of a long series of experiments, instituted for the purpose of
ascertaining the influence of the physical agents upon animal life.
These agents comprehend the atmospheric air, water, and temperature;
the two first constituting the media in which all animals exist, and
the last influencing in common the inhabitants of both media. It
is true, this is a subject by no means new, for it has engaged the
attention of experimenters from the earliest days of science. But
Dr. Edwards has diligently and patiently sought to investigate the
subject himself, to correct previous errors, and to embody the facts
which he has accumulated into a more complete and regular system than
heretofore adopted. In this attempt he has been eminently successful,
and has effected more perhaps than all who preceded him, availing
himself, nevertheless, of the experience of former inquiries.
The extent of his book, and the number of the experiments [p139]
are indeed somewhat appalling, but his clear and distinct method of
arrangement greatly facilitates the reader’s endeavours to master the
extensive subjects of his pages. As a book of reference it should
find a place in the library of every scientific society, and no
individual devoted to philosophy should omit the possession of it.
The agency of the air around us, water, and heat and cold, have
often been the objects of _chemical_ inquiry, from their known
great influence upon the animal economy. The changes effected by
the phenomena of animal life upon these agents have been accurately
examined, and partly reduced to a mathematical precision of
calculation.
Spallanzani and others have viewed the subject as it regards
physiology, but with such results as left the field open to
subsequent investigation. Dr. Edwards seems to have seized upon the
deficiencies of his predecessors, and, by going over their ground,
and extending his own inquiries, he has arrived at most interesting
and important results. These he has divided into four parts, as they
relate to the different orders of the animal creation. The first
part includes some of the lower animals, particularly tenacious of
life, and of cold blood, such as frogs, toads, and salamanders. The
second part is devoted to other animals of cold blood, and of the
vertebrated order, as fish, and those reptiles which include lizards,
snakes, and turtles. The third part refers to warm-blooded animals;
and the fourth part of the work is dedicated to the influence of the
physical agents upon the human race and vertebrated animals. To these
the author has added the discoveries of modern times, relative to
electricity on the animal economy, in an Appendix. A collection of
tables is appended to the work, exhibiting the principal series of
his experiments, as they regard the relative influence of physical
agents on the duration of life, and the phenomena resulting from
their mutual action.
The great importance of the four grand divisions of the work forbids
our hastily reviewing them, and we will endeavour to condense so much
of the information they contain as may forward the objects of our
analysis. Dr. Edwards thus announces the arrangement of his work:—
“Ces recherches auront donc rapport à l’air dans les conditions de
quantité, de mouvement et de repos, de densité et de raréfaction;
à l’eau liquide et à la vapeur aqueuse; à la température, dans ses
modifications de degré et de durée; à la lumière et à l’électricité.
Ces causes agissent à la fois sur l’économie animale, ordinairement
d’une manière sourde et imperceptible; et toujours [p140]
l’impression qu’on reçoit est le résultat de toutes ces actions
combinées.”
“Lors même que, par l’intensité de l’une d’elles, il nous arrive de
distinguer la cause qui nous affecte, l’observation de l’effet se
borne le plus souvent à la sensation, et les autres changemens qui
l’accompagnent nous échappent. On conçoit par la que l’observation
la plus attentive des phénomènes tels que la nature nous les
présente, ne saurait démêler dans cette combinaison d’actions
l’effet propre à chaque cause, ni reconnaître des effets qui ne
seraient pas révélés par la sensation.
“Il est une méthode qui règle les conditions extérieures, qui fait
varier celle dont on veut apprécier l’action, et qui fait juger,
par la correspondance entre ce changement et celui qui survient
dans l’économie, du rapport de cause et d’effet: c’est la méthode
expérimentale; c’est celle que j’ai suivie. Pour en tirer parti il
fallait, d’une part, déterminer l’intensité de la cause, d’autre
part celle de l’effet. La physique nous fournit ordinairement les
moyens de remplir la première indication.”
