itching of the nose and anus may be expected to attend worms in the stomach or intestines, and the itching of the glans penis and anus a slight inflammation at the neck of the bladder from a stone. These instances shew, that vibrations run freely along the surfaces of uniform membranes; and this is farther confirmed in the last case, by the check which a pressure made in perinæo, or anywhere upon the urethra, will give to the itching in the glans.
Titillation is nearly related to itching. It is excited by slight impressions upon the more sensible parts, frequently renewed; and this may show, that it arises from an increase of vibrations. The impressions must be so slight as not to excite a contraction in the neighbouring muscular fibrils, and also frequently renewed, that the increase of vibrations may diffuse itself farther and farther perpetually; and thus the whole nervous system may, in some cases, be at last put into a pleasurable state, approaching to the limits of pain, and passing within them at times.
Hence it appears, that as soon as children have learnt to cry, or yield a sound, from nascent pains, and from pleasures just passing into pains, titillation may excite short, alternate, nascent cries in them, i.e. laughter, but not before. If the impressions be made upon the chest, the effect will be quicker and stronger, because these impressions have a direct influence upon the muscles concerned in laughter.
If a feather be passed slightly to and fro between the lips, it will excite a titillation there, which will diffuse itself into the neighbouring parts of the upper lip and chin, and pass into an itching in them. The original titillation between the lips may be allayed by pressing them together, and the consequent itching by pressure and friction, as in other cases. All these things suit with the hypothesis of vibrations, and of their free diffusion.
In like manner, the free diffusion of vibrations, and their influence in contracting the neighbouring muscles, may be inferred from the vomitings, which are excited by tickling the fauces with a feather.
As friction and titillation agree with heat in increasing vibrations, so pressure agrees with cold in checking them. Thus pressure abates the uneasiness from itching, as mentioned above; and binding, or otherwise gently compressing parts in pain, or inflamed, i.e. parts in which the vibrations are excessive, will, for the most part, afford relief; whereas friction would increase the pain to a great degree. But the pain usually becomes more violent the instant the compression is removed. For the mere elasticity of the parts alters their figure, i.e. produces internal
