place of the stimulus. The manner in which this is effected, I conceive to be as follows:—The repeated or continued action of the stimulus diffuses vibrations from the place of its action over the whole membrane, which, by their reciprocal influences, become equal, or nearly so, in every part of it, and are, at last, so exalted, as to contract every part. As soon as this contraction takes place, the vibrations in its small particles must cease, for reasons given above. They will therefore be propagated almost instantaneously over the neighbouring muscles, from the nervous communications between the membrane and the neighbouring muscles; by which all changes made in the nerves of the membrane must affect those of the neighbouring muscles. As therefore during the vivid vibrations of the particles of the membrane, we must suppose some to be propagated into the neighbouring muscles, agreeably to the first article of this proposition, so upon their sudden cessation, such a change may reasonably be supposed, in the communicating nervous fibrils, as shall agitate the æther contained in them with much more vivid vibrations than before; and these vibrations must now pass into the muscles alone, since the contraction of the membrane hinders them from returning into it. I shall hereafter produce several examples of this process, in detail. It may suffice, at present, just to mention the action of sneezing, and to desire the reader to compare this action, in a cursory way, with the foregoing account.
Fifthly, I have, in the last article, shewn how a cessation of vibrations in the particles of a membrane may increase those in the neighbouring muscles. But it seems also, that a cessation of vibrations in any other considerable part of the body, from whatever cause it proceeds, has a like tendency; and that this tendency is deducible from the change made in the nerves of the part affected, and thence propagated into the communicating branches, or even into the whole medullary substance. The yawnings and stretchings of persons disposed to sleep, the convulsive respiration of those that are just fallen asleep, and the convulsive motions which attend the extinction of the senses in epileptic fits, and the near approaches of death, may be derived, perhaps, in part, from this source, in part from some of the foregoing.
The particular detail of this obscure and intricate matter will be attempted in the proper places of the next chapter, which will contain the application of the general positions concerning sensation and motion, in this, to each of the most remarkable phænomena, considered separately. I will, however, present the reader here with a short sketch, to enable him to form some notion of the manner and plausibility of the attempt.
