gradually undermined, is when irritative
motions continue to be produced in consequence of stimulus, but are not
succeeded by sensation; hence the stimulus of contagious matter is not
capable of producing fever a second time, because it is not succeeded by
sensation. See Sect. XII. 3. 6. And hence, owing to the want of the general
pleasurable sensation, which ought to attend digestion and glandular
secretion, an irksomeness of life ensues; and, where this is in greater
excess, the melancholy of old age occurs, with torpor or debility.

From hence I conclude, that it is probable that the fibrillae, or moving
filaments at the extremities of the nerves of sense, and the fibres which
constitute the muscles (which are perhaps the only parts of the system that
are endued with contractile life) are not changed, as we advance in years,
like the other parts of the body; but only enlarged or elongated with our
growth; and in consequence they become less and less excitable into action.
Whence, instead of gradually changing the old animal, the generation of a
totally new one becomes necessary with undiminished excitability; which
many years will continue to acquire new parts, or new solidity, and then
losing its excitability in time, perish like its parent.

V. From this idea the art of preserving long health and life may be
deduced; which must consist in using no greater stimulus, whether of the
quantity or kind of our food and drink, or of external circumstances, such
as heat, and exercise, and wakefulness, than is sufficient to preserve us
in vigour; and gradually, as we grow old to increase the stimulus of our
aliment, as the irritability of our system increases.

The debilitating effects ascribed by the poet MARTIAL to the excessive use
of warm bathing in Italy, may with equal propriety be applied to the warm
rooms of England; which, with the general excessive stimulus of spirituous
or fermented liquors, and in some instances of immoderate venery,
contribute to shorten our lives.

_Balnea, vina, venus, corrumpunt corpora nostra_,
_At faciunt vitam balnea, vina, venus!_

Wine, women, warmth, against our lives combine;
But what is life without warmth, women, wine!

* * * * *

SECT. XXXVIII.

OF THE OXYGENATION OF THE BLOOD IN THE LUNGS, AND IN THE PLACENTA.

I. _Blood absorbs oxygene from the air, whence phosphoric acid changes
its colour, gives out heat, and some phlogist