Mrs. (Jane Haldimand) Marcet.

Conversations on chemistry .. online

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pand ; and, on the other hand, when the chest contracts, it com-
presses the lungs, and forces the air out of them. • ^

Caroline. The lungs, then, are like bellows, and the chest is the
power that works them.
*^ (Fig. 35.

JIfrt. B. Precisely so. Here is a curious App«r»t» to murtntte the med
little figure which will assist me in explain-
ing the mechanism of breathing.

Caroline, What a droll figure! a little
head fixod upon a glass bell, with a bladder
tied over the bottom of it.

Mrs. B. You must observe that there is
tmother bladder within the glass, the neck
of which communicates with the mouth of
the figure — this represents the lunes con-
tained within the chest ; the other bladder,
which you see Js tied loose, represents a
muscular membrane, called the dia-
phragmy which separates the chest from
the lower part of the body. By the chest,
therefore,! mean that ^arge cavitjr ip the
upper part of the body contained withiu the
ribs, the neck, and the diaphragm^ this
membrane is muscular, aUd capable of con-
traction and dilatation. The contraction
may be imitated by drawing Itie bladder
tight ov<er the bottom of the receiver, as I
can easily do by squeezing it in my hand,

when the air in the bladder, which repre- ^^

sents the lungs, will be forced out througl^ ^^ Gi^^unrBrBUdderr.-

the mOUllf of the figure. — >6tentiiif langi. C. BUdd«r re-

prttentlnf the Diaphnf i£^ 1.

Emily, See, Caroline, how it blows the flame of the c^dle in

Jtff^i. B, By letting the.bladder loose again, we imitate tbe^la-
tation of the diaphragm, and the cavity of the chest being enlarged,
the lungs expand, and the air rushes in to fill them.

EmUy, This figure, I think, gives a very clear* idea of the process
•of breathing.

Mrt,B. It illustrates, tolerably well, tb^ action of the lungs and
diaphragm ; but thode are not the only powers concerned in tne en-
Jargement or diminution of the cavity of the chest ; the ribs are also
possessed of a muscular motion for the same purpose ;/they'are al-
ternately drawn in, edgeways, lo assist the contractioniand stretch-

, 1377. On what does the expansion and contraction -of the lungs
depend? >

1378. What part of the body is called the chestf

1379. How would you explain figure 36 f

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ed out, Uke the hoops of a barrel, to contribute to the dilatation of
the cbesp

Emil^ I always supposed that the elevalion and depressiisP of
the ribs trere thepoosequence, not tbcupause, of breathing.

Mrt, B. It is exactly the reverse. (The muscular action of the
diaphragm, together with that of t\^ riBs, are the causes of the conr
traction and expansion of the che8» and the air rushing iprto, and
being expelled from the lungs, aretmlycorMe^t^eTu^fiipf those actions.
Caroline. 1 confess that I thought the act of breathing began by
opening the mouth for the air to rush in, and that it was the air
alone, which, by alternately rushing in and out, occasioned the di*
lat^tions and contractions of the lungs and chest.

JUrs. B. Try the experiment of merely opening your mouth :
the air will not rush in, till by an internal muscular action you pro-
duce a vacuum — yes, just so, your diaphragm is now dilated, and
the ribs expanded. But you will not be able to keep them ion? in
that situation. Your lungs and chest are already resuming their
former state, and expelling the air with which they had just been
filled. This mechanism goes on more or less rapidly : but, in gene-
ral, a person at rest and in health will brcath^etween fifteen and
twenty-five times in a mmut^ ^

We may now proceed to the chemical effects of respiration ; but,
for this purpose, it is necessary that you should previously have
some notion of the xirculatiorh of the blood. Tell me, Caroline,
what do you understand bv the circulation of the blood ?

