motions, with an increase of vibrations. Pressure may also increase vibrations in the internal parts, contiguous to those where it checks them; or even in the external ones, if it be so great as to occasion any considerable distention there. And thus there may be a variety of vibrations occasioned by the several kinds and degrees of pressure, sufficient to correspond to all the variety of sensations excited thereby.
Muscular contraction most commonly attends and is attended by pressure, as in the common motions of handling and walking, whereby we overcome the vis inertiæ of our own bodies, and of those which we have occasion to move or stop. Hence all the sensations which we receive from the vis inertiæ of matter, must be derived from these two sources of muscular contraction and pressure.
Now it has been observed already, that muscular contraction checks the vibrations in the contracted fibres, and increases them in the neighbouring parts. And it is easy to conceive, that the sensation corresponding to this alteration of vibrations may sometimes fall within the limits of pleasure, sometimes go beyond them. In young animals, also after sleep and rest in all, it is usually pleasant; after much labour, or sprains, and in inflammations, painful; and this, whether the disordered muscle itself, or its antagonist, be contracted. For there must be an increase of vibrations in the disordered muscle both before it can be itself contracted, and also in consequence of the contraction of its antagonist; as has been shewn before.
Numbness, being a diminution of sensibility, ought, according to the doctrine of vibrations, to proceed from such causes, as either indispose the parts for the reception of vivid vibrations, or hinder their free ascent to the brain. Agreeably to this, a compression made upon the nerve, which leads to any part, will occasion a numbness in that part, the nerve below the compression being unfitted thereby to receive vibrations freely, and the nerve above incapable of transmitting freely such as are excited. A compression of a blood-vessel may have a like effect, because it must lessen that heat, and intestine motion, which a free circulation would communicate to the part. The compression usually made upon the skin, when we press a nerve or blood-vessel, will also contribute. And external cold will hasten the effect, when joined to the just-named causes; or produce it alone, if intense, or long continued. In like manner, numbness, from a compression made upon the nerves or blood-vessels, is much favoured by sleep, because the parts are then indisposed both to receive and to transmit vibrations.