In the true spirit of philosophical investigation, Dr. Edwards, in
the first place, proceeds to examine the action of physical agents
upon the simplest forms, and least elaborately developed organised
beings, extending his inquiries upwards, in the scale of the animal
world, to man, the most perfect creature, and the ultimate object of
all physiological researches.
The peculiarity of constitution belonging to cold-blooded reptiles,
among which there is so little mutual dependance of organs, renders
these the best tests of the relative and proportionate influence
of the different agents, the intense action of which is liable to
destroy the more perfect animals; and the great development of the
nervous system in the higher orders gives them a wider and more
acute range of sensibility. It is difficult, at all times, and often
impossible, to insulate corporeal functions among the warm-blooded
classes, so as to ascertain the amount and limits of physical agency.
The four classes of vertebrated animals, or such as are furnished
with true spines, afford ample means of comparative illustrations;
and these departments have engaged the author’s attention, in order
to display the result of the action of the same agent exercising a
uniform influence upon constitutions very differently constructed.
The _air_, for example, exercises its influence uniformly upon he
four mentioned classes of vertebratæ, and their different families
are similarly exposed to the action of the atmosphere by respiration.
Curious and interesting as is this subject, it is singular [p141]
that, while it was among the first to be noticed, it has been
the latest in producing satisfactory results. Among the opposing
causes of the advancement of knowledge in this department, the
ignorance of our ancestors in _chemical_ science seems to be the
principal. Without chemical aid it is perfectly useless to attempt
the investigation. The composition of the air respired must be well
understood; the different gases must be carefully examined, or the
physiological inquiry will be darkened and obscured.
Dr. Priestley laid the foundation of our chemical knowledge of
gases in their relation to respiration; but some time elapsed
before it was understood in what manner the air was connected with
animal organisation. Oxygen gas, one of the known constituents of
atmospheric air, was Priestley’s discovery, in its effect upon the
blood, of converting this fluid from a dark purple to a bright
crimson. Lavoisier founded a chemical theory upon this discovery
of the agency of air, which was subsequently applied by Goodwin to
physiology. The latter author demonstrated, by a series of excellent
and correct experiments, that the exclusion of atmospheric air
produces death in animals, in consequence of the dark-coloured blood
usually circulating in the veins being prevented from becoming
crimsoned. The state in which any animal may be thus placed, is known
by the term ASPHYXY, and by which is to be understood a deficient
or suspended aërification of the blood, from whatever cause it may
proceed that the atmospheric air is prevented from access to the
blood as it circulates through the lungs.
The great French anatomist, Bichat, pursued this subject still
farther, and published a treatise on Asphyxy. He sought, by numerous
experiments, to determine the threefold relation of the air to the
nervous system, respiration, and the circulation; and he arrived at
this great and important conclusion, that the VENOUS OR DARK BLOOD
CIRCULATING THROUGH THE BRAIN, CREATES A CESSATION OF THE FUNCTIONS
OF THAT ORGAN, AND THAT IN CONSEQUENCE THE HEART LOSES ITS ACTION.
This discovery shows us at once the direct cause of asphyxy in all
its different degrees, according, in effect, to the vitiated state of
the blood from its deficient or suspended aërification.
Le Gallois also investigated the subject of asphyxy; and he found
that, when _dark blood circulated through the spinal marrow, the
motions of the heart ceased_; and thus he not only determined the
relations of the nervous system to atmospheric air, but also those of
the respiration and the circulation, [p142] explaining the action of
the air upon animals physiologically.
In this inquiry warm-blooded animals were almost exclusively referred
to.
Spallanzani certainly investigated the action of the air on animals
of cold blood, but less in relation to the three grand objects of
Bichat and Le Gallois; and Spallanzani had the misfortune to live in
an age when neither chemistry nor physiology had made such advances
as the present age has produced.