Caroline. I am delighted that you have come to this subject ; for
it is one that has long excited my .curiosity. But 1 cap.pot conceive
how it is connected with respiration. The idea that I have of the
circulation is, that the blood runs from tbie heart through the veins
all over the body, and back again to the heart-

J^Irs. B. I could hardly have expected a better definition from
you : i^is, however, not quite correct"; for you do not distinguish
the arteries from the veins, which, as we have already observed,
are two^stinct sets of vessels, each having its own peculiar func-
tions, ^iie arteries convey the blood from the heart to the extre-
mities of the body ; and the veins bring it back into the hea^

This sketch will give you an idea of ^the manner in whicKsome
of the principal veins and arteries of the;human body branch out
of the neart, wbigh may be considered as AfCommon centre to both
sets of vessels. /The heart is a kind of strong elastic bag, or mus-
cdlar cavity, wmch possesses a power of dilating itself, for the pur-
p<)6cs of alternately receiving and-^xpelling the blood, in order to
carry on the process of circulation^

Emily. Why are the arteries in this drawling paiited red, and the
veins purple?

J^irs. B. It is to point out the difference ofuhe colour of the blood
in these two sets of vessels.

1380. What office do the ribs perform ?

1381. What causes the contraction and expansion of the chest f
1332. How many times will a person, well and at rest, breathe

Uk a minute ?
1383. In what manner is the circulation of hlood carried on ?
.1384. How is the heart described ? ^ i

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Caroline, Bat if it is the same blood which flovrs from the arte-
ries into the veins, how can its colour be changed ? ^

Mrt, B. This change arises from yarious circumstances, /in the
first place, during its passag^e through the arteries, the blood under-
goes a considerable alteration, some of its constituent parts being
gradually separated from it for the purpose of nourishing the bodj,
and of supplying the various secretions. In consequence of this,
the florid arterisd colour of the blood changes by degrees to a deep
purple, which is its constant colour in the veins On the other
hand, the blood is recruited during its return through the veins by
the fresh chyle, or imperfect blood, which has been produced by
food ; and it receives also lymph from the absorbent vessels, as we
have before mentioned. After having undergone these several
changes, the blood returns to the heart in a state very different
from that in which it left it. It is loaded with a greater proportion
of hydrogen and carbon, and is no longer fit for the nourishment
of the b(2[y, or other purposes of circulation^

Emily. And in this state does it mix in tire heatt With the pure
.florid blood jvhicb runs into the arteries ?

Jtfr*. J?./N(V The heart is divided into two cavities, or corn-
partitions, called the right and tefl ventricles. The left ventricle is
•the receptacle for the pure arterial blood, previous to its circula-
tion; whilst the venous, or impure blood, which returns to the
heart after having circulated, is received into the right ventricle,
previous to its purification, which I shall priesendy explain.

Caroline. I own that I always thought the same blood circulated
again and again through the body, without undergoing any ehange.

Mrt. B. Yet you must have supposed that the blood circulated
for some purpose.

Caroline, 1 knew that it was indispensable to'lifb; but had no
4dea of its real functions.

Jtfri. B, But now that you understand that |he blood conveys
nourishment to every part of fhe bod]^, and supplies the various
secretioifi) yoii must be sensible that it cannot constantly answer
these obj^ts without being proportionally renovated and purified.

Caroline. But does notlbe chyle answer this purpose f

Mn, B, Only in part. Qt renovates the nutritive principles of
the blood, but does not relieve it from the superabundance or water
and carbob with which it is encumberedA

JSint/v. Hp'Vj then, isthi^ffecled? /

Jtfrt.^.(By Respiration^ This is one of the grand mysteries
which modem chemistry has disclosed. When the venous blood en-
ters thte right veiitricle oi the heart, it contracts, by its muscular
power, and throws the blood through a large Vessel 4nto the lungs,
which are contiguous, and through which it Circulates by millions of

