On a new theory of nervous action as regards the transmission of sensation along the nerves / by Robert M'Donnell.

  • M'Donnell, Robert.
Date:
[1870]
Catalogue details

Licence: Public Domain Mark

Credit: On a new theory of nervous action as regards the transmission of sensation along the nerves / by Robert M'Donnell. Source: Wellcome Collection.

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    condition, transmit the wave which produces the idea of simple contact, or that which produces the idea of heat—or, again, the same nerve tubules in the optic nerve which propagate the undulations of red may also propagate, in normal vision, those which excite the idea of yellow or blue, and so for other senses. I advocate this undulatory theory of sensation in preference to the theory of distinct conductors— lstly. Because it is simple. 2ndly. Because it is strongly supported by analogy, when compared with wave propagations in other departments of science. 3rdly. Because it appears to he in harmony with a large number of recognized physiological facts, which seem inexplicable upon the theory of distinct conductors. It would he. obviously impossible, within the limits of one com¬ munication, to discuss such a theory in its application to the various senses. I wish merely to bring before the Academy, at present, a general statement of the grounds upon which this hypothesis rests; and I shall hope, hereafter, in several communications, to elucidate its applicability to the transmission of the sensations peculiar to each special sense. 1st. When compared with the theory of distinct conductors, the undulatory theory is obviously simpler as regards anatomical detail. Anatomy has not given any evidence that with an ordinary compound nerve there exist different kinds of conductors—to the highest powers of the microscope all such tubules are identical in appearance. Aay more, we now know that nerves may he so spliced (if I may use the expres¬ sion) on to one another, that sensitive nerves may he made continuous with those which convey the commands of the will to muscles. As regards the analogy between this theory of nerve action and the wave theory of light, I do not pretend to say that it holds in every respect: there are obvious points of difference. If we infer that light and heat do. not consist of particles emitted by a hot body, our natural alternative is to suppose that they are undulations of a medium per¬ vading space. This hypothesis furnishes by far the best explanation of many very curious phenomena in light and heat, and is now generally received. This medium, which we suppose to pervade space likewise, with more or less freedom, pervades transparent and diathermanous bodies; but nerve tissue being neither transparent nor diathermanous, it is not to be conceived that the undulations of this medium are trans¬ mitted along nerve tissue, as if thro ugh glass or rock salt: on the con¬ trary, the vibrations of light and heat are transferred from the medium in question to the axis cylinder of the nerve tubule in a form capable of being propagated to the sensorium. . As I conceive, the analogy lies chiefly in this:—as we know, various solid and liquid bodies exercise a selective absorption both for heat and light, in virtue of whichkertain rays are set apart to be stopped, while certain others are allowed to proceed; after an analogous fashion, certain nerves exercise a so-called selective power, permitting certain
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