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THE MECHANICAL PROPERTIES OF THE ANIMAL BODY CONSIDERED
OF THE SOLID STRUCTURE OF ANIMAL BODIES SUBSTITUTES
ON THE POSITION OF THE HEAD OF ANIMALS, AND ITS RELATION
TO THE SPINE; TO ILLUSTRATE THE PROPOSITION, THAT ALL
PARTS OF THE SKELETON ARE CORRELATED, IN ADAPTATION
APPROPRIATE SENSIBILITIES INDUCE COMBINED MUSCULAR AC-
TIONS, FOR THE PROTECTION OF THE VITAL ORGANS
ACCOUNT OF THE AUTHOR'S DISCOVERIES
THE NERVOUS SYSTEM.
BY ALEXANDER SHAW,
IN the following Treatise, especially in the fifth and seventh Chapters, and the Appendix, there is frequent reference to the distinct qualities of the Nerves, in illustration of the properties of the Hand. But the Author has only alluded distantly to the original inquiries which he himself pursued in the Nervous System. The Discoveries for which Physiology is indebted to him, in that branch, are among the most valuable that have at any time been made. In the fundamental changes they have wrought on the theories held upon the subject, they are on a par with the Discovery of the Circulation of the Blood by Harvey. It may, therefore, be an appropriate introduction to this Volume, to give a brief account of the principal results of his researches.
When the author commenced his investigations, the subject of the functions of the Brain and Nerves was involved in great darkness. The extent of ignorance may be judged of, when it is stated that the distinction between the nerve which gives Motor power to the muscles, and the nerve which conveys Sensation from the skin, had not at that time been ascertained,
The opinion universally held was, that the nerves generally, to whatever part of the body they were distributed, possessed not only those two properties, but various other less clearly-defined ones; all of which, it was supposed, were obtained promiscuously from the brain, which was regarded as a common central source of every imaginable nervous endowment. And we may perceive that those who held that view, of motor power and sensation belonging conjointly to the same nerve, did not consider it impossible for two kinds of nervous influence, essentially distinct from each other, to be conveyed along its fibrils, one taking one direction, and the other a direction exactly opposite, at the same instant; for it cannot be doubted that the nervous agency which excites the muscles proceeds outwardly from the brain, or centrifugally; while that which communicates sensation proceeds inwardly, or centripetally.
We may be surprised that a view which appears now so incongruous should have held its ground so long. Yet there is a fact which will assist in accounting for the error. The body generally, from head to foot, is supplied, with scarcely an exception, by the extensive series of "Spinal nerves." Now, it happens that every one of that numerous class possesses the power of bestowing both motion and sensation. For example, if any nerve whatever that goes to the arm or leg be cut across, the immediate effect will be total loss of both functions in the part corresponding to the distribution of the nerve. Accordingly, when physiologists observed the same effects constantly produced by such experiments, they naturally concluded that the two properties were inseparably united in every nerve.
As an additional source of deception, it may be mentioned, that if a nerve be removed from the body, and its internal structure carefully examined, the thread-like fibrils of which it is composed will all appear exactly alike; nothing will be found in their size, colour, or texture, to indicate that there should be any difference in their functions.
It is to the author that physiology is indebted for the overthrow of those erroneous opinions, and for the discovery of the true principle on which the functions of the nervous system are to be investigated. To him the honour is exclusively due of having demonstrated, for the first time, that the nerve of Motion is distinct from the nerve of Sensation; and that when a nerve,
apparently simple, possesses both properties, it is a sign that it is really compound, and consists of fibrils derived from distinct divisions of the brain or spinal cord.
The process of investigation by which he made that pre-eminently great discovery may be briefly explained. It commenced in his adopting certain original views concerning the functions. of the principal centres of the nervous system. He opposed the prevalent opinion, that the same common properties belonged indiscriminately to, all parts of the Brain and Spinal cord. He conceived that, however undefined and irregular were the various subdivisions of these important organs, and however assimilated in structure, so as apparently to forbid the notion of their having any essential differences between them, they were, in fact, centres of distinct kinds of nervous agency; but that all were subject to a connecting and presiding influence exercised by the Brain as a whole.
Now, from this theory, of inferior organs, each endowed with a distinct power, being contained within the brain and spinal cord, the author was led to take a corresponding new view of the functions of the nerves. He conceived that each of those nerves which arose from a distinct organ, would possess the power of conveying to or from the body the particular influence with which the organ itself was endowed.
Accordingly, a method of ascertaining the functions of the nerves, never before thought of by physiologists, was suggested to his mind. Heretofore, the only attempts made to distinguish their uses had been by performing experiments on the trunks of the nerves at a distance from their origins, and where they had formed frequent connexions, in their course, with numerous others coming from totally different parts of the brain. But the mode he adopted, and which was the key to all his discoveries, was that of examining the nerves at their roots-close, that is to say, to the divisions of the brain, or of the spinal cord, from which they took their rise.*
The first nerves to which he applied that original method of research were those just adverted to as conferring motion and sensation conjointly, viz., the Spinal nerves. After a time, he carried his inquiries into the nerves of the Brain; and prosecuted them in a similar manner, by taking their origins as his * Idea of a New Anatomy of the Brain. 1811.