1385. How does the arterial differ from the venous blood ?

1386. Does the venous' blood mix with the ^rterial'in its return
to the heart ?

1387. How is tiourishment conveyed to different paits ofthe body ?

1388. How does the chyle serve to renovate and purify the blood?

1389. How is the' blood relieved '<rom its superabundance of wa-
Her and carbon ?

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small ramifications. Here it eomes ia cootaci^ with the air which
we bredtdie. The action of the air on the blood in the lungs, is in-
deed, concealed from our immediate observation ; but we are «ble
to form a toleralxle accurate judgment of it from the chans^es which
it effects, not^nly in the blood, but also on the air expired.
rThe air, after passing through the lungs, is found to contain all the
nitrogen inspired, but to have lost part of its oxygen, and to have
acquired a p(n*tion of waUry vapor, and of carbonic acid ^aV"'
Hence it is inferred, that/wben the air comes in contact with-m
venous blood in the lungs^the oxygen attracts from it the supera-
bundant quantity of -carbon with which it has impregn^ed itselr dur-
ing tlie circulation, and converts it into carbonic acidl This gas-
eous acid, together with the redundant moisture frdm the lungsf
being then expired, the blood is restored to its former purity, that is,
to the state of arterial blood, -and is thus again, enabled to perform
its various functions.

Caroline, This is trul j wonderful ! of all that we have yet learn-
ed, I do not recollect any thin^ that, has appeared to me so curiovs
•and interesting. 1 almost believe that I should hke to study anato-
my now, though I have hitherto had so disgusting an idea ef it.-^
Pray, to whom are we indebted for these beautiful discoveries ^

Jur$. JB./PriesUy and Crawford, in this country, and Lavoisier,
in France, are the principal inventors of the theory of respiration a
Of late years the subject has been farther illustrated and simplifiea
hy the accurate experiments of Messrs AUyn and Pepys. But the
sUn more important and more admirable discovery of the circula-
tion oLthe bipod was made long before by our immortal country^
man,(H^rvey. /J Jl ^v \ :i

EmUy, Indebdi 1 never heard any thing that delighted me so
much. as this theory of respiration. J3ut I hope, Mrs. B., tliat you
will enter a little more into particulars before you dismiss so inter-
esting a subject. We 16ft the blood in the lungs to undergo the
salutary change ; but how does it thence s{)read to all the parts of
<he body • ^ *

Jtfrir. B. /After circulating through tbeltfngs, tlie blood istx)llect<»
ed into founlarge vessels, by which it is conveyed into the left ven-
tricle of the heart, whence it is propelled to aul the diferent parts
of the body by a large artery, wnich gradually ramifies into mil-
lions of smaU arteries through the whole frame. From tlie extremi^-
ties of tbese little ramifications the blood is transmitted to the veins,

* Not in actual contact. In this case it is obvious there tirould be
nothing to confine the blood and prevent its flowing out. The air
cells are ^parated from the blood vessels by an extremely thin

f The quantity of ^moisture discharged 'htf the lung^ in 24 hoars^
may be computed at eigbt or nine ounces.

1390. What effect does respiration have on the air we breathe?

1 391. What becomes ef the blood when it has become purified im
ciroolatinc through the Vcmgs ?

1392. Who were the inventors of the received theory of resf^
tion ?

139a W ho discovered the circulation <of the blood ?
1394. After the blood is purified in the lungs, how is it spread t» '
the various parU of the body f ^^^^^^^^ by Google


which briDgf it^back to the heart vaod lang^wio go rouDd again and
again in the manner we have just dcfscribeaJ Yoa see, therefore,
that the blood actuallyundergoes two circuladons ; the one, through
the lungs, by which it is converted into pure arterial blood ; the
other, a general circulation by which nourishfhent is conveyed to
every part of the body; and these are both egually indispensable
to the support of animal life.

Emily, But whence proceeds the carbon with which the blood is
impregnatefi when it comes into the longs f

Mrs, R. ^arbon exists in a greater proportion in blood than in
organized animal matter. The blood, therefore, after suppl3ring its
various secretions, becomes loaded with an excess of carbon, which
is^jarried off by respiration ; and the formation of new chyle from
the food affords a constant supply of carbonaceous matte^

Caroline. I wonder what quantity of carbon may bo^xpelled
from the blood by respiration in the course of 24 hours ?

Mrs, B, It appears by the experiments of Messrs. Allyn and
Pepys tbat^out 40,000 cubic inches of carbonic acid ^s, are emit-
ted from thVlungs of a healthy person, daily l which is equivalent
to -eleven (nincas of solid carbon every %i hour^

Emily, What an immense Quantity! And*^pray how much of
carbonic acid g^s do' we expel from our lungs at each respiration ?

