the nature and structure of the lungs. You recollect that the venous
blood, on being expelled from the right ventricle, enters the lungs to
go through what we may call the lesser circulation; the large trunk or
vessel that conveys it branches out, at its entrance into the lungs,
into an infinite number of very fine ramifications. The windpipe, which
conveys the air from the mouth into the lungs, likewise spreads out into
a corresponding number of air vessels, which follow the same course as
the blood vessels, forming millions of very minute air-cells. These two
sets of vessels are so interwoven as to form a sort of net-work,
connected into a kind of spongy mass, in which every particle of blood
must necessarily come in contact with a particle of air.
CAROLINE.
But since the blood and the air are contained in different vessels, how
can they come into contact?
MRS. B.
They act on each other through the membrane which forms the coats of
these vessels; for although this membrane prevents the blood and the air
from mixing together in the lungs, yet it is no impediment to their
chemical action on each other.
EMILY.
Are the lungs composed entirely of blood vessels and air vessels?
MRS. B.
I believe they are, with the addition only of nerves and of a small
quantity of the cellular substance before mentioned, which connects the
whole into an uniform mass.
EMILY.
Pray, why are the lungs always spoken of in the plural number? Are there
more than one?
MRS. B.
Yes; for though they form but one organ, they really consist of two
compartments called lobes, which are enclosed in separate membranes or
bags, each occupying one side of the chest, and being in close contact
with each other, but without communicating together. This is a beautiful
provision of nature, in consequence of which, if one of the lobes be
wounded, the other performs the whole process of respiration till the
first is healed.
The blood, thus completed, by the process of respiration, forms the most
complex of all animal compounds, since it contains not only the numerous
materials necessary to form the various secretions, as saliva, tears,
&c. but likewise all those that are required to nourish the several
parts of the body, as the muscles, bones, nerves, glands, &c.
EMILY.
There seems to be a singular analogy between the blood of animals and
the sap of vegetables; for each of these fluids contains the several
materials destined for the nutrition of the numerous class of bodies to
which they respectively belong.
MRS. B.
Nor is the production of these fluids in the animal and vegetable
systems entirely different; for the absorbent vessels, which pump up the
chyle from the stomach and intestines, may be compared to the absorbents
of the roots of plants, which suck up the nourishment from the soil. And
the analogy between the sap and the blood may be still further traced,
if we follow the latter in the course of its circulation; for, in the
living animal, we find every where organs which are possessed of a power
to secrete from the blood and appropriate to themselves the ingredients
requisite for their support.
CAROLINE.
But whence do these organs derive their respective powers?
MRS. B.
From a peculiar organisation, the secret of which no one has yet been
able to unfold. But it must be ultimately by means of the vital
principle that both their mechanical and chemical powers are brought
into action.
I cannot dismiss the subject of circulation without mentioning
_perspiration_, a secretion which is immediately connected with it, and
acts a most important part in the animal economy.
CAROLINE.
Is not this secretion likewise made by appropriate glands?
MRS. B.
No; it is performed by the extremities of the arteries, which penetrate
through the skin and terminate under the cuticle, through the pores of
which the perspiration issues. When this fluid is not secreted in
excess, it is _insensible_, because it is dissolved by the air as it
exudes from the pores; but when it is secreted faster than it can be
dissolved, it becomes _sensible_, as it assumes its liquid state.
EMILY.
This secretion bears a striking resemblance to the transpiration of the
sap of plants. They both consist of the most fluid part, and both exude
from the surface by the extremities of the vessels through which they
circulate.
MRS. B.
And the analogy does not stop there; for, since it has been ascertained
that the sap returns into the roots of the plants, the resemblance
between the animal and vegetable circulation is become still more
obvious. The latter, however, is far from being complete, since, as we
observed before, it consists only in a rising and descending of the sap,
whilst in animals the blood actually _circulates_ through every part of
the system.
We have now, I think, traced the process of nutrition, from the
introduction of the food into the stomach to its finally becoming a
constituent part of the animal frame. This will, therefore, be a fit
period to conclude our present conversation. What further remarks we
have to make on the animal economy shall be reserved for our next
interview.
