subjected to the same subtle laws. And the exquisite structure of animal bodies in so many other respects, makes it easier to conceive, that the organ of organs, viz. the medullary substance, should be endued with a proper subtle ultimate structure, for the purpose of retaining a state that is frequently impressed. One may guess also, that it is better suited to this purpose during its growth, i.e. in passing from infancy to adult age, than afterwards; as this would be very agreeable to the phænomena.
Fourthly, Suppose now the vibrations A, B, C, D, &c. belonging to each of the senses, to be excited, and repeated in such order and manner as usually happens to the new-born infant upon its entrance into this new scene of things. It is evident, that these will have a greater power to overrule the natural state N, than the vibrations A from one single object could have: for A affected only one region of the medullary substance primarily; whereas A, B, C, D, &c. affect all the regions primarily in their turn. It is evident also, that the secondary vibrations, or those which are propagated from the region of the medullary substance primarily affected into the rest, will be overruled, in great measure, in each region, by the primary vibrations peculiar to that region. Lastly, It is evident, that of the vibrations which are excited in each region, no one can prevail over all the rest, but each must leave an effect, in proportion to its strength and frequency. We may conceive, therefore, that each region of the medullary substance will have a tendency generated in it to vibrate with vibrations of the same frequency (but weaker in degree) as those which the several appropriated objects impress upon it respectively; and that diminutive vibrations resembling them will rise in succession in each region. For each region may easily be conceived to lean sometimes to the vibrations from one object, sometimes to those from another, according to the strength, frequency, and novelty of the impression, the then present disposition of the nervous system, association (of which in the two next propositions), and other such-like causes. And for the same reason, as in every sense the idea of some one object of that sense must prevail over all the rest, we may conclude, that sometimes the ideas belonging to one sense, sometimes those belonging to another, will prevail over the rest.
Or thus: Some vibrations there must always be in the medullary substance, on account of its heat, and the pulsation of the arteries which pass through it. These cannot be the natural ones N, because they will soon be overruled by the great force and variety of impressions made on the new-born infant, which must also dispose each region of the brain to lean to some or other of those vibrations which are excited in it primarily. Hence we may conceive, that a very complex set of vibrations, arising from the mixture and combinations of degree, kind, place, and line of direction, exists always in the medullary substance, being kept up by its heat, and the pulsation of its arteries, when
