ty, and may thus be compared to the erection of
the penis, or of the nipples of the breasts of women; while new particles
become added at the same time; as in the process of nutrition above
described.

When only the natural growth of the various parts of the body are produced,
a pleasurable sensation attends it, as in youth, and perhaps in those, who
are in the progress of becoming fat. When an unnatural growth is the
consequence, as in inflammatory diseases, a painful sensation attends the
enlargement of the system.

IV. This apposition of new parts, as the old ones disappear, selected from
the aliment we take, first enlarges and strengthens our bodies for twenty
years, for another twenty years it keeps us in health and vigour, and adds
strength and solidity to the system; and then gradually ceases to nourish
us properly, and for another twenty years we gradually sink into decay, and
finally cease to act, and to exist.

On considering this subject one should have imagined at first view, that it
might have been easier for nature to have supported her progeny for ever in
health and life, than to have perpetually reproduced them by the wonderful
and mysterious process of generation. But it seems our bodies by long habit
cease to obey the stimulus of the aliment, which should support us. After
we have acquired our height and solidity we make no more new parts, and the
system obeys the irritations, sensations, volitions; and associations,
with, less and less energy, till the whole sinks into inaction.

Three causes may conspire to render our nerves less excitable, which have
been already mentioned, 1. If a stimulus be greater than natural, it
produces too great an exertion of the stimulated organ, and in consequence
exhausts the spirit of animation; and the moving organ ceases to act, even
though the stimulus be continued. And though rest will recruit this
exhaustion, yet some degree of permanent injury remains, as is evident
after exposing the eyes long to too strong a light. 2. If excitations
weaker than natural be applied, so as not to excite the organ into action,
(as when small doses of aloe or rhubarb are exhibited,) they may be
gradually increased, without exciting the organ into action; which will
thus acquire a habit of disobedience to the stimulus; thus by increasing
the dose by degrees, great quantities of opium or wine may be taken without
intoxication. See Sect. XII. 3. 1.

3. Another mode, by which life is