Mrs. (Jane Haldimand) Marcet.

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which is taken with ,the food is seldom assimilated, exceptingl^hen
the female nourishes her young ; it is then all secrated into the milk,
as a, provision for the tender bones of the niirslinn ' "

Emily, So that whatever becomes superfluoira in one being, is
immediately wanted by another ; and the child acquires strength
precisely by the species of nourishment which is no longer necessa-
ry 1o the mother. Nature is indeed, an admirable ^onomist I

Caroline. Pray, Mrs. B., does, not the disease in the bones of
children, called the rickets, proceed from a deficiedcy of phosphat
of lime ? ' y* .

Mrs, B, I liave heard that(tbis disease mav arise from two causes ;
it is sometimes occasioned by the growth of the muscles* being too
rapid in proportion to that of the bonesT) In this case the weig^bt
of the flesh is greater than the bones can s\]pport, and presses upon
them so as toprotiiuoe a swelling of the joints, which is the |^reat
indication of the rickets.
fThe other cause of this disorder, is supposed to be an imperfect di-



1333. What are the ingredients that enter into the composition
of bones?

1334. What gives to bones their hardness and durability ?

1335. What is the state of jK>Des in the early periods of apimal life?

1336. Whence is the phosphat of lime necessary in the formation
of bones obtained ?

1337. How are teeth formed ?

1338. What gives the horns of quadrupeds and the beaks and
quills of birds their hardness ?

1339. When is the phosphat of lime assimilated in adult animals f

1340. How is the disease called rickets occasioned/^^^w|^

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OF THE AKIM AL ECONOinr. 297

gfestioa and assimilation of the food, attended with an excess of
acid, ivtiich couoteracts the formation of phosphat of liinS^ In both
instances, the re fore/care should be taken to alt^r the ^ild^ dint,
not merely by incre^ng^ the quantity of aliment containingp phos-
phat of lime, but also by avoiding' all food that is apt to turn acid on
the stomach, and to produce indigestion. But the best prca^nra-
tive agtiinst complaints of this kind, is no doubt, good nuraloff :
when a child has plenty of air and exereisev the digestion and as-*
similation will be properly performed, no acid will be produced to
interrupt thesefunctionSj^nd the muscles and bones will grow to-
g-etlier in just proportiocS^ /

Caro/in<?. I hare often neard the rickets attributed to bad nurs-
ing^, but I never coold have guessed what connexion there Was be-
tween exercise and the formation cf the bones*

«A/r«. B. Exercise is generally beneficial to all the animal func-
tions. If man is destined to labour for his subsistence, the bread
which he earns is scarcely n>or& essential to his health and preserv-
ation than -the exertions by which he obtains it. Those whom
the gifts of fortune have placed above the necessity of bodily la-
boTir, are compelled to take exercise in some mode or other, and
when they cannot convert it into an amusement, they must submit
to it as a tisisk, or their health will soon experience the effects of
their indolence.

Emily. That will qever be my case ; for exercise, unless it, be-
comes fatii^ue, always gives me pleasure ; and^o far from bein^ a
task, is to me a source of daily enjoyment. I often think what a
blessing' it is, that exercise, which is so conducive to health, should
be so delightful ; whilst fatigue^ which is rather hurtful instead of
pleasure, occasions painful sensations. So that fatigue, no doubt,
was intended to moderate our bodily exertions, sis satiety puts a
limit to our appetites.

Mrs.B, Certainly. ^But let us not deviate too far from our sub-
ject. (The bones are connected together by li^ments, which con<h
sist 0)f a white, (hick flexible substance, adhering to their extremi-
ties so far a^i^oj^ure the joints firmly, though without impeding
their motiorp (A nd the jomts are, moreover, covered by a solid,
smooth, elastic^hite substance, called cartHage, th^ use of which
is to allow, by its smoothness and elasticity-, the bones to slid^ easily
over one another, so that the joints may peWbrm their office.without
difficulty or detriment)

(pver the bones ihe'wiutclei are placed ; they consist of bundles
oPfibres, which terminate in a kind of string, or ligament, by
which they are fastened to the bones'. • The muscles are the organs
bf motion ; by their power of dilatation and contraction, they p^t
into action the bones, which apt as levers in all motions of the body,
and form the solid support of its various parts!\ The muscles are
of various degreesof strength or consistence, 'In different species
of animals. Tlie mammiferous tribe, or those that suckle.their
young, seem, in this respect, to occupy an intermediate place be-
tween birds and cold-blooded animals, such as reptil(»s and fishes.