CONVERSATION XXVI.
ON ANIMAL HEAT; AND ON VARIOUS ANIMAL PRODUCTS.
EMILY.
Since our last interview, I have been thinking much of the theory of
respiration; and I cannot help being struck with the resemblance which
it appears to bear to the process of combustion. For in respiration, as
in most cases of combustion, the air suffers a change, and a portion of
its oxygen combines with carbon, producing carbonic acid gas.
MRS. B.
I am much pleased that this idea has occurred to you: these two
processes appear so very analogous, that it has been supposed that a
kind of combustion actually takes place in the lungs; not of the blood,
but of the superfluous carbon which the oxygen attracts from it.
CAROLINE.
A combustion in our lungs! that is a curious idea indeed! But, Mrs. B.,
how can you call the action of the air on the blood in the lungs
combustion, when neither light nor heat are produced by it?
EMILY.
I was going to make the same objection. --Yet I do not conceive how the
oxygen can combine with the carbon, and produce carbonic acid, without
disengaging heat?
MRS. B.
The fact is, that heat is disengaged.* Whether any light be evolved,
I cannot pretend to determine; but that heat is produced in considerable
and very sensible quantities is certain, and this is the principal, if
not the only source of ANIMAL HEAT.
[Footnote *: It has been calculated that the heat produced by
respiration in 12 hours, in the lungs of a healthy person, is such
as would melt about 100 pounds of ice.]
EMILY.
How wonderful! that the very process which purifies and elaborates the
blood, should afford an inexhaustible supply of internal heat?
MRS. B.
This is the theory of animal heat in its original simplicity, such
nearly as it was first proposed by Black and Lavoisier. It was equally
clear and ingenious; and was at first generally adopted. But it was
objected, on second consideration, that if the whole of the animal heat
was evolved in the lungs, it would necessarily be much less in the
extremities of the body than immediately at its source; which is not
found to be the case. This objection, however, which was by no means
frivolous, is now satisfactorily removed by the following
consideration:-- Venous blood has been found by experiment to have _less
capacity for heat_ than arterial blood; whence it follows that the
blood, in gradually passing from the arterial to the venous state,
during the circulation, parts with a portion of caloric, by means of
which heat is diffused through every part of the body.
EMILY.
More and more admirable!
CAROLINE.
The cause of animal heat was always a perfect mystery to me, and I am
delighted with its explanation. --But pray, Mrs. B., can you tell me
what is the reason of the increase of heat that takes place in a fever?
EMILY.
Is it not because we then breathe quicker, and therefore more heat is
disengaged in the system?
MRS. B.
That may be one reason: but I should think that the principal cause of
the heat experienced in fevers, is, that there is no vent for the
caloric which is generated in the body. One of the most considerable
secretions is the insensible perspiration; this is constantly carrying
off caloric in a latent state; but during the hot stage of a fever, the
pores are so contracted, that all perspiration ceases, and the
accumulation of caloric in the body occasions those burning sensations
which are so painful.
EMILY.
This is, no doubt, the reason why the perspiration that often succeeds
the hot stage of a fever affords so much relief. If I had known this
theory of animal heat when I had a fever last summer, I think I should
have found some amusement in watching the chemical processes that were
going on within me.
CAROLINE.
But exercise likewise produces animal heat, and that must be quite in a
different manner.
MRS. B.
Not so much so as you think; for the more exercise you take, the more
the body is stimulated, and requires recruiting. For this purpose the
circulation of the blood is quickened, the breath proportionably
accelerated, and consequently a greater quantity of caloric evolved.
CAROLINE.
True; after running very fast, I gasp for breath, my respiration is
quick and hard, and it is just then that I begin to feel hot.
EMILY.
It would seem, then, that violent exercise should produce fever.
MRS. B.
Not if the person is in a good state of health; for the additional
caloric is then carried off by the perspiration which succeeds.
EMILY.
What admirable resources nature has provided for us! By the production
of animal heat she has enabled us to keep up the temperature of our
bodies above that of inanimate objects; and whenever this source becomes
too abundant, the excess is carried off by perspiration.
MRS. B.
