direction is in each case indicated by the arrow. The central end _b_, if irritated, will produce sensation; whereas the peripheral end _a_ produces neither sensation nor movement. The central end _d_ produces neither sensation nor movement; the peripheral end _c_ produces movement. 25. Two facts are proved by these experiments. First, that the co-operation of the centre is necessary for Sensation, but not for Movement. Although normally all the muscles of the trunk are moved only when their centre has been excited, yet any irritation applied directly to the muscle nerve, even when separated from its centre, produces a movement. And to this we may add that a slighter stimulus will move the muscle by direct irritation of the nerve, than by indirect irritation through the centre; a slighter stimulus also will suffice when applied to the nerve than when applied to the muscle itself. 26. The second fact proved is known as _Bell’s Law_, that the sensory and motor channels are respectively the posterior and anterior nerves. The fact is indisputable, but its theoretic interpretation can no longer be accepted in its original form. Bell supposed the two nerves to be different in kind, endowed with different specific energies, the one sensitive, the other motor. The majority of writers still express themselves as if they adopted this view. We shall, however, presently see reason for replacing it by the more consistent interpretation which assigns one and the same property to both nerves, marking their distinction by the terms afferent and efferent; the one set being anatomically so disposed that it conveys stimuli from the surfaces to the centre, and the other set conveying stimuli from the centre to the muscles, glands, and other cells.[89] 27. Bell’s discovery was rapidly generalized. The principle of localization was extended to all nerves, and of course to the posterior and anterior columns of the spinal cord, which indeed were assumed to be continuations of the nerves. Bell, who was greater as an anatomist than as a philosopher, always maintained that anatomical deduction was superior to experiment. But this was to misunderstand the reach of deduction, which is only valid to the extent of its premises.[90] In the present case, the premises assumed that the posterior columns were continuations of the posterior roots, and carried impressions to the brain, the anterior columns carrying back from the brain the “mandates of the will.” Experiment has, however, decisively shown that it is _not_ through the posterior columns that sensory impressions travel to the brain, but through the central gray substance.