sleep is said to be induced,
because the whole of the brain both within the head and spine becomes
compressed by the retrocession of the fluid within the tumour. But by what
means a compression of the brain induces sleep has not been explained, but
probably by diminishing the secretion of sensorial power, and then the
voluntary motions become suspended previously to the irritative ones, as
occurs in most dying persons.

Another way of procuring sleep mechanically was related to me by Mr.
Brindley, the famous canal engineer, who was brought up to the business of
a mill-wright; he told me, that he had more than once seen the experiment
of a man extending himself across the large stone of a corn-mill, and that
by gradually letting the stone whirl, the man fell asleep, before the stone
had gained its full velocity, and he supposed would have died without pain
by the continuance or increase of the motion. In this case the centrifugal
motion of the head and feet must accumulate the blood in both those
extremities of the body, and thus compress the brain.

Lastly, we should mention the application of cold; which, when in a less
degree, produces watchfulness by the pain it occasions, and the tremulous
convulsions of the subcutaneous muscles; but when it is applied in great
degree, is said to produce sleep. To explain this effect it has been said,
that as the vessels of the skin and extremities become first torpid by the
want of the stimulus of heat, and as thence less blood is circulated
through them, as appears from their paleness, a greater quantity of blood
poured upon the brain produces sleep by its compression of that organ. But
I should rather imagine, that the sensorial power becomes exhausted by the
convulsive actions in consequence of the pain of cold, and of the voluntary
exercise previously used to prevent it, and that the sleep is only the
beginning to die, as the suspension of voluntary power in lingering deaths
precedes for many hours the extinction of the irritative motions.

21. The following are the characteristic circumstances attending perfect
sleep.

1. The power of volition is totally suspended.

2. The trains of ideas caused by sensation proceed with greater facility
and vivacity; but become inconsistent with the usual order of nature. The
muscular motions caused by sensation continue; as those concerned in our
evacuations during infancy, and afterwards in digestion, and in priapismus.

3. The irrita