It is by the same law of nature that we are enabled, in all climates,
and in all seasons, to preserve our bodies of an equal temperature, or
at least very nearly so.
CAROLINE.
You cannot mean to say that our bodies are of the same temperature in
summer, and in winter, in England, and in the West-Indies.
MRS. B.
Yes, I do; at least if you speak of the temperature of the blood, and
the internal parts of the body; for those parts that are immediately in
contact with the atmosphere, such as the hands and face, will
occasionally get warmer, or colder, than the internal or more sheltered
parts. But if you put the bulb of a thermometer in your mouth, which is
the best way of ascertaining the real temperature of your body, you will
scarcely perceive any difference in its indication, whatever may be the
difference of temperature of the atmosphere.
CAROLINE.
And when I feel overcome by heat, I am really not hotter than when I am
shivering with cold?
MRS. B.
When a person in health feels very hot, whether from internal heat, from
violent exercise, or from the temperature of the atmosphere, his body is
certainly a little warmer than when he feels very cold; but this
difference is much smaller than our sensations would make us believe;
and the natural standard is soon restored by rest and by perspiration.
It is chiefly the external parts that are warmer, and I am sure that you
will be surprised to hear that the internal temperature of the body
scarcely ever descends below ninety-five or ninety-six degrees, and
seldom attains one hundred and four or one hundred and five degrees,
even in the most violent fevers.
EMILY.
The greater quantity of caloric, therefore, that we receive from the
atmosphere in summer, cannot raise the temperature of our bodies beyond
certain limits, as it does that of inanimate bodies, because an excess
of caloric is carried off by perspiration.
CAROLINE.
But the temperature of the atmosphere, and consequently that of
inanimate bodies, is surely never so high as that of animal heat?
MRS. B.
I beg your pardon. Frequently in the East and West Indies, and sometimes
in the southern parts of Europe, the atmosphere is above ninety-eight
degrees, which is the common temperature of animal heat. Indeed, even in
this country, it occasionally happens that the sun’s rays, setting full
on an object, elevate its temperature above that point.
In illustration of the power which our bodies have to resist the effects
of external heat, Sir Charles Blagden, with some other gentlemen, made
several very curious experiments. He remained for some time in an oven
heated to a temperature not much inferior to that of boiling water,
without suffering any other inconvenience than a profuse perspiration,
which he supported by drinking plentifully.
EMILY.
He could scarcely consider the perspiration as an inconvenience, since
it saved him from being baked by giving vent to the excess of caloric.
CAROLINE.
I always thought, I confess, that it was from the heat of the
perspiration that we suffered in summer.
MRS. B.
You now find that you are quite mistaken. Whenever evaporation takes
place, cold, you know, is produced in consequence of a quantity of
caloric being carried off in a latent state; this is the case with
perspiration, and it is in this way that it affords relief. It is on
that account also that we are so apt to _catch cold_, when in a state of
profuse perspiration. It is for the same reason that tea is often
refreshing in summer, though it appears to heat you at the moment you
drink it.
EMILY.
And in winter, on the contrary, tea is pleasant on account of its heat.
MRS. B.
Yes; for we have then rather to guard against a deficiency than an
excess of caloric, and you do not find that tea will excite perspiration
in winter, unless after dancing, or any other violent exercise.
CAROLINE.
What is the reason that it is dangerous to eat ice after dancing, or to
drink any thing cold when one is very hot?
MRS. B.
Because the loss of heat arising from the perspiration, conjointly with
the chill occasioned by the cold draught, produce more cold than can be
borne with safety, unless you continue to use the same exercise after
drinking that you did before; for the heat occasioned by the exercise
will counteract the effects of the cold drink, and the danger will be
removed. You may, however, contrary to the common notion, consider it as
a rule, that cold liquids may, at all times, be drunk with perfect
safety, however hot you may feel, provided you are not at the moment in
a state of great perspiration, and on condition that you keep yourself
in gentle exercise afterwards.
EMILY.
But since we are furnished with such resources against the extremes of
heat or cold, I should have thought that all climates would have been
equally wholesome.
MRS. B.
