the fingers, are in general more subject to irritation, itching, and inflammation, and endued with a greater degree of sensibility, than the other parts. Now this phænomenon agrees with the doctrine of vibrations, inasmuch as such parts must, according to the Newtonian hypothesis, be surrounded with an æther of a greater density than that within their pores, and which also grows denser and denser, in a regular manner. For one may conceive, that the vibrations communicated to this denser æther will be stronger in proportion to its density; and consequently, that they will agitate the small particles of the extreme parts also with vibrations stronger than ordinary.
It is true, indeed, that the sensibility of each part does depend, in great measure, on the number, structure, and disposition, of the nervous papillae, which are the immediate organ in the senses of feeling, taste, and smell; but then we may remark, that the same observation holds in respect of these nervous papillae. For they are also extreme and pointed parts; and that especially, if we suppose, which seems probable, that when any part is in a state of exquisite sensibility, the nervous papillae are erected, (in some such manner as the hairs of the neck and back in certain animals, when enraged,) so as to recede from each other, and, consequently, to admit the denser æther between them. They may also, upon the same occasions, be made turgid, by the constriction of their bases, and thus have their sensibility or power of receiving vibrations, increased by distention.
We may remark likewise, in pursuing this method of reasoning, that the æther which lies contiguous to the medullary substance in the ventricles of the brain, is denser than that which lies in the medullary substance itself. May we not therefore conjecture, that one use of the cavities of the ventricles is to increase and keep up all the vibrations propagated from the external nerves into the medullary substance of the brain, by means of the denser æther lodged in those cavities; that blood and serum extravasated, and lying in the ventricles, suffocate sensations, by excluding this denser æther, as well as by pressing on the medullary substance; and lastly, that those brutes whose olfactory nerves have cavities within them continued from the ventricles, are more acute than men, in perceiving odours, and distinguishing them from each other, in part, upon this account?
Boerhaave is, indeed, of opinion, that the opposite sides of the ventricle always touch each other, so as to leave no cavity. But it seems more reasonable to suppose, that a subtle vapour, which is exhaled from the vessels of the investing membrane, and whose particles, like the vapour of water, have a repulsive power, in respect of each other, prevents the absolute mutual contact of the opposite sides, in common cases. And the same thing is favoured by the experiments tried upon the Parisian beggar. Since the brain in him could be somewhat pressed in, it seems that the skull was not entirely full before.
