contiguous denser æther. For we must suppose, that the vibrations of the rarer æther within the nerves extend themselves a little way into the denser surrounding æther, and even become stronger to a certain distance; after which they become weaker again, and are, at last, quite suppressed by the increase of density in the æther, and by their own diffusion.
Thirdly, It is customary, in endeavouring to feel exquisitely, to rub the ends of the fingers against the tangible object. Now this friction may, by exciting vibrations, and a consequent contraction in certain muscular fibrils belonging to the papillæ, distend and erect these, and thereby increase their sensibility.
Fourthly, There is much here to be ascribed to practice and habit, i.e. to association; and it is chiefly on this account, that the sensations of the ends of the fingers give us so much more precise information concerning the tangible qualities of bodies, than those of the ends of the toes, since the structure of the nervous papillæ is alike in both. It ought to be considered as a part of this reason, that, according to the principles laid down in the last chapter, we may get a voluntary power of erecting the papillæ without friction, or of increasing and fixing the distention during friction, in order to feel with greater exquisiteness and precision.
The sense of feeling may also be distinguished into that of the external surface of the body, and that of the cavities of the nose, mouth, fauces, alimentary duct, pelvis of the kidnies, ureters, bladder of urine, gall-bladder, follicles, and ducts of the glands, &c. The sensibility in the last is much greater than in the first, because the impressions can more easily penetrate through the soft epithelium, with which the internal cavities are invested, than through the hard cuticle, because the compact fibrous membrane of the true skin does not suffer the vibrations to pass freely up the nerve through its own substance, but rather diffuses them along its surface, and because the moisture of the epithelium dissolves, and thereby renders active all the saline particles, which touch the internal cavities. In the mouth and nose this sensibility is so great, and attended with such distinguishing circumstances, as to have the names of taste and smell assigned respectively to the sensations impressed upon the papillæ of these two organs. And as the sensations of the alimentary duct have a near relation to, and connexion with, those of the mouth, I shall refer them to the head of taste. But the sensations of the other internal cavities may be comprehended more properly under feeling.
It ought also to be observed here, that the lips, nipples, and external parts of generation, have a more exquisite sensibility than the other external parts; partly from the structure of their papillæ, and partly from the thinness of the cutis, and softness and thinness of the cuticle. The extreme sensibility of the cornea and tunica conjunctiva of the eye may arise from the manner in which the nerves are here exposed, and the tension of these parts.