Messrs. Humboldt and Provençal have, indeed, supplied much of this
deficiency, by their researches into the respiratory functions of
fishes. Nevertheless, the ground was still open, and our author has
justly appreciated the extent of former inquiries, and observed that
the phenomena of cold-blooded animals were too extraordinary to be
noticed lightly, and required much more extensive observation than
was previously bestowed upon them. With this impression, he proceeded
to form an estimate of the comparative influence of the air and water
upon the nervous and muscular systems of cold-blooded animals, which
the singular modifications of life among reptiles in particular
afford ample means of ascertaining.
We know that these animals possess the extraordinary property
of existing a considerable time after the removal of the heart,
with the free exercise of their senses and of voluntary motion,
notwithstanding the suppression of the circulation. Dr. Edwards
accordingly selected _salamanders_ for his first investigations,
and removed the heart, with the bulb of the aorta. Two of these
were exposed to the free action of the air, and the other two were
submersed in water previously deprived of air by boiling; a similar
temperature being maintained in each medium. In four or five hours,
those submersed in the non-arëated water ceased to be active, unless
irritated, when they still appeared to retain voluntary power. One
died in eight, and the other in nine hours. The salamanders in air
lived from twenty to twenty-six hours and upwards. These comparisons
were frequently repeated, and upon frogs and toads, with the same
results, showing the experiments in air to be far more favourable
to their existence than with the animals submersed in the water.
Eight hours were about the maximum of the duration of life among
the animals submersed in the water, and twenty-nine among those
exposed to the air; so that, independently of respiration, the air
is thus proved to be the most proper [p143] medium for the action of
their nervous and muscular systems, in their insulated state, the
respiration and the circulation of the blood being both suspended. As
a further corroboration of the superior vivifying property of the air
over simple water, when the same animals were plunged into unaërated
water during a certain time, as soon as they were, removed into the
atmosphere, they instantly revived; and their nervous and muscular
systems were acted on according as they were placed in either medium.
Dr. Edwards also confirmed the observation of Goodwin relative to
the effect produced on the colour of the blood. Properly speaking,
the asphyxy comes on the instant the air is excluded, the shades of
difference in the colour of the blood being referrible to the air
left in the lungs after cessation of respiration.
The next point to determine was the influence of the air upon the
same animals exercising the respiratory function, and retaining their
circulation, compared with those deprived of these functions.
The difference of time in the two cases developes the influence which
the general circulation of the blood, free from aërial contact,
exercises upon the nervous system.
To ascertain this point, an equal number of frogs, deprived of the
power to exercise their respiratory and circulating functions,
together with others left entire, were respectively plunged into
disaërated water. At times the difference in favour of the untouched
animals was twenty-four hours in favour of the duration of life.
Similar trials with toads and salamanders produced the same results.
In each case _asphyxy_ came on; but the existence of the animals
which lived without the respiration and circulation was much
shortened. Thus the relative powers of life between the sole and
insulated action of the nervous system, and its action combined with
the circulation of dark blood, were estimated. The inference to be
deduced, therefore, is, that although disaërated blood furnishes
but an ephemeral sort of existence, it nevertheless exercises a
comparatively favourable influence upon the nervous and muscular
systems, since it tends to the prolongation of the action of these
animal functions.
Dr. Edwards next proceeds to investigate the phenomena of _asphyxy_
produced by _strangulation_, or the mechanical obstruction to the
access of air to the lungs, and consequently to the blood. The same
animals were employed. When the windpipe was rendered impervious by
ligature, the muscles of the animals seemed to be paralysed directly;
and although their motions became subsequently revived at times,
[p144] they never altogether recovered their perfect freedom. As a
comparative illustration, an equal number of frogs were submersed
in water, all of which died in about ten or eleven hours, while
those which were strangled lived from one to five days. Salamanders
continued active longest, and one did not cease to exist till the
eleventh day, although during this time he was in a complete state of
_asphyxy_ from perfect strangulation.
