I shall first inquire, says Dr. Rennie, what are the effects of climate
on healthy constitutions, as respects heat, cold, moisture, and
vicissitudes; including also the diurnal and annual revolutions.
Cold applied to the body acts as a direct sedative. It diminishes the
nervous sensibility, represses the activity of the circulation, detracts
from the sum of the animal heat, and thereby diminishes stimulation. In
the cessation of excitement and sensibility that ensues, the whole vital
actions are moderated, existing irritation is soothed; and in the same
manner as sleep recruits the wasted powers, so does cold restore and
invigorate the nerves when overstimulated, and in fact promotes the tone
and vigour of the whole body; when again a warmer atmosphere succeeds a
colder, the animal heat increases in its sum, the surface of the body is
re-excited, nervous sensibility returns, and a reaction of the
circulation takes place; so that the blood diffuses itself in greater
abundance towards the remote and superficial parts of the body, and the
secretions are also promoted.
Alternations of cold and heat therefore in healthy constitutions within
certain limits, are salutary; promoting, on the one hand, the vigour and
tone of the body; on the other, the due activity and excitement of the
various functions.
The temperature occasioned by day and night, and also those more
progressive and slow alternations of heat and cold, on the large scale,
attending the annual revolution of the seasons, are a natural provision
admirably adapted to effect these objects as described; constituted as
our bodies are, such a constant and regular succession of heat and cold
is just such as the necessities of the human frame require. The
alternations of day and night, of winter and summer, are far from being
merely incidental and unimportant circumstances in the general
adaptation of the earth to man's constitutional wants; neither do they
bear reference solely to the productions of the earth for his use. They
exert a continual and direct influence on his constitution, calculated
to aid the vigorous and healthy performance of the various functions of
the body each in its due degree and order, and they conduce mainly to
the perfection and longevity of the species.
Let us therefore trace the effects of these changes on the human body.
During the winter, the prevailing cold acts as a universal sedative and
tonic, soothing the nervous excitement and sensibility, allaying the
activity of the circulation, moderating the functions of the skin, and
diminishing the various secretions.
As the Spring opens, the sun gains daily in influence, generating a
gradually increasing atmospheric warmth. The body therefore becomes
subject from this heat to a reactive effect, during which the nervous
sensibility and circulation are gradually re-excited, the blood is more
equally diffused towards the surface and extremities of the body, and
the secretion by the skin is increased.
If the cold of winter were to continue unmitigated from year to year,
without the genial influence of summer, the human race, as is apparent
in polar regions and upland mountainous districts, would degenerate into
dwarfishness.
If the heat of summer were continually maintained the whole year round,
a tendency to degeneracy of the race would be also observed, as we see
in tropical latitudes. It is in the medium betwixt these extremes, where
a moderate and regular winter cold is succeeded by a mild, genial summer
temperature, that the species approaches most to perfection in stature,
health, strength, and longevity.
In observing also the influence of day and night on the constitution,
there is a sedative effect produced in the morning before the sun is up,
a reactive tendency promoted towards noon under the solar influence, and
again towards evening this reaction is repressed by the sedative effect
of the evening cold; and this sedative effect is at its maximum at
midnight. Hence those who sit up late feel unusually chilly and
depressed towards midnight, partly owing to exhaustion from want of
sleep, but chiefly from the total absence of solar influence in the
atmospherical temperature. In regular habits this sedative effect is
never thoroughly experienced; for before midnight, the constitution,
enveloped in warm blankets, has experienced the reaction arising from
the accumulation of heat in bed. Whence the common remark, that one
hour's sleep before midnight is worth three after that hour, is actually
true to a certain extent. By early retirement to rest, the sedative
effect on the constitution, to an extent such as to disturb the
functions, is escaped.
If we connect these two influences, the annual and diurnal successions
of cold and heat, in their joint effect, we find, that about, or a
little after the summer solstice, the influence of the sun being at its
maximum, the nervous sensibility, heat, circulating excitement, and
cutaneous secretions of the body, are also at their maximum. The
temperature of the day and night differ so little, that the sedative
effects of evening and morning are not sufficient to restore the frame
by soothing the sensibilities, overexcited and irritable from the
previous warmth. Whence the languor and irritability felt in summer,
when the heat is long continued, and the nights are spent in
restlessness and anxious oppression. Exhaustion and relaxation of the
frame are the consequence.
As the autumnal equinox verges on, the mornings and evenings get cooler
in relation to the mid-day heat; and about the equinox, the difference
in the temperature of mid-day and midnight is at its maximum. We have
therefore a powerful sedative effect in the morning, which braces and
invigorates the body; a powerful reactive effect at mid-day, which
rouses and stimulates the actions and sensibilities of the frame; and
again towards evening a sedative effect, from the increasing cold
reaching its maximum at midnight.
As the season passes on from the Equinox towards the winter solstice,
the heat of the sun daily diminishes, and the cold gains a daily
preponderance. The sedative effect on the body goes on progressively
increasing, being less and less counteracted by any genial influence
from the solar heat at mid-day; whence the gloom and depression so
universally experienced by the nervous in November and December, which
is more and more felt till the shortest day. So soon as the minimum of
solar influence and maximum of sedative effect on the body has passed
over, the sun gradually acquires more of meridian influence, and a daily
increasing ascendancy over the prevalent cold. The human constitution at
the same time is subject to a proportionate reactive disposition; which
reaction is felt most at noon, and it daily becomes more and more
apparent till the vernal equinox, when we have the difference betwixt
the meridian and midnight temperature again at a maximum. We have daily
a powerful sedative effect in the morning, a powerful meridian reaction,
which again subsides into a sedative condition on the access of the
evening. This daily effect on the constitution is exactly similar to
that at the autumnal equinox, only it occurs under different
circumstances. In autumn it is connected with departing heat and
progressively increasing cold; in Spring it is connected with
progressively diminishing cold and advancing heat. After the vernal
equinox, the difference in the meridian and midnight temperature
gradually diminishes; the daily sedative effect at morning and evening
becomes less and less apparent as general atmospheric warmth prevails,
till towards the summer solstice, the general effect on the constitution
is stimulation and excitement by atmospheric heat.
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