VII. _Of the Sense of Extension._

The organ of touch is properly the sense of pressure, but the muscular
fibres themselves constitute the organ of sense, that feels extension. The
sense of pressure is always attended with the ideas of the figure and
solidity of the object, neither of which accompany our perception of
extension. The whole set of muscles, whether they are hollow ones, as the
heart, arteries, and intestines, or longitudinal ones attached to bones,
contract themselves, whenever they are stimulated by forcible elongation;
and it is observable, that the white muscles, which constitute the arterial
system, seem to be excited into contraction from no other kinds of
stimulus, according to the experiments of Haller. And hence the violent
pain in some inflammations, as in the paronychia, obtains immediate relief
by cutting the membrane, that was stretched by the tumour of the subjacent
parts.

Hence the whole muscular system may be considered as one organ of sense,
and the various attitudes of the body, as ideas belonging to this organ, of
many of which we are hourly conscious, while many others, like the
irritative ideas of the other senses, are performed without our attention.

When the muscles of the heart cease to act, the refluent blood again
distends or elongates them; and thus irritated they contract as before. The
same happens to the arterial system, and I suppose to the capillaries,
intestines, and various glands of the body.

When the quantity of urine, or of excrement, distends the bladder, or
rectum, those parts contract, and exclude their contents, and many other
muscles by association act along with them; but if these evacuations are