That is true, in a certain degree, with regard to those who have been
accustomed to them from birth; for we find that the natives of those
climates, which we consider as most deleterious, are as healthy as
ourselves; and if such climates are unwholesome to those who are
habituated to a more moderate temperature, it is because the animal
economy does not easily accustom itself to considerable changes.
CAROLINE.
But pray, Mrs. B., if the circulation preserves the body of an uniform
temperature, how does it happen that animals are sometimes frozen?
MRS. B.
Because, if more heat be carried off by the atmosphere than the
circulation can supply, the cold will finally prevail, the heart will
cease to beat, and the animal will be frozen. And, likewise, if the body
remained long exposed to a degree of heat, greater than the perspiration
could carry off, it would at last lose the power of resisting its
destructive influence.
CAROLINE.
Fish, I suppose, have no animal heat, but only partake of the
temperature of the water in which they live?
EMILY.
And their coldness, no doubt, proceeds from their not breathing?
MRS. B.
All kinds of fish breathe more or less, though in a much smaller degree
than land animals. Nor are they entirely destitute of animal heat,
though, for the same reason, they are much colder than other creatures.
They have comparatively but a very small quantity of blood, therefore
but very little oxygen is required, and a proportionally small quantity
of animal heat is generated.
CAROLINE.
But how can fish breathe under water?
MRS. B.
They breathe by means of the air which is dissolved in the water, and if
you put them into water deprived of air by boiling, they are soon
suffocated.
If a fish is confined in a vessel of water closed from the air, it soon
dies; and any fish put in afterwards would be killed immediately, as all
the air had been previously consumed.
CAROLINE.
Are there any species of animals that breathe more than we do?
MRS. B.
Yes; birds, of all animals, breathe the greatest quantity of air in
proportion to their size; and it is to this that they are supposed to
owe the peculiar firmness and strength of their muscles, by which they
are enabled to support the violent exertion of flying.
This difference between birds and fish, which may be considered as the
two extremes of the scale of muscular strength, is well worth observing.
Birds residing constantly in the atmosphere, surrounded by oxygen, and
respiring it in greater proportions than any other species of animals,
are endowed with a superior degree of muscular strength, whilst the
muscles of fish, on the contrary, are flaccid and oily; these animals
are comparatively feeble in their motions, and their temperature is
scarcely above that of the water in which they live. This is, in all
probability, owing to their imperfect respiration; the quantity of
hydrogen and carbon, that is in consequence accumulated in their bodies,
forms the oil which is so strongly characteristic of that species of
animals, and which relaxes and softens the small quantity of fibrine
which their muscles contain.
CAROLINE.
But, Mrs. B., there are some species of birds that frequent both
elements, as, for instance, ducks and other water fowl. Of what nature
is the flesh of these?
MRS. B.
Such birds, in general, make but little use of their wings; if they fly,
it is but feebly, and only to a short distance. Their flesh, too,
partakes of the oily nature, and even in taste sometimes resembles that
of fish. This is the case not only with the various kinds of water
fowls, but with all other amphibious animals, as the otter, the
crocodile, the lizard, &c.
CAROLINE.
And what is the reason that reptiles are so deficient in muscular
strength?
MRS. B.
It is because they usually live under ground, and seldom come into the
atmosphere. They have imperfect, and sometimes no discernible organs of
respiration; they partake therefore of the soft oily nature of fish;
indeed, many of them are amphibious, as frogs, toads, and snakes, and
very few of them find any difficulty in remaining a length of time under
water. Whilst, on the contrary, the insect tribe, that are so strong in
proportion to their size, and alert in their motions, partake of the
nature of birds, air being their peculiar element, and their organs of
respiration being comparatively larger than in other classes of animals.
I have now given you a short account of the principal animal functions.
However interesting the subject may appear to you, a fuller
investigation of it would, I fear, lead us too far from our object.
EMILY.
Yet I shall not quit it without much regret; for of all the branches of
chemistry, it is certainly the most curious and most interesting.
CAROLINE.
But, Mrs. B., I must remind you that you promised to give us some
account of the nature of _milk_.
MRS. B.
True. There are several other animal productions that deserve likewise
to be mentioned. We shall begin with milk, which is certainly the most
important and the most interesting of all the animal secretions.