Emily, The different degrees of firmness and solidity in the; mus-

1341. What precaution may be taken against this disease .'

1342. How are the bones of an animal system confined together ?

1343. By what arc the joints covered .* ^ I

1344. By what are the organs of motion } Dgtzed byCnOOgle



OF TItt AnnCAJL ECONOMT.

cles of these several species of animals, proc^, I imagine, from
the different nature of tbe food on which they subsist.

Jtfr«. B. No, that is not supposed to be the case ; for the human
species, who are of the mammifi^rous tribe, live on more substantial
food than birds ; ^and yet the latter exceed them in muscnlar
strength. We shall, hereafter, attempt to acc^ount for this differ-
ence ; but let us now proceed in the examination of the animal
functions. <

The next class of orguns is that of the veueUoi the body, tbe of-
fice of which is to convey the various fluids throughout the frame.
These vessels are innumerable. The most considerable of them
are thos^through which the blood . circulates j which are^ of two
kinds /the aHmgrwhicYi convey it from the heart to the extremi-
ties of The bod^^d the vnm which bring it back into the heai^

Besides theseftEere area numerous set of sipall transparent ves-
sels, destiued^to absorb ana convey different fluids into tbe bloo^
Ibey are generally called the abiorbenl or lympkaiU vessels ; but it
is to a portion of them oply, that the function of conveying into
ibe blood the fluid called lymph is assigned.

Emily. Pray what is the nature of that fluid ?

Mrs. B. The nature and use of the lymph hdve, I believe, ntever
been perfectly ascertained ; but it is supposed to consist of matter
that has been previously animalized, and which after answering the
purpose for which it was intended, must, in regukur rotation, make
way for the fresh aupplies pmduced by nourishment. The lym*
phatic vessels pump up this fluid from every part of the system, and
convey it into the vem» to be mixed with the blood which ^run8
through them, apd which is commonly called ve;nous blood.

Caroline. But does it not again enter into the animal system
through that channel ?

Mrf,B, Not entirely; for the venous blood does not return into
the circulation until it iias undergone a peculiar change, in which
it throws off whatever is become useless.

Another set of absorbent vessels pump up the ehylt fronv tbe
stomach and intestines, and convey it, after many circumvolutions,
into- the great vein near the heart.*

Emily. Pjay, what is chyle?

Jtfiri. J5./lt is the substance into which food is converted by di-
gestion^ \^ ♦ .

Carmne, One set of the absorbent vessels, then, is employed in
bringing away the old materials which are no longer fit for use :
whilst the other set is busy in conveying into the blo<xi the new ma-
terials that are to replace them .'

* This is a mistake. The chyle is conveyed into the trunk of the
absorbent system, called by anatomists the thoracic duct. This runs
in a serpentrae direction along tbe internal side of the back bone,
up to.tbe tu&e^tan vein, which lies under the collar bone. loto
this vein the chyle is discharged, and mixes with the blood, and be-
fore it reaches the heart, it is converted into blood itself.— C.

^^* Eor what purpose are the arteries ?
1 346. For what purpose are the veins ?
i«^Z' ^!*at are lymphatic vessels?. .
1348. What is chyle?

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OF THE AlOHAL ECONOMY. 299

Emily. Whata^reat variety of ing^redieDts moet enter into the
composition of the blood !

Jars, B, You must observe that there is also a great variety of
substaoces to be secreted from it. AVe may compare the blood to a
general reoeptaoleor storehouse for^ll kinds of commodities, which
are afterwards fa6luoned, arranged, and disposed of, as circumstan-
ces requir^

There i«4nother set of absorbent vessels in females, which is des-
tined to secrete milk for thd-noiirisbment of 'the young.

Emily. Pray is not milk very analagous in its composition to
blood ; for, since the nursing derives its nourishment from that
source only, it must 'contain every principle which Ui6 animal sys-
tem requires.



Mrs, B. 'Very true. |(lilk is found, bv its analysis, to contain the
priijcipal materials of «n1inal matter, albumen, oil,- and phosphat of
Hm^ so that the suckling has'but little troubleHo ^digest and assi-



milate this nourishment. But we shall examine the composition of
milk more fully afterwards. , ' '

In many parts of the body, numbers ofsmall vessels are collected
together in little bun(lleK called ^/anJij/from a ILtatin w6rd, mean-
ing acorn, on account OU^ resemblanee which some of them bear
in shape to that fruit) ffl^e (unctions oL the glands is io secrete, or
separate certain maftera from the blood)

The secretions are not only mechanical, but«hemical separations
from the blood ; fbr the substances thus formed, though contained
in the blobd, are not ready combined in that fluid. The secjretions
are o(1wo kind|^ those which form peculiar ^animil fluids/as bile,
tearS} saliva,. ^c<S£ and those which prodocei the general maieriak
of the animal ^yftem, for the purpose of recruitibg and nourishing
the seve^ organs of the bodjf t such fik albumen, gelatine, and
fibrine ; the, latter may be distinguisbea^ by the name of nufyitive
$ecretioni%

Caroline, li&kn quite astonished to bear vthat all the secretions
8h«uld be derived from the bipod.

