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Bell other only by delicacy of structure and by a corresponding delicacy of sensation. It is imagined that impressions thus differing in kind are carried along the nerves to the sensorium and presented to the mind, and that the mind, by the same nerves which receive sensation, sends out the mandate of the will to the moving parts of the body.' His own conclusions were, 'that the nerves are not single nerves possessing various powers, but bundles of different nerves, distinct in office;' and 'that the nerves of sense, the nerves of motion, and the vital nerves, are distinct throughout their whole course.' These conclusions were established by the fact that, 'on laying bare the roots of the spinal nerves, I found that I could cut across the posterior fasciculus of nerves which took its origin from the posterior portion of the spinal marrow without convulsing the muscles of the back, but that, on touching the anterior fasciculus with the point of the knife, the muscles of the back were immediately convulsed.' 'I now saw,' he adds, 'the meaning of the double connection of the nerves with the spinal marrow.' His apprehension of the meaning of this observation was at first obscured by a recollection of the old doctrine that all nerves were sensitive, and for a time he spoke of two great classes of nerves distinguishable in function, the one sensible, the other insensible (letter dated 6 Dec. 1814). But he had established beyond doubt the existence of sensory and of motor nerves. Majendie (Journal de Physiologie, Paris, 1822, ii. 371) claims to have first shown this experimentally in 1821, but he is refuted by the printed record of Bell's experiment in 1811, as is admitted by Béclard in his most recent account of the controversy (ib., Paris, 1884, p. 405), where, speaking of Bell's discovery, Béclard says: 'Il n'est pas douteux qu'il a résolu, le premier, cette question par la voie expérimentale.' It was not till 1826 that Bell's discovery was complete in its modern form. He thus explains it (letter, 9 Jan. 1826): 'It shows that two nerves are necessary to a muscle, one to excite action, the other to convey the sense of that action, and that the impression runs only in one direction, e.g. the nerve that carries the will outward can receive no impression from without; the nerve that conveys inward a sense of the condition of the muscle cannot convey outward; that there must be a circle established betwixt the brain and a muscle.' His investigations were completed from 1821 to 1829, in a series of papers read before the Royal Society, and were published, with some slight alterations, in a separate volume in 1830, entitled 'The Nervous System of the Human Body.' Before his time nothing was known of the functions of the nerves, and the reason of the relation between hemiplegia or paralysis of one vertical half of the body and injury of the brain was explained through groundless hypotheses. A few vague expressions in earlier writers have been quoted as showing that something was known; but whatever the words, the interpretation of them was never given till after Bell's discovery had made the whole subject clear. Bell himself states, with perfect fairness, in his republication, all the details known before the time of his discoveries (Nervous System, pp. vii, viii). 'Dr. Alexander Monro discovered that the ganglions of the spinal nerves were formed on the posterior roots, and that the anterior roots passed the ganglion. Santorini and Wrisberg observed the two roots of the fifth pair of nerves. Prochaska and Sœmmering noticed the resemblance between the spinal nerves and the fifth pair, and they said, "Why should the fifth nerve of the brain, after the manner of the nerves of the spine, have an anterior root passing by the ganglion and entering the third division of the nerve?"'

Bell's great discovery, thus gradually completed, was that there are two kinds of nerves, sensory and motor; that the spinal nerves have filaments of both kinds, but that their anterior roots or origins from the spinal cord are always motor, their posterior roots sensory. He further (Phil. Trans. 28 May 1829) demonstrated that the fifth cranial nerve is a motor as well as a sensory nerve, and that while the fifth supplies the face with sensory branches, the motor nerve of the facial muscles is the portio dura of the seventh nerve. From this discovery of its true function, the portio dura is often spoken of by anatomists as Bell's nerve. His discoveries as to the fifth and seventh nerves were suggested by their anatomical relations, confirmed by observation of the results following accidental injuries in man, and completely established by experiments on animals. These experiments were a cause of delay; for in a letter dated 1 July 1822 (Letters of Sir C. Bell, p. 275) he says: 'I should be writing a third paper on the nerves, but I cannot proceed without making some experiments, which are so unpleasant to make that I defer them. You may think me silly, but I cannot perfectly convince myself that I am authorised in nature or religion to do these cruelties.' Bell's discoveries were the greatest which had been made in physiology since Harvey had demonstrated the circulation of the blood, and Bell was only 