Mrs, B, The opantity of air which we take into our lungs at
each inspirationnsabout 40 cubic inches, which contains a little less
than 10 cubic inches of oxyg^^ and of those 10 hiches, one-eighth
is converted into carbonic ac1a gas on passing once through the
lungs,^ a change sufficient to prevent air which has only been
breathed once ^om suffering a taper to burn in it.

Caroline, Pray how does air come -in contact with the blood in
the lungs?"

Mrs, B, I cannot answer this question without entering into an
explanation of the nature and structure of the lung^. You re-
collect that/tlie venous blood, on being expelled from the right ven-
tricle enteiV the lungs to go -through what we may call the lesser
circulation ; the large trunk or vessel conveys its branches out, at
its entrance into the lungs, into an infinitebumber of very fine ram-
ifications. The wind-pipe, which conveys the air from the .mouth
into the lungs, likei^i$<e spreads out into a corresponding number of
air vessels^ which follow the same course as the blood vessels, form-
ing 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%ind
of spongy mass, in which every par-t(cle of blood must necessarily
come in contact with a particle^of ain

^ The bulk of carbonic acid gas formed by respiration, is exactfy
the same as that of the ox} gen gas which disappears.

1395. Whence proceeds the carbon with which the blood is im-
'pregnated on its return to the \\xrx^% P

1396. What quantity of carbon is expelled from the blood by res-
piration in 24 hours ?

1397. What is the quantity of air we take into our longs at each
respiration ?

1398. How does the air come in contact with the blood in the
viungs ?

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Caroline. But since the blood and the air are contained in differ-
ent vessels, hpw can they come in contact ?

•Mn. B, (Tbey act on each other through the membrane which
forms the c^s of these vessels ; for although this membrane pre-
vents the blood and the air from mixing together in th^Jungs, yet it
is no impediment to their chemical action on each oth^l"

Emily, Are the lungs composed entirely of blood vSsels and air
vessels ?

Mrs, B, I believelpey are, with the addition only of nerves and
of a small quantity of the cellular substance before-mentioned,
which connects the whole into a uniform mass^

Emily, Pray, why are the lungs always spvken of in the plural
number!* Are theremore than one ?

Mrs, B, Yes ; forlBough they form but one organ, they really
consist of two .compartments, csdled lobes^ which are enclosed in sep-
arate membranes or bags, each occupying one side of Ihe 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
p^ess of respiration till the first is healed.
/The blood,)thus completed, by the process of respiration, forms
die 4nost complex of all animal compounds, since it contains not on-
ly the numerous materials necessary to form the various secretions,
as saliva, tears, &c. but likewise all those that are required to nour-
ish the several parts of the body, as the muscles, bones, nerves,
glands, &c.f

^ It is not absolutely certain that the change which the blood un-
dergoes in the lungs is entirely owine to the loss of carbon ; since
experiments shew that any animal 8ut)stance, even the hand, when
confined in a portion of atmospheric air, lessens the quantity of ox-
ygen and produces a corresponding' quantity of carbonic acid. It
IS possible then, that the carbon produced by respiration, may be
owJDg merely to the contact between the air and the lungs.— C.

-fThe process of secretion does not consist merely in the separa-
tion of certain materials from the blood by the secreting organ ;
' but in many instances, entirely new products are formed, no traces
of which ha^e been detected m the bloodJ For instance, the solid
matter of the bones is derived from the blood, yet not a particle of
phosphat of lime, (a substance composing the basis of the bone,) is
found in it. It appears, then, that the gland* which are the organs
of secretion, have the power of producing from the ultimate atoms
of the blood, the variety of products peculiar to each. Thus, the

f lands situated about the eyes secrete the tears^ a saline, pellucid
uid ; while the liver secretes from the same source, the bile, a
greenish, opake, bitter, and extremely nauseous substance. It is
most probable that we shall ever remain in profound ignorance, of
any mode of imitating these operations. —

