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178 and that glands will secrete without the intervention of nerves. It is also probable that the sensorium may be stimulated without the direct intervention of nerves. But it is not less certain that the ordinary stimulus which awakens the activity of muscles, glands, and nerve-centres, is the stimulus of nerves. How this is effected we cannot say. What the peculiar property of the nerves may be, baffles science. It may be electricity; it may be a correlation of that force; it may be a special "nerve-force," something sui generis. To avoid every hypothesis, and yet to secure a specific name, I proposed the term Neurility, as corresponding with the terms Sensibility and Contractility; the term, having met with some acceptance, may be used throughout this paper.

In the course of investigation, it appeared to me that many of the vexed questions of nerve-physiology would rapidly receive answers, if the perplexing ambiguities of phraseology were to give place to a more rigorous nomenclature. For example, it is difficult to come to an understanding respecting the motor and sensory nerves, so long as we continue to talk as if we believed that "motility" resides in the spinal chord, and that the posterior roots are "sensitive." Motor force no more resides in the spinal chord, than explosive force resides in the lighted match; the motor-force is in the muscles, the explosive force is in the gunpowder; and when eminent physiologists are at great pains to detect the "seat of motility" (siège de la motricité) in the grey matter of the chord, they are perplexing a subject already difficult enough. I do not assert that competent physiologists ever believe that the seat of motility is elsewhere than in the muscles; what they mean is, doubtless, that the centre, from which the stimulus issues which will excite the muscles, is in the spinal chord. But how easily the ambiguous language leads to ambiguity of conception may be seen in a hundred examples; and it may be, to a great extent, avoided by rigorously demarcating the phenomena of Sensibility, Neurility, and Contractility, as the actions of three different organs: nerve-centres, nerves, and muscles.

Müller puts this question:—"Is the nervous principle, or force of the motor fibres, different in its quality from that of the sensitive fibres? or are what are here called the motor and sensitive principles, actions of the same nervous principle, differing only indirection—being centrifugal in the motor, and centripetal in the sensitive fibres?" Put into the language of the essay, this question will run thus:—Are there two Neurilities, one motor, and the other sensory (with the possibility of a third—the secretory)? Or does the Neurility in each nerve act only in one direction, from a centre along the motor nerve; to the centre along a sensory nerve?

That there are two Neurilities is extremely improbable, nor is there a shadow of evidence in its favour. Neither motions nor sensations belong to the nerves themselves, but to the muscles and centres, stimulated by the nerves. It is only in the looseness of unsystematized phraseology, that we speak of "sensitive impressions" being "conveyed to the