of experimental inquiry, and the young recently hatched exhibited
a _lower_ temperature than the grown birds. After removal from the
shelter of their nests into a mild atmosphere of 17°, in one hour
they cooled down from 36° to 19°, thus losing 17° in an hour. At an
elevation of 22° the same results were obtained, and they cooled down
to within one degree of the surrounding air. The plumage of birds
has little if any influence upon their temperature. The production
of heat lies within, and not on the surface of the animal; and if
it be strongly developed, the removal of natural coverings does not
influence the heat produced; and if it be weak, their addition will
not prevent cooling. Birds recently escaped from the shell cooled
to within two degrees of the air, whereas the unplumed adult birds
scarcely lost one degree.
The distinctive character of warm-blooded animals to preserve an
uniformity of heat has no reference to _bulk_. The _eagle_ maintains
the same temperature as the _wren_ or the _tom-tit_, taking them at
the same age, and placing them under the same circumstances; but if
cooling measures be adopted, the lesser body parts with its heat
faster than the larger, though ultimately they arrive at the same
point. The dimensions of animals are infinitely varied; but the giant
reaches no higher standard than the dwarf, nor sinks to a lower
temperature.
In estimating the temperature of young animals, it must be taken
into account that they are born at different periods of organic
developement. Some come earlier into the world than others, and
some are more perfectly formed than others at their birth, and more
capable of helping themselves. This variation produces a different
standard of heat after birth, and especially creates a variety of
temperature among birds when tested at the same epochs of their
existence. The season in which animals are produced also modifies
their temperature.
The influence of age in modifying temperature is common both to
mammiferæ and birds. Young and healthy sucking pigs cooled faster
than their parent, their generating means of heat being more feeble.
Animals of warm blood possess [p300] the power of supplying heat at
its _maximum_ when first born; they then part with it by degrees,
and, as they advance in age, their heat becomes gradually augmented
again till it reaches the adult standard.
Dr. Edwards next proceeds to discuss the phenomena of animal
temperature more exclusively regarding adults, and especially among
those singular creatures of the mammiferæ which form an exception to
the general law of nature respecting the uniformity of temperature
as to warm-blooded animals. These beings are what are termed
_hybernants_, such as the _dormouse_, the _hedgehog_, the _bat_, the
_marmot_, &c., natives of Europe; which remain dormant during winter
without any external signs of life and motion. The change which
these undergo reduces them from the state of warm-blooded animals to
that of cold. Unlike the rest of their class, the autumnal season
lowers their temperature by degrees, till in winter it reaches so
low as to be scarcely higher than the surrounding air. Their powers
fail gradually, and their losses of heat are not repaired, till at
length their respirations become slow and feeble, and the heart
languidly urges the blood through the arteries. In this state there
is an imperfect aërification of the blood, and a partial state of
asphyxia, producing continued repose of the nervous and muscular
system. But the temperature of these animals sinks no lower than
the air, and remains sufficient to maintain a passive existence,
till the returning spring raises their heat again, and they become
lively and active till autumn; but even in spring these animals are
characterised by producing less heat than others of their class.
If we seek to know the cause of this curious variety, we can only
refer it to peculiarity of constitution, which is instituted by
nature as adapted to animals placed in situations of rigorous cold,
and where they cannot procure sustenance but in spring and summer.
Our author imitated the process of hybernation by artificial cold,
and produced the same effects; and when he restored animation by
gradual warmth, he found the animals as lively as before.
John Hunter and others have written on the natural history of
hybernants, and Dr. Edwards regards only their temperature. The
experiments on hybernants by artificial cold prove this fact, that
hybernation is attributable to other causes than to the reduction and
deprivation of nutriment; for the animals submitted to the ordeal
of cold were well [p301] fed, and in the lively season of advanced
spring. The deprivation of food seems to be a local consequence
provided for by the phenomenon of hybernation, and not its exciting
cause. Nor does there appear to be any change of organization in
these cases, but a state of constitution exists which we are unable
to account for further.
We have, in the next place, a series of experiments showing the
influence of the _seasons_ upon animal temperature with the
warm-blooded; by which it seems that they produce a variety of
results: and it is demonstrated that animals of warm blood in general
undergo some constitutional changes with the periodical returns of
the seasons. When, for example, the highest degree of temperature is
attained, animals no longer produce heat; so that their temperature
continues below that of the air in the hot season. And, in the cold
season, if the cold be not too rigorous, the animal’s age offers
a proportionate resistance to the cooling effects of the air as
the approach to maturity is attained. An elevated and a depressed
temperature thus produce contrary effects upon the internal powers
of generating animal heat, a high temperature arresting them, and a
low one promoting them. _Thus we cannot fail to observe the beautiful
adaptation of means to final causes_.
