_. Like all good hypotheses, it has been fruitful; and Helmholtz still holds it to be of extraordinary importance for the theory of perception. Although combated by a few physiologists, it has kept its place firm in the general acceptance; no doubt because it forms a ready explanation of the facts. But, as I often have to remark, _explanation_ is not _demonstration_.[108]