Emily. I thought that the bile was produced by the liver.

Jdrs. B. So it is ; Imtthe Uvpr is nothing more than a very large
gland, Vhich sdcretesf the' bile fronw the blood.

The last of the animal organs which we have ^mentioned are the
nerves ;miese<fiLre'the«vehicle8 of sen^tion, every other part of the
body beTDg, of itself; tdtidly insensible^

darotine. Th^y must, then, be sprfiid through 'every, partof Uie
frame, for wevre every where susceptible of feeling.

Etfiili/, Exeeptiog the nails and the hair.

Jlfr^.'B. And tho«e are almost the only parts' in which nerves
canoDtbe disiSover^. /The common source ofall the nerves is the
hrainj thence they dcawend, some of them through different aper-
tures^n the skull, but th^^eatest part through the backbone, and
extend themselves by innumerable ramifications throughout the

1349. ToVhatTnay thePblodd be compared?

1350. Of what does milk consist ?

1351. From'.#hat do the gkinde derive their name ?

1352. What is their use ?

1353. How many kinds of secretionirij^ithere, and wb&t are they f

1354. What are the nerve* ? Digitized by GooqIc
^1355. What is the common source ef tfac^nenres ? ^



300 OF TUB ANPUiL BOONOMY.

whole bodj. They spread themselves oyer the niMSoies, pene^rikte
the e^lands, wind round the vascular system, and even pierce iolo
the iQterior of the bones'. It is most probable through them that the
communicatioQ is carried on betiveen the mind^and the other parti
of the body ; but in what manner they are aoted on by the mind,
and made to. react on the body, is still a profound secret. Many
hypotheses have \)een formed on this very obscure subject, but ihey
are. all equally improbable, and it would be useless for us to waste
our time in conjeqtures on an inquiry, which, in all probability, is
beyond the reach of human capacity.

Caroline, But you ha?e not mentioned those, particular nerves
that form the senses of hearing', seeing, smelling, and tasting?

Mrs. B, They are considered as being of the same nature as
those which are dispersed over every part of the body, and consti-
tute the general sense of feeling. The different sensations which
they producCtiTrise from their peculiar situation and connexion with
the several organs of taste, sfnell, and hearing^

Emily. But these senses appear totally dimsrentfrom that of feel*
ing? ,

Mrs. B, They are all of them sensations, but variously modified-
according to the nature of the different organsr in which the nerves
are situated. For, as we have formerly observed, it is by contact
only that the nerves are affected. —Thus odoriferous particles must
strike upon the nerves of the nose, in order to excite the bense of
smelling ; in ihe same manner that taste is produced by the parti-
cular stibstance coming in contact with the nerves of the tongue.
It is thus also that the sensation of sound is. produced by. the concus-
sion of the air striking against the auditory nerve; and si^ht-is the
effect of the light falling upon the optic nerve. These various J^en-
ses, therefore, are affected only by the actual Contact of the particles
of matter, in the same manner as that of feeling.

The different organs bf the animal body, though easily separated
and perfectly diStinct(^re loosely connected together by^akind of
spongy substance, in texture somewhat resemblmg net- work, call-
ed the cellular membrane ; and the whole is covered by the skin^

The Mfi. asivell as the bark of vegetables, is formed o£^fH«e
- coats. The external on^ is called the cuticle or epidermii ; \he se-
'Cond which is called i}ne(mucous minibranSi ts of a thin, soft texture,
and consists of a mucpus substance, which, in ne^pes is black, and
is the caus'e of their skin appearing^ of that colouu

Caroline. Is then the external skin of negroes white like ours ?

Mrs. B. Yes ; but asftEe cuticle is transparent, as well as porous,
the blackness of the mucpus membrane is visible through ih The
extremities of:thh nerves are spread over this skin;* so that^e sen-
sation of feeling is transmitted through the cuticle. The jnteniat
covering of the muscles, which is properly the skin, is the thickest,

1356. How are the nerves made subservient to the piirposcs of
hearing, seeing, smelling, and tasting f

1557. By what are the different parts of the animal body connect-
ed together ?

1358. Of how many coats is the skin formed, and what are thev
called ? "^ »

1359. Where is the colour of the sk-n?

.1360. If the.colour is in the second coat, why is it so easily seexr?