Dumeril once found that a salamander lived a long time after
decapitation, even when the cicatrix of the wound was healed so as to
stop all access of air to the lungs.
In comparing the effects of strangulation with those of submersion
or drowning, it is to be supposed either that these animals exist a
limited period without the necessity of the nervous system being in
contact with atmospheric air, or that the air influences their blood
through the integuments of the body. Accordingly Dr. Edwards put this
to the test by making experiments upon _cutaneous respiration_.
Spallanzani found that the exposure of cold-blooded animals to the
air was attended with exudation of _carbon_, a phenomenon similar
to that of respiration. There appears, however, to be some source
of error in these experiments, for Spallanzani removed the lungs,
and this operation rendered the animal liable to the absorption of
air and loss of blood. Dr. Edwards sought to effect the same purpose
by a different and more successful measure. He also confined frogs
in vessels of atmospheric air, and fastened bladders round the head
and neck, tight enough to stop the entrance of air to the lungs. At
the expiration of two hours the air was examined in the bladder,
and it was found to contain an excess of _carbonic acid_. The same
result was obtained from salamanders. It appears, therefore, that
while air is in contact with the skin, _carbon_ is given out; but
whether this be the effect of exhalation merely, or that _oxygen_ is
actually absorbed, and _carbon_ transpired, is a question which led
to further inquiries. Dr. Edwards, therefore, inclosed cold-blooded
animals in _solid substances_, in order to determine the influence of
dark-coloured blood, free of all external agency, in the production
of chemical changes, and to observe its sensible effect upon the
nervous system.
In the year 1779 three toads were confined in a box hermetically
sealed, and so deposited in the Academy of Sciences. Eighteen months
after, the box was opened, and one toad was found dead. These animals
have been found alive in blocks of coal after an imprisonment of some
years, and have [p145] also been sealed up during similar periods
without perishing. Possibly some hole or crevice might have admitted
a little air. But, in Hevissant’s experiment of 79, care seems to
have been taken to obviate this suspicion.
Dr. Edwards, however, determined to put the question to the test.
He enclosed ten out of fifteen frogs in thick wooden boxes, and
filled the interstices with plaster, covering them over with the
same substance, the toads lying each in a central hole or bed. The
other five toads were at the same time submersed in water, and at
the expiration of eight hours they were found to be dead. In sixteen
hours more, one toad was taken from a box and found to be lively, and
was reconsigned to its prison. On the sixteenth day the toads in the
boxes were discovered alive, and thus the fact was established that
these animals can live far longer in a state of asphyxy confined in
solid substances, than when submersed in water. This was confirmed by
repeated trials on salamanders, frogs, and toads. The frogs perished
quickest.
Thus an extraordinary fact is established, as regarding reptiles,
since it affords an exception to the general rule that _all animals
require a_ CONSTANT _supply of fresh air for the maintenance of their
existence_.
Similar trials were repeated in sand, and with the same results.
Dr. Edwards found that although a certain quantity of air enters the
boxes and sand, yet that it is far too little to maintain life. His
conclusion, therefore, stands, that animals of the kind employed can
live longer in _solid substances_ than in a limited quantity of _dry
air_.
It remains, however, to be considered in what manner these animals
have their lives extended beyond those exposed to the action of a
body of air. Dr. Edwards supposes the _moisture_ of the sand to be
one cause, since in the dry air the animals become _desiccated_, the
cutaneous transpiration being lost in one case, and retained in the
other, by the exclusion of air. A rapid and abundant transpiration
from the body, united with deficiency of air, seems to be a greater
cause of dissolution than confinement in solid substances wherein
there is no waste by transpiration.