Upon the subject of _asphyxia_ in warm-blooded animals, Dr. Edwards
found a great dependence between animal heat and the faculty of
living without contact with the air, a state in which the blood is
not aërated by respiration, and which is sustained by hybernants
while in the dormant condition. Having submersed animals in water
of various temperatures successively, so as to bring them under the
influence of variable temperature, he found the _descending_ scale
of temperature the most hurtful. The _ascending_ heat was that which
prolonged life most. Between 20° and 10° the results were similar to
those between 20° and 40°.
Animals, then, of warm blood in a state of asphyxia hold their
existence on two principal conditions relative to heat; one regarding
the different measures by which some develope their heat, and the
other the degree of external temperature. The first is proper to
animals naturally, the second fortuitous.
Upon the _respiration_ of both young and adult animals the author
arrives at a conclusion opposite to that of common opinion, which is
founded on the notion of the heat in young animals being higher than
that of the matured. Finding, [p302] however, as already noticed,
that the parent exceeds the temperature of its offspring after
birth, it is naturally concluded that its consumption of air is also
greatest. This was experimentally confirmed, and is in unison with
other facts. In the first part of this work the vertebratæ of cold
blood were also found to consume least air in proportion to their
diminution of temperature. Temperature seems to act uniformly with
all the vertebratæ, and their consumption of air is in proportion.
The _mammiferæ_ have a lower temperature than _birds_, and they
consume less air than the latter. Fish and reptiles consume less air
than the warm-blooded, and possess a lower temperature.
The influence of the _seasons_ upon _respiration_ is considered
in the sixth chapter. Many changes occur in the atmosphere during
the revolutions of the seasons, varieties in the temperature, and
the pressure and density of the air. Dr. Edwards shows that the
faculty of producing heat with warm-blooded animals is greater in
_winter_ than in _summer_, the constitution of animals being adapted
to their individual climates; and in reference to the relation of
this faculty to the consumption of air, it is presumable, all other
circumstances being alike, that the consumption ought to be increased
with the faculty of developing heat, and the experiments justify the
presumption.
Upon the subject of _transpiration_, it is shown that the air not
only exercises a vivifying effect upon the constitution, but one
little less important in removing a vaporous substance from the
surface of the body, and which is separated from the fluids before
its conversion into vapour, and known by the name of perspiration
or sweat, which transpires from the skin. The variations in the
temperature of the air possess great influence over this function.
Experiments on this subject were detailed most fully in our last
Number, relative to cold-blooded animals; and therefore these need
not now be repeated in respect to the warm-blooded, for the results
are exactly similar, as to transpiration in equal and successive
periods, the comparative influence of dry and moist states of the
air, and the effects of air in motion and in repose. Inspection of
the table annexed to the work displays the similarity of the effects
produced by the same physical agents upon cold and warm blooded
animals, and this accordance serves to afford mutual support to the
different investigations.
We are now arrived at the fourth and last part of this [p303] work.
Much, however, of this part appertains to what has been already
detailed upon other animals. But the modifications of heat in the
human being, from the period of birth to maturity, will be found
highly interesting. They accord precisely with the results obtained
among the lower animals and mammiferæ; and present analogical proofs
of the general application of principles laid down in the preceding
portions of our notices.
While, however, we trace analogy throughout the animal kingdom, it
must be remembered that there are infinite sources of variation
arising from the extensive variety of species modifying those
principles, which are governed by a general harmony of effect. Of
all animals, man exhibits this variety the most, possessing, as
he does, attributes above all the groups of his class, from his
intellectual properties, speech, &c., rendering his race unique and
superior to all others. Our curiosity cannot, therefore, be allowed
to rest satisfied with the general application of principles,
until we have observed their modifications in the human being as
well as in brutes. It is highly interesting to inquire into the
conditions of human phenomena, and examine the forces which man
opposes in his intelligent character to the physical agents around
him. He is equally liable to their influence, exists by their
contact, and yields, like other members of the animal kingdom, to
their destructive tendency. The essential distinctions appertaining
to his economy are thus the more necessary to be understood. His
_organization_ affords him no shelter from the operations of physical
laws beyond that of brutes; but the superiority of his nature may
be supposed to modify their influence from causes referrible to his
_sensibility_. These have formed the subject of Dr. Edwards’s inquiry.
Man’s state and condition, at his birth, place him in very different
circumstances from those at which he subsequently arrives. Here,
therefore, we see an extensive field of inquiry; and it is suggested
whether, in the infantile state, man generates _less heat_ than in
more matured existence. Dr. Edwards has shown that the young of
mammiferæ generally, being born at the period when their eyes are
open, produce _less_ heat than adults. It is, therefore, presumable
that the generating powers of heat differ in the two states of
existence which man goes through, the infantile and mature.
