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Page:Popular Science Monthly Volume 21.djvu/192

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182
THE POPULAR SCIENCE MONTHLY.

the spinal nerves) by a double root, to be the nerve of ordinary (or voluntary) motion for the muscles of the face generally, as well as of sensation for its sensory surfaces. The analogy of the fifth pair to the spinal nerves (which was no new idea) seemed to him to be further indicated by the existence of a "ganglion" upon its larger root, corresponding with that which is seen on the posterior roots of the spinal nerves. Following up this train of reasoning, he instituted experiments with the view of determining what function the fifth pair had in virtue of its double root, which the seventh pair had not. And as he found that division of the seventh pair, while partially paralyzing the muscles of the face, did not in any perceptible degree impair its sensibility, while section of either of the three divisions of the fifth pair destroys the sensibility of the part of the face it supplies, he came to the conclusion that the sensory endowments of the fifth pair are due to its possession of a double root; a conclusion which he strengthened by the consideration that the third, fourth, and sixth nerves—which, being distributed exclusively to the muscles of the eyeball, can not be supposed to have any but motor endowments—all arise by single roots.

In this way, Bell was led to assign to the two roots of the spinal nerves the same double function which he attributed to the two roots of the fifth pair of nerves of the head; and thence to assign the sensory function to the posterior roots, because, like the second root of the fifth, they bore ganglia before uniting with the motor roots.[1] Now, to say that Bell, by this train of reasoning, discovered the motor and sensory functions of the anterior and posterior roots of the spinal nerves, is utterly preposterous. He had not even truly determined (as the event proved) the true functions of the fifth and seventh nerves of the head. And the extension of his conclusions regarding the double roots of the fifth pair, to the spinal nerves generally, had rather the character of a happy guess than of a logical sequence. No scientific physiologist at the present time would think himself justified in putting forward such an extension as more than a suggestion, to be confirmed or negatived by experimental evidence. And let it not be forgotten, moreover, that it was experiment alone which afforded Bell any reason whatever for attributing a sensory function to the gangliated root of the fifth pair; and that, without this basis, the question of the spinal nerves remained exactly in the condition in which he had taken it up.

It is, indeed, not a little curious that in the two memoirs (1821 and 1822) in which Bell presented to the Royal Society the results of

  1. It is a significant indication of the chaotic ignorance which prevailed on this subject "sixty years since," that, as Bell himself informs us, he found himself met, when first groping at the notion of the sensory endowments of the posterior roots of the spinal nerves, by the current doctrine that the function of the ganglia is "to cut off sensation," i. e., to allow these nerves to minister to the "vital and involuntary motions," without our being made conscious either of those movements or of the impressions which excite them.