OF THE ANIMAL ECONOMY. 301

»
the toug^hest, and the most resisting of the whole ; i^ is this memr
brane which is so essential in the arts, bj forming leather when
combined with tannin.

The skin which covers the animal bodj, as well as those mem-
branes that form the coats of* the vessels, (consists almost exclusive-
ly of gelatine ; and is capable of being converted into glue, size,
or jelm

Th^^^vities between the muscles and the skin are usually filled
with fat, which lodges in the cells of the membranous net before
mentioned, and gives to the external form (especially in the human
figure) that roundness, smoothness, and softness, so essential to
beauty.

Emily, And the skin itself is, I think, a very ornamental part of
the human frame, both from the fineness of its texture, and toe va-
riety and delicacy of its tihts.

Jttrf. J3. This variety and harmonious gradatwp of colours, pro-
ceed, not so much from the skin itself, aS frowfme internal organs
which transmit their several colours through it; these tints being
only softened and blended bv the colour of the skin, which is uni-
formly of a yellowish whit^

Thus modified, the darlfiiess of the veins appears of a pale blue
colour, and the floridness of .the arteries is changed to a delicate
pink. In the mo^t transparent parts, (be skin exhibits the bloom of
the rose, whilst where it is more opaque, its own colour predomin-
ates ; aqd at the joints, where the bones are most prominent, their
whiteness is often discernible. . In a word, every part of the human
frame seems to contribute to its external ornament ; and this not
merely by producing a pleasing variety of tints, but by apeculiar
kind of beauty which belongs to each individual part. Thus it is
to the solidity and arrangement of the bones that the human figure
owes the grandeur of its stature, and its fiitn and dignified deport^ .
ment. The muscles delineate the fbrm, and stamp it with energy
and grace, and the soft substance which is spread over them smooths
their ruggednete, and gives to the contour the gentle undulations
of the line qC beauty. Everv^ organ "of sense is a peculiar and sep-
arate ornament ; and the skin, which polishes the surface, and
gives it that charm of colouring so inimitable by art, finally con-
spires to render the^ whole the fairest work of the creation.

But now that we have ^n in what manner the animal frame is
form«d, let us observe how it provides for its support, and how the
several organs, which foi^n so complete a whole, are nourished and
maintained.

This will lead bs to a more particular explanation of the internal
organs : here we shall not meet with so much apparent beauty, be-
cause these parts were not intended by nature to be exhibited to
riew ; but the beauty of design, in the internal organization of the
animal frame, is, if possible, still more remarkable than that of the
external parts.

We shall deferthis subject till our next interview.



1361. Of what does the skin consist ?

1362. On what is the human complexion or colour depending be-
sides the skin ? r^^^^u

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ON AIOilALISATION.



CONVERSATION XXV.

^N ANIHALISATION, NUTHITIOBI, AND RESPIKATION^

Mrs. B. We have now learnt of what materials the animal sys-
tem is composed, and have formed some idea of the nature of its or-
ganization. In order to complete the subject, it remains for us to
examine in what manner it is nourished and supported.

Vegetables, we have observed, obtain their nourishment from va-
rious substances, either in their elementary state, or in a very sim-
ple state of combination ; as car>on, water, and salts, which they
pump ap from the soil ; and carbonic acid and oxygen, which they
absorb from the.atmosphere,

Animals, on the contrary, feed on substances of the most compli-
cated kind ; for they derive their sustenance, isome from the animal
creation, others from the vegetable kingdom, and some from both.

Caroline. And there is one species of animals, which, not satisfied
with enjoying either kind of food in its simple state, has invented
the art of combining them together in a thousand ways, and of ren-
dering: even the mineral kingdom subservient to its refinements.

Emily ' Nor is this all -, for our delicacies are collected from the
•various climates of the earth, sp that the four quarters of the globe
are often obliged to contribute to the preparation of our simpleat
dishes.

Caroline. Butthe very complicated substances which constitute
the nourishment of animals, do not, 1 suppose, enter into the sys-
tem in their aictual state of combination ?

