want of transparency, like water converted into snow; there is reason
to suppose, that a defect of secreted moisture simply may be the cause of
this kind of opacity, as explained in Cataracta, Class I. 2. 2. 13.

M. M. Whatever prevents the inirritability and insensibility of the system,
that is, whatever prevents the approach of old age, will so far counteract
the production of grey hairs, which is a symptom of it. For this purpose in
people, who are not corpulent, and perhaps in those who are so, the warm
bath twice or thrice a week is particularly serviceable. See Sect. XXXIX.
5. 1. on the colours of animals, and Class I. 1. 2. 15.

12. _Callus._ The callous skin on the hands and feet of laborious people is
owing to the extreme vessels coalescing from the perpetual pressure they
are exposed to.

As we advance in life, the finer arteries lose their power of action, and
their sides grow together; hence the paleness of the skins of elderly
people, and the loss of that bloom, which is owing to the numerous fine
arteries, and the transparency of the skin, that encloses them.

M. M. Warm bath. Paring the thick skin with a knife. Smoothing it with a
pumice stone. Cover the part with oiled silk to prevent the evaporation of
the perspirable matter, and thus to keep it moist.

13. _Cataracta_ is an opacity of the crystalline lens of the eye. It is a
disease of light-coloured eyes, as the gutta serena is of dark ones. On
cutting off with scissars the cornea of a calf's eye, and holding it in the
palm of one's hand, so as to gain a proper light, the artery, which
supplies nutriment to the crystalline humour, is easily and beautifully
seen; as it rises from the centre of the optic nerve through the vitreous
humour to the crystalline. It is this point, where the artery enters the
eye through the cineritious part of the optic nerve, (which is in part near
the middle of the nerve,) which is without sensibility to light; as is
shewn by fixing three papers, each of them about half an inch in diameter,
against a wall about a foot distant from each other, about the height of
the eye; and then looking at the middle one, with one eye, and retreating
till you lose sight of one of the external papers. Now as the animal grows
older, the artery becomes less visible, and perhaps carries only a
transparent fluid, and at length in some subjects I suppose ceases to be
pervious; then it follows, that the crystalline lens, losing some fluid,
and