less oxygen; and their movements in water are slower than those of
aerial animals from the greater resistance of the element. There
besides seems to be no obstacle to the growth of aquatic animals; as
by means of the air-bladder, they can make their specific gravity the
same as that of the water in which they swim. And the moisture of the
element seems well adapted to counteract the rigidity of their fibres;
and as their exertions in locomotion, and the pressure of some parts
on others, are so much less than in the bodies of land animals.

2. But as all excessive stimuli exhaust the sensorial power, and
render the system less excitable for a time till the quantity of
sensorial power is restored by sleep, or by the diminution or absence
of stimulus; which is seen by the weakness of inebriates for a day at
least after intoxication. And as the frequent repetition of this great
and unnatural stimulus of fermented liquors produces a permanent
debility, or disobedience of the system to the usual and natural kinds
and quantities of stimulus, as occurs in those who have long been
addicted to the ingurgitation of fermented liquors.

And as, secondly, the too great deficiency of the quantity of natural
stimuli, as of food, and warmth, or of fresh air, produces also
diseases; as is often seen in the children of the poor in large towns,
who become scrofulous from want of due nourishment, and from cold,
damp, unairy lodgings.

The great and principal means to prevent the approach of old age and
death, must consist in the due management of the quantity of every
kind of stimulus, but particularly of that from objects external to
the moving organ; which may excite into action too great or too small
a quantity of the sensorial power of irritation, which principally
actuates the vital organs. Whence the use of much wine, or opium, or
spice, or of much salt, by their unnatural stimulus induces consequent
debility, and shortens life, on the one hand, by the exhaustion of
sensorial power; so on the other hand, the want of heat, food, and
fresh air, induces debility from defect of stimulus, and a consequent
accumulation of sensorial power, and a general debility of the system.
Whence arise the pains of cold and hunger, and those which are called
nervous; and which are the cause of hysteric, epileptic, and perhaps
of asthmatic paroxysms, and of the cold fits of fever.

3. Though all excesses of increase and decrease of stimulus should be