Now, it will be obvious to any reflective person how very important all information we can acquire respecting this central empire over the impressions of our sense-nerves may become when we try to estimate the conditions of human knowledge. If it be true that the Mind imposes laws of activity on the nervous system even when receiving impressions from it, then the necessity we are under of _thinking_ in accordance with certain inly imposed laws receives a most striking illustration. And the inference from it carries an _a fortiori_ probability since our thoughts lie nearer to our mental centre than any of our sense-impressions. [99] Nerves of common feeling are acutely sensitive when divided, and the patient animal under a Majendie or a dentist utters a sharp shriek. The case is different with motor nerves, with those of the sympathetic system, and with (what is more to our purpose) nerves of sensation. It seems clear that mechanical injuries, or even touches, excite them in the direction of their own special functions. Auditory nerves feel a shock as a sound,--optic nerves receive it as a sudden and brilliant light. We are doubly assured from these effects of the true functions belonging to the several sets of nerves. Disease and injury are great discoverers of what ought to be healthy susceptibilities. In such cases, however, they prove also something more agreeable to think upon. They prove that suffering is confined within definite limits, and that _economy_ of pain forms part of the universal design, for the sensitive animal as well as the sensitive man. If all our nerves shrank equally with equal tenderness, life would be a history of protracted agony. Yet one might have expected, _primâ facie_, that a fibre which telegraphs shapes and colours with their blendings, would eloquently tell the story of its own occasional anguish. And our whole nervous framework might have been conceived as an instrument of torture. It has not been so constituted. _Per contra_, the nerves of common feeling assert their own vocation.--"A brazen canstick turned" sets the teeth on edge, and troubles the skin with horripilation. Believers in ghosts--and also disbelievers--are aware that some sights "Make knotted and combined locks to part, And each particular hair to stand on end." For extended information on this subject compare Additional Note C. [100] Aristotle so described it before Mr. Bain and other modern writers, "[Greek: to gar horaton esti chrôma]," De Anima II. 7. 1. As Kampe carefully observes, "so ist die Farbe (nicht die gefärbten Körper) das Eigenthümliche des Gesichtssinns." See also his note, Erkenntnisstheorie des Aristoteles, p. 88. [w] Compare Helmholtz on "The Sensation of Sight," _Lectures_, pp. 256, 7, and 259.