But the power of producing heat differs among adult animals, and it
is desirable to know the limits of this faculty, [p304] Moreover,
this power differs in different parts of the body; so that, when
experiments are made, we should always apply the thermometer to
the same part of the body. Among twenty adult persons, Dr. Edwards
found the average temperature 36°.12: in infants from a few hours
to two days old, 34°.75 was the average. Thus we perceive that the
temperature of human infants is _inferior_ to that of adults. In
infants born previous to the usual period, two or three hours after
birth their heat was at 32° of Reaumur’s scale. So far we perceive a
similarity in man to the mammiferæ in general.
We have next a chapter on the effects of _cold_ upon mortality at
different ages. It is highly interesting to observe the care of
animals towards their offspring, in protecting them against the
effects of cold instinctively at a period before their own powers of
generating heat enable them to resist its baneful tendency.
Dr. Edwards endeavours to investigate the subject of cold, so as to
discover its limit of action. He examined the young of mammiferæ and
birds, the former born with closed eyes, and the latter unfledged.
He exposed them separately and apart to the air, so as to be
independent of each other’s warmth, and they exhibited a temperature
below their natural standard at the period of birth, even when a
degree of artificial heat was applied beyond that of adult birds.
The final result of these experiments was, that the application of
heat may be conducive to their developement, but is not indispensable
to their preservation. The author discovered, that the diminution
of temperature is not equally injurious at all ages. The _younger_
the animal, the _less_ is the injury sustained by cold, because the
faculty of producing heat is less powerful with the young than with
the matured animal, the power increasing as the animal grows, and
also with the increase of cold.
Still, however, this subject is open to inquiry, for the great
variety of species, and other circumstances belonging to the animal
creation, so modify the phenomena as to create an almost endless
field of investigation. When warm-blooded animals are exposed by
their parents to the atmospheric influence at an early age, they
are better provided against the perils of cold, being born with an
abundant source of heat. But, if the cold exceeds their powers of
generating heat, the mortality is so much readier. Hence arises
the danger of animals being born in the winter season. [p305] Two
circumstances are distinguishable, the refrigeration of the body,
and the temperature it is capable of sustaining. The cooling is so
much less injurious with the young. If two young animals of the same
species be cooled down equally, the youngest suffers the least. But,
in order to lower to the same number of degrees the temperature of
bodies of different ages, the external heat should be lowered in
proportion to the advancement of the animal towards maturity, in
order to compensate for the difference which the modification of age
produces.
While it is true that the younger animals suffer least from cold,
it is, at the same time, to be considered that they cool down more
rapidly. On this principle depends the mortality of our domestic
fowls and other animals, the management of which requires so much
observation and experience in order to rear them. In regions where
the temperature is liable to great alterations in the course of the
year, man and other vertebrated animals of warm blood are liable
to suffer in their health; for, though _cold_ should produce the
resistance derived from the necessary constitutional developement of
heat, this increase of caloric, having its limits, often exposes the
constitution to the effects of too great reduction of temperature, as
is exemplified in the frozen regions of the North Pole, in Siberia,
and in Russia.
The young of mammiferæ, in general, were found by Dr. Edwards to
differ very materially in the duration of their lives, in a state
of asphyxia, often being limited to from five to eleven minutes,
according to their developement at birth, the most advanced in
organization living the longest period. The author proved these facts
by placing animals in a state of asphyxia under water; and it is
remarkable that, in all his experiments, the _voluntary_ motions were
always first destroyed, the _involuntary_ outliving them. With dogs,
cats, and rabbits, sensibility existed only three or four minutes. A
puppy showed _automatic_ signs of life nearly half an hour. The best
divers appear to be able to remain under water from three to four
minutes.
When animals are entirely deprived of aërial contact, it may be
inquired, what are the principal functions exercised? When the air
circulates through the lungs, it imparts to the blood a peculiar
quality, by which its colour becomes changed. Deprived of this
influence from the air, the blood acquires a dark colour, and
the _nervous function_ is [p306] simultaneously affected. Among
_reptiles_, Dr. Edwards found that life could be maintained by this
dark blood; but it is questionable whether the circulation of venous
or dark blood can promote life in animals of the warm-blooded kind.
Temperature certainly modifies their capability of existence. Under
20°, they live longest; at 0°, their existence is shortest. The
vitality of the nervous system seems to be thus directly influenced
by temperature.