The author’s inquiries are next directed to the influence of
_temperature_ upon animals of cold blood, and two and forty
experiments are practised upon this subject, from the month of July
to September following, during which period frogs were submersed
in aërated water, with a view of settling the duration of life,
acted on by varieties of temperature. The [p146] continuance of
life, generally, in these experiments, varied from one to two hours
and twenty-seven minutes. The mean term of life was one hour and
thirty-seven minutes, as averaged in July, and in September one hour
and forty-five minutes, the two extremes of the seasons approximating
the effects. The duration of the frog’s existence was greatest in the
greatest depression of temperature. Thus at ten degrees the duration
of life was more than double what occurred at sixteen or seventeen
degrees, and at zero it was about triple. As the heat was increased,
the duration of life was diminished; at forty-two the frogs died, and
in the lowest temperature they lived longest.
It appeared that at zero the frogs did not become stiffened, but
retained their motion, and their resistance to the frozen state is
the cause of the continuance of their existence at a low temperature.
The cause of this resistance is to be found in their peculiarity of
constitution. Toads produced similar results.
It may be alleged that frogs naturally live in climates at from forty
to forty-two; but, it is to be observed, that they are then placed in
a situation of liberty to come to the surface of the water to respire
when they please; whereas in these experiments their respiration is
limited, from their inability to reach the surface.
Taking a wider range of temperature, Dr. Edwards sought to ascertain
the influence of the _seasons_. In July and September frogs were
found to live from one to two hours and twenty-seven minutes in
aërated water at fifteen and seventeen degrees. In November they
died at the end of more than double this period, under the same
temperature, and all other circumstances being similar excepting the
season. As the autumn advanced life was prolonged.
To what are we to ascribe the modifications of the seasons? Probably
to circumstances appertaining to the intensity of light, to
electricity, to temperature, to the pressure of the atmosphere, to
dryness and moisture, &c.? Such existing causes naturally suggest
themselves. But it appears that little or no account can be rendered
as to pressure, since its variations were too trifling during the
two seasons. Moisture could not effect an influence, because the
experiments were performed in water. The motion of the air was also
obviated. Of all the suggested modifications _temperature_ alone
acted, and this, as it related to the surrounding air, was rendered
ineffectual by artificial temperature. The animals, therefore,
could only be affected as to the temperature of the [p147] seasons
_by that which preceded the experiments_. The modifications of the
seasons, therefore, appeared to influence the cold-blooded animals
used in the experiments in this point of view only. Accordingly we
have this remarkable result, that the animals lived twice as long in
autumn as in the summer preceding, when plunged in water of equal
temperature. The _seasons_ evidently influence their constitutions,
so as to extend the duration of life independently of other causes,
that is, from summer to autumn. Dr. Edwards endeavoured to ascertain
if it proceeds from atmospheric temperature, and he found that frogs
lived in aërated water at ten degrees, during November, from five
or ten to eleven, and even to forty hours, in some instances, the
last term being about double the duration of life in water of the
same degree in summer. This proves the remarkable dependence of the
frog’s life under water, and the temperature of the month preceding.
Two curious facts are thus developed by experiments instituted at
different seasons. First, the influence of the temperature of the
water in which the animals were placed; and secondly, the influence
of the temperature of the air during certain periods preceding the
experiments, for in autumn the duration of life was about double
that of summer, and in winter he found the term to equal autumn,
the temperature of the air being in each comparative experiment
artificially raised to the same degree.
It appears from the foregoing experiments that frogs, toads,
and salamanders, exist in water according to its _lowness of
temperature_, and that their lives are prolonged _by the temperature
which precedes the experiment being lowered_. It then becomes a
question, what are the limits of this influence? This is to be
ascertained by observing the greatest duration of life among animals
deprived of external air by submersion in water; and noticing at
the same time all the favourable circumstances dependent on the
concurrent temperature in prolonging life among the cold-blooded
animals.
A point relative to the natural history of frogs first presents
itself to our notice. Spallanzani is of opinion that frogs do not
pass the winter under water, but retire in October from their native
rivers into moist sands, in which they make openings to breathe the
air through, called by the Italian fishermen _il respiro della ranà_.