66. The facts which the hypothesis of specific energies is called on to explain are more consistently interpreted on the admission of a common property in nerve-tissue, manifesting different degrees of excitability, and entering into different mechanisms, so that the functional results differ. A nerve which may be stimulated from the skin will not respond at all, or not in the same way, if the stimulus be applied under the skin. Are we to suppose that the specific energy resides in one part of the nerve, and not in another?[109] That the optic nerve responds to stimuli which will not sensibly excite a motor nerve, depends on the terminal structures through which the stimulation is excited; for the optic nerve itself, apart from the retinal expansion, is as insensible to light as the motor nerve is. And the specific sensation, or movement, which _results_ from stimulation of a nerve depends not on the nerve, but on the mechanism of which the nerve is one element. Sensations of touch, temperature, and pain are assuredly specific; they are as unlike each other as a sensation of taste is unlike a sensation of smell. Yet the same nerves, variously stimulated, produce all three sensations.
67. We conclude, therefore, that the phrase “specific energy” is an elliptical expression for the particular office of a nerve. In this meaning there is no obscurity. The optic nerve is not a vasomotor nerve, the skin nerve is not a muscle nerve; the auditory nerve is a nerve of special sensation, the vagus is a nerve of systemic sensation; and so on. Neither movement nor sensation belongs to the nerves themselves.
68. The principles laid down in the preceding chapter are equally applicable to the central system. But here greater difficulties await us. We cannot expect traditional views to be easily displaced, when they have taken such hold on the mind, as is the case with regard to Sensibility. To admit that all nerves have a common property, and that their functional relations depend on the organs which they innervate, demands small relinquishment of cherished opinions. But to admit that all nerve-centres have a common property, and that their functional relations depend on their anatomical connections, is to sweep away at once a mass of theoretic interpretations which from long familiarity have acquired an almost axiomatic force. That the brain, and the brain only, is the source and seat of Sensibility is the postulate of modern Physiology.