Milk, like all other animal substances, ultimately yields by analysis
oxygen, hydrogen, carbon, and nitrogen. These are combined in it under
the forms of albumen, gelatine, oil, and water. But milk contains,
besides, a considerable portion of phosphat of lime, the purposes of
which I have already pointed out.
CAROLINE.
Yes; it is this salt which serves to nourish the tender bones of the
suckling.
MRS. B.
To reduce milk to its elements, would be a very complicated, as well as
useless operation; but this fluid, without any chemical assistance, may
be decomposed into three parts, _cream_, _curds_, and _whey_. These
constituents of milk have but a very slight affinity for each other, and
you find accordingly that cream separates from milk by mere standing. It
consists chiefly of oil, which being lighter than the other parts of the
milk, gradually rises to the surface. It is of this, you know, that
butter is made, which is nothing more than oxygenated cream.
CAROLINE.
Butter, then, is somewhat analogous to the waxy substance formed by the
oxygenation of vegetable oils.
MRS. B.
Very much so.
EMILY.
But is the cream oxygenated by churning?
MRS. B.
Its oxygenation commences previous to churning, merely by standing
exposed to the atmosphere, from which it absorbs oxygen. The process is
afterwards completed by churning; the violent motion which this
operation occasions brings every particle of cream in contact with the
atmosphere, and thus facilitates its oxygenation.
CAROLINE.
But the effect of churning, I have often observed in the dairy, is to
separate the cream into two substances, butter and butter-milk.
MRS. B.
That is to say, in proportion as the oily particles of the cream become
oxygenated, they separate from the other constituent parts of the cream
in the form of butter. So by churning you produce, on the one hand,
butter, or oxygenated oil; and, on the other, butter-milk, or cream
deprived of oil. But if you make butter by churning new milk instead of
cream, the butter-milk will then be exactly similar in its properties to
creamed or skimmed milk.
CAROLINE.
Yet butter-milk is very different from common skimmed milk.
MRS. B.
Because you know it is customary, in order to save time and labour, to
make butter from cream alone. In this case, therefore, the butter-milk
is deprived of the creamed milk, which contains both the curd and whey.
Besides, in consequence of the milk remaining exposed to the atmosphere
during the separation of the cream, the latter becomes more or less
acid, as well as the butter-milk which it yields in churning.
EMILY.
Why should not the butter be equally acidified by oxygenation?
MRS. B.
Animal oil is not so easily acidified as the other ingredients of milk.
Butter, therefore, though usually made of sour cream, is not sour
itself, because the oily part of the cream had not been acidified.
Butter, however, is susceptible of becoming acid by an excess of oxygen;
it is then said to be rancid, and produces the sebacic acid, the same as
that which is obtained from fat.
EMILY.
If that be the case, might not rancid butter be sweetened by mixing with
it some substance that would take the acid from it?
MRS. B.
This idea has been suggested by Sir H. Davy, who supposes, that if
rancid butter were well washed in an alkaline solution, the alkali would
separate the acid from the butter.
CAROLINE.
You said just now that creamed milk consisted of curd and whey. Pray how
are these separated?
MRS. B.
They may be separated by standing for a certain length of time exposed
to the atmosphere; but this decomposition may be almost instantaneously
effected by the chemical agency of a variety of substances. Alkalies,
rennet*, and indeed almost all animal substances, decompose milk by
combining with the curds.
Acids and spirituous liquors, on the other hand, produce a decomposition
by combining with the whey. In order, therefore, to obtain the whey
pure, rennet, or alkaline substances, must be used to attract the curds
from it.
But if it be wished to obtain the curds pure, the whey must be separated
by acids, wine, or other spirituous liquors.
[Footnote *: Rennet is the name given to a watery infusion of the
coats of the stomach of a sucking calf. Its remarkable efficacy in
promoting coagulation is supposed to depend on the gastric juice
with which it is impregnated.]
EMILY.
This is a very useful piece of information; for I find white-wine whey,
which I sometimes take when I have a cold, extremely heating; now, if
the whey were separated by means of an alkali instead of wine, it would
not produce that effect.
MRS. B.