64. The first criticism to be made on the hypothesis is that it commits the error of confounding function with property, assigning as a specific quality of the nerve the reaction of the organ innervated. Thus Müller speaks of the specific energy as “the essential condition of the nerves in virtue of which they see light and hear sound.” But the optic nerve no more _sees_, than the liver-nerve _secretes_ bile. That the optic nerve is one element in the mechanism on which vision depends, is all that we can say, Müller declares that it is not sufficient to assume each nerve to be so constituted that it has a susceptibility to certain stimuli rather than to others; but that “with Aristotle we must ascribe to each a peculiar energy as its vital quality. Sensation,” he adds, “consists in the sensorium receiving through the medium of the nerves a _knowledge of certain qualities_,--a condition, not of the external bodies, but of the nerves themselves,”--and these qualities are different in different nerves. In other words, he assumes a special substance for each special energy. The sensation of color depends on the special Visual substance (_Sehsinnsubstanz_); the sensation of sound on the Auditory substance (_Hörsinnsubstanz_); and so on. 65. We have here an hypothesis analogous to that of Innate Ideas, or _a priori_ Forms of Thought. It is, in fact, only a reproduction of that conception carried into the sphere of Sense. No one thinks of assigning specific energies to the several muscles, yet a movement of prehension is as different from a movement of extension, a peristaltic movement is as different from a movement of occlusion, as a sensation of sound is from a sensation of color. If movement is common to both of the one class, feeling is common to both of the other: the forms and mechanism are different and specific. Muscles have the common property of contracting under stimulation; whatever be the nature of the stimulus, each muscle has its own particular response, or mode of reaction: the flexor always bending, never extending the limb; the sphincter always closing, never opening the orifice. The movements of the heart are not the same as those of the eye; both are unlike the movements of the intestine. There are muscles which respond to some stimuli, and not to others. Those of the eye, or of the vocal chords, respond to impulses which would leave the masseter or biceps unstirred. According to Marey, the hyoglossus of a frog will become tetanic under a stimulus of only ten pulses in a second; whereas the gastrocnemius of that same frog resists a stimulus of less than twenty in a second. We find the retina responding to ethereal pulses which leave the auditorius unaffected; we find the muscles of a gnat’s wing so exquisitely susceptible that the wing beats eight thousand times in a second,--a delicacy in comparison with which even our muscles of the eye are coarse.