If you propose to me that curious physiological question,—in what consists the function of a nerve—in oscillation, or in undulation of a fluid, in electricity, or in magnetism? or how the nerve carries this impression to the brain? or if you desire me to meet the subtle objection, which Dr. Reid advanced against the opinion of Aristotle and of the more modern psychologists,—I might weary you with conjectures like those of Newton and Hartley, that some ethereal fluid was, by the impulse of peculiar stimulus to its nerve, the cause of the senses; or that the mental phenomena are an imparting, or influence of the immaterial soul by corporeal vibration; or that dreams are “_motions of fibres_:” and at length, with humility confess this to be a mystery we cannot yet fathom. And this I do the more willingly, as it may prove my devotion to the _proper limits of our study_; moreover, the question itself is not essential to my argument. Yet it is certain that external impressions of every object or subject, reach the brain through the medium of a nerve; and when the _same fibrils_ of those nerves, or _that spot_ of brain on which the original image rested, are _again_ irritated by their proper stimulus, or by the same or a similar body, _an association is produced, and memory is the result_. For the insurance of this sense of touch, and feeling, and perception, it is essential that the impression at the end of a nerve shall be perfectly transmitted along its course to the brain, so that the brain shall be _conscious_, or sensible of this impression. For if a nerve be cut asunder, or a ligature be placed on any portion of it between the skin and the brain, _the sensation instantly ceases_. It is not essential, however, that the contact should take place at the moment of the perception; and the explanation of this involves one of the most curious phenomena of the body’s feeling; and, indeed, the metaphysical mystery of the nature of memory, which is too abstruse a point to be touched by us here. After amputation, the patient may still complain of pain, and heat, and cold, _in the dissevered limb_; he experiences the memory of a sensation; he feels, as it were, the ghost of his arm or leg. On the night succeeding the operation, the groaning patient has often cried out to me, with pain in the toe or finger of that limb; and when he is moved or shifts his position, he will attempt to hold his leg, or will beg his nurse to take care that she does not touch or run against it. Nay, I have frequently, on asking a patient how he felt, even after the lapse of many months from the operation, been answered, that he was well, but had not lost the pain in his leg; or that his leg or his arm were lying by his side, when perhaps the limb was undergoing the process of maceration in the dissecting-room, or the bones were bleached and dangling in the museum.