Jtfr*. JB./So far-from it, that they not only undergo anew ar-
rangement\>f their, parts, but a selection is made of such as are
' inost proper for the nounslnnent of the body, and those only enter
into the system, and are animalise^
Emily. And by what organs is tWs process performed ?
Mrs. B. Chiefly by the stomach, which is the organ of digestiooi
and the prirt*e regulator of the animal framed

Digetiion is the first step towards nutrition, ift consists in redu-
cing into one homogeneous mass the various substances that are
taken as nourishmeot ; it is performed by first chewing and mixing
the solid aliment with the saliva, which reduces it to a soft mass, in
which state it is conveyed iAto the stomacB^ wherept is more com-
pletely dissolved by the gastric juice.k '. • **. •

This fluid (which is secreted infb the- stomach by appropriate
glands) is so powerful a solvent, that scarcely any substances will
resist its action.

Emily. The coats of the stomach, however, cannot be attacked
by it, otherwise we should be in danger of having them destroyed
when the stomach was empty.

1363. What is the subject of the 25th conversation ?

1364. Do the substances wliich constitute the nourishment of an-
imals enter into their system, in their actual state of combination i

1365. Where is the digestion performed ?
r366» What is the first operatiton in digestion ?
1367. What office is performed by the gastric mice ?

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ON SUPIBATION. 30^

JSrt, B. They are probably not subject to its action \ as leo^, at
least, as life continues. But it appears, thatX^hen the j^tric ji^oe
has no foreign substance to act upon, it is capable of occasioning a
degree of irritation in Ibe coats of the stomach, irbich produces tne
sensation of bunge^ ^he gastric juice, together with the heat and
muscular action of^the stomach, conyerts the aliment into an uni-
form, pulpy mass, called chymej; This passes into the intestines,
where it meets-wilh the bile and some other fluids, by the agencv.of
which, and by the operatioiM>f^ther causes hitherto unknown/lbe
chyme is changed into chylo^ much thinner substance, someirhat
resembling mild -which is'^minped by immense numbers of small
absorbent Fessds spread over the internal surface of the intestines.
These, after many circumvolutions, gradually meet and unite into
lar^e branches, till they at length collect the chyle into one vessel, .
which pours its contents into the great vein near the heart, by which
means the food, thus prepared, enters into the circulation.

Caroline. But I do not yet clearly understand how tb^ blood, thai
formed, nourishes the body and supplies all the secretions?
. Jdrs. B, Before this can be explained to vm, you must first allow
me to complete the formationof the, blood, w^^e chylaroay, indeed,
be considered as forming thechief ingredient of blood? but this fluid
if hot perfect until it has passed through the lungs, and undergone
(together with the blood that has already circulated) certain neces-
sary changes that are effected by rbspiaatioic .

Caroline, I am very glad that you are going to explain the natnre
of respiration : I have often longed to understand it; for though we
talk incessantly o( bireathing, I nevjer knew precisely what purpose
it answered.

JIfri. B, It is, indeed, one of the most intdresting processes ima-
grinable ; but in order to understand this function well, it will be
necessary to enter into some previous explanations. Tell me,
Emily, what do you understand by respiration i

Emily. Respiration, I oonceivei^onsists siroplyJn alternately in-
jptrtfig air into the lungs, and expirthg it from ttie^

Jlfr«. B. Tour answer will do very well as a gf neral definition.
But, in order to form a tolerably clear notion of the various phenom-
ena of respiration, there are many circumstances to b4 taken into
(&)nsideration. ^

In the first place, there are two things to be distingoisbed in res-
piration, the tneehanteal and the diemical part of the process.

The mechanism of breathingmepends on the alternate expansions

id contractions of the chest, m which the lungs are contained)

hen the chest dilates, the cavity is enlarged, and the air rushes m
at Che mouth, to fill up the vacuum formed by this dilatatig^y^hen
it contracts, the cavity is diminished and the air forced ouvVgain.

1368. How is the sensation of hunger produced ?

1369. At what state of animalisation is the aliment called chyme f

1370. Into w^t is it next changed ?

1371. How does chyle differ from chyme f

1372. What forms the chief ingredient of blood ?

1373. What is respiration }

1374. On what depends the mechanism of breathing?

1375. What takes place when the chest dilates ?

1376. What tiakes place when it contracts ?

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aiK

/Wl



304



ON BESPIBATION.



of breatyn;.



Caroline. 1 tboug^ht that.it was the lungs that contracted and ex-
panded in breathing. ^ , ,^

Mrs, B. They do likewise ; butQheir action is only the conse-
quence of that of the chest. The lungs, together with the heart and
largest blood vessels, in a manner fill up the cavity of the chesj:
they could not, therefore, dilate, if the chest did not previously ex-



Online LibraryMrs. (Jane Haldimand) MarcetConversations on chemistry .. → online text (page 37 of 43)