Of all the phenomena of animal life, those relative to the blood’s
state in asphyxia are, perhaps, the most interesting and curious,
from loss of consciousness, sensation, and voluntary motion attending
its disoxygenated state. If, however, animals differ so materially
under the influence of a deprivation of air, as to the duration of
such existence, we may imagine a corresponding difference relative to
their respirations modified by species, age, &c. Air, the _pabulum
vitæ_, is not equally consumed by all, but in different proportions;
at least, such is the presumption from the experiments upon animals
of warm blood. The relative proportions of this difference are sought
to be ascertained. Warm-blooded animals of equal size and age, at
their liveliest period of age, were the objects of comparative
inquiry. We must refer the reader to the table at the end of the
work for the results. A marked difference is observable between
the quantity of air consumed by the cold-blooded animals and that
required for the support of the warm-blooded; and each has an
organization appropriated to the individual distinctions. Thus the
structure of the reptile and fish entails the lesser consumption of
air, compared with that of the mammiferæ and birds. Fish consume
least air, reptiles stand next, then the mammiferæ, and, lastly,
birds consume most. The two last, however, very nearly approach
each other; so do also the two first; and the distinction between
the organization and the consumption of air is most strongly marked
between the fish and reptiles on the one hand, and the mammiferæ and
birds on the other, which, indeed, has caused their separation into
two distinct groups, by the appellation of _cold_ and _warm blooded
animals_,—a distinction which clearly separates the whole of the
vertebrated animals into two groups, bearing different physiological
characters in their relations to animal heat and respiration.
The mere temperature of the blood in each group is insufficient for
our knowledge of their distinctive characters. We [p307] further
find them characterised by a consumption of air in union with their
heat, so as to unite these two functions, and thus render them
dependent upon the same organs. Dr. Edwards has further shown, that
from birth to maturity the production of heat goes on increasing
with the consumption of air. And thus age (as well as the seasons)
has been shown to be a modifier of animal heat; for, as the hot
season advances, the consumption of air becomes diminished, and when
the cold sets in, it increases; and this decrease and increase are
accompanied by corresponding developements of heat.
In cases of fainting, of hysteric and asthmatic fits, the principle
here laid down, as to the balance between the air consumed and heat,
is instinctively acted upon by the most ignorant persons, who open
all the doors and windows to admit cold air, and dash cold water in
the patient’s face. The addition or continuance of heat increases the
affection. The application of cold produces instant relief. The state
of asphyxia is relieved, the senses return, the pulse beats at the
wrist, and the respiration goes on naturally. The _cooling_ renders
the air, unfit before, fitted for the purposes of life.
The effects of temperature upon the respiratory movements are
indicated also in those constitutional changes which diminish the
production of heat and the consumption of air. Organic affection
of the heart or lungs may produce this change, which entails the
necessity of a change of climate, or an alteration of temperature
artificially, to restore the balance between the air and the animal
heat.
A very elaborate and complete argument, and series of experiments,
are devoted to the subject of _transpiration_, and the effect upon it
of the influence derived from repose of the body and sleep, by the
air’s motion or stillness, and by the pressure of the atmosphere.
We have, however, pursued the interesting points touched upon
so far as to render it impossible to enter at present upon this
portion of the work. The importance of the subject demands a fuller
investigation and report than we have now room for; and we must,
therefore, defer it to another opportunity. [p308]
_Experiments on_ THOUGHT. By a Correspondent.
There is a very common prejudice respecting the rapidity of thought,
which is imagined by many to be almost unlimited: and the opinion
is very worthily illustrated by a reference to the oriental tale of
a man’s being bewitched into the belief that he had passed through
a period of seven years duration, and full of the most striking
vicissitudes; all in the time that he employed in dipping his head
in a pail of water. Now there is no doubt that we often dream of a
period of many years while we are only sleeping an hour; that is, we
dream of an impression of a long continued existence, or perhaps of
some detached fact scattered through such a period: but if any person
will write down all that he can possibly recollect, of the separate
imaginations that have passed through his mind in the dream, he will
find that he will be able to read them over with ease in less than
five minutes.
It is probable that there may be considerable diversity in the
rapidity of thought in different persons, as there is in that of
muscular motions: but there is no reason to think the diversity
greater. A healthy young man can run a mile in five minutes: a good
pedestrian in four; but no man ever ran a mile in three minutes;
and perhaps no horse in two. There is reason to think the rapidity
of thought does not differ more materially than this in different
individuals.
The rapidity of thought seems, however, more intimately connected
with that of muscular motion than by analogy only: for they appear in
some cases to be absolutely identical.
I have often been able to count ten in a second, in audible English
words; not distinctly, indeed, but so as to assure myself that I do
hear the ten words in their proper order; and to repeat the sounds
for several consecutive seconds. If I say the words _to myself_ only,
that is, if I think them over, I cannot repeat them ten times in less
than about nine seconds: I can never, for example, keep pace with my
pulse, though it sometimes beats as slowly as seventy in a minute:
nor can I, by any effort, think over the numbers from one to twenty
in two seconds.