1399. If the blood and air are contained' in sepatate vessels, how
can they come in contact ?

1400. Are the lungs entirely composed ot blood and air vessels ?

1401. Why are the lungs spoken of in the plural number?

1402. What forms the most complex of all the animal compounds?

1403. What is said of secretion in the note / ^ .

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Emiljf. There seerps to be a sing^ular analogy between the blood
of animals and the sap of vegetables ; foi^ch of these fluids coo-
tains the several materials destined for theSratrition of the nutner-
oas class of bodies to -which they respectively belon?^

Mrs. B. Nor is the production of these flaids in Tne animal and
veg^etable 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 itscircalation ; 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 f ^

Mrs, B, u^rom a peculiar organization, the secret of which no
one has yeH)een able to unfold) But it must be ultimately hj
means of the vital principle that ooth their mechanical and chemi-
cal 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 ;^t is performed by the extremities of th^ arteries,
which penetrateih rough the skin and terminate under the cuticle,
through the pores of which the perspiration issu^^ (When this flu-
id is not secreted in excess, it is insensible^hecmsett is dissolved
by the air as it exudes from the pores); Tbut when it is secreted
faster than it can be dissolved it becormi muibUf as it assumes its
liquid stateS

Emily, This secretion bears a striking resemblance to the trans-
piration of the sap of plants. They both consist of the most fluid
parts, and both etude 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
()een ascertained that the sap returns into the roots of the plants,
the lesemblance between the animal and vegetable circulation is
become still more obvious. The laiter, however, is far from being
complete, since, as we observed before, it consists only in arising
and descending of the sap, whilst in animals the blood actually cir-
eulates through every part of the system.

We have now, f 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.

1404. What analogy is there between the blood of animals and
^vegetables ?

1405. Whence do the several organs derive thoir respective
powers ?

1406. How does perspiration take place ?

1407. When is perspiration insensible ? r^^^r^l^

1408. When does it become seotible ? °^ * "^' '' boogie




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.
Fornn respiration, as in most cases of combustion, the air suffers a
change, and a portion of its oxygen combines with carbon, pro-
ducing carbonic acid gas^

Mrs, B. I am much pleased that this idea has occurred to you ;
these two processes appear so very analagous, that it has been sup-
posed 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 in-
deed ! But, Mrs. ^., 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. 1 was going to make the same objection. — ^Yet I do not
conceive how the oxygen can combine with the carbon, and pro-
duce carbonic acid vvithout disengaging heat ?

Mrs. B. The fact is that heat is disengaged.^* Whether any light
be evolved, 1 cannot pretend to determine ;/but that heat is pro-
duced in considerable and^very sensible quantities is'certain.; and
this is the principal, if not the only source of animal heat^

Emily. How wonderful, that the very process which purines and
elaborates the blood, should afford an inexhaustible supply of in-
ternal heat ?

Mrs. B. This is the theory of animal heat in its original simplici-
ty, such nearly as it was first proposed by Black and Liavoisier. It
appeared equally clear and ingenious ; and was at firsts enerally
adopted. But it was objected on second eonsidcratioriythat if the
whole of the animal heat was evolved in the lungs, it would necessa-
rily be much less in the extremities of the body, than immediately at
its source ; which is not found to be the cas^ This objection, how-
ever, which was by no means frivolous, is now satisfactorily removed
by the following consideration t-fVenous blood has been found by

* It has been calculated that the beat produced by respiration in
12 hours, in the lungs of a healthy person, is such as would melt
about 100 pounds of ice.

1409. What analogy is there between respiration and com-

1410. What is the principal source of animal heat ?

1411. What objection has been made to the hypothesis which
ascribes animal heat to respiration '

1412. If the whole of animal heat is evolved in the lungs, why is
it not less at the extremities of the body than at its source f t

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experiment to have leas capacity for heaty than arterial blood ;
wbeoce it follows that the blood, in g^radually passing from the ar-
terial to the renous state, during the circulation, parts with a por-
tion of caloric, by means of whi^h heat is diffused through every

Online LibraryMrs. (Jane Haldimand) MarcetConversations on chemistry .. → online text (page 38 of 43)