CAROLINE.

They must then be spread through every part of the frame, for we are

every where susceptible of feeling.

EMILY.

Excepting the nails and the hair.

MRS. B.

And those are almost the only parts in which nerves cannot be

discovered. The common source of all the nerves is the brain; thence

they descend, some of them through different holes of the skull, but the

greatest part through the back bone, and extend themselves by

innumerable ramifications throughout the whole body. They spread

themselves over the muscles, penetrate the glands, wind round the

vascular system, and even pierce into the interior of the bones. It is

most probably through them that the communication is carried on between

the mind and the other parts of the body; but in what manner they are

acted on by the mind, and made to re-act on the body, is still a

profound secret. Many hypotheses have been formed on this very obscure

subject, but they are all equally improbable, and it would be useless

for us to waste our time in conjectures on an enquiry, which, in all

probability, is beyond the reach of human capacity.

CAROLINE.

But you have 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 constitute the general sense

of feeling. The different sensations which they produce arise from their

peculiar situation and connection with the several organs of taste,

smell, and hearing.

EMILY.

But these senses appear totally different from that of feeling?

MRS. B.

They are all of them sensations, but variously modified according to the

nature of the different organs 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 sense of smelling; in the same manner that

taste is produced by the particular substance coming in contact with the

nerves of the palate. It is thus also that the sensation of sound is

produced by the concussion of the air striking against the auditory

nerve; and sight is the effect of the light falling upon the optic

nerve. These various senses, therefore, are affected only by the actual

contact of particles of matter, in the same manner as that of feeling.

The different organs of the animal body, though easily separated and

perfectly distinct, are loosely connected together by a kind of spongy

substance, in texture somewhat resembling net-work, called the cellular

membrane; and the whole is covered by the skin.

The _skin_, as well as the bark of vegetables, is formed of three coats.

The external one is called the _cuticle_ or _epidermis_; the second,

which is called the _mucous membrane_, is of a thin soft texture, and

consists of a mucous substance, which in negroes is black, and is the

cause of their skin appearing of that colour.

CAROLINE.

Is then the external skin of negroes white like ours?

MRS. B.

Yes; but as the cuticle is transparent, as well as porous, the blackness

of the mucous membrane is visible through it. The extremities of the

nerves are spread over this skin, so that the sensation of feeling is

transmitted through the cuticle. The internal covering of the muscles,

which is properly the skin, is the thickest, the toughest, and most

resisting of the whole; it is this membrane which is so essential in the

arts, by forming leather when combined with tannin.

The skin which covers the animal body, as well as those membranes that

form the coats of the vessels, consists almost exclusively of gelatine;

and is capable of being converted into glue, size, or jelly.

The cavities 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 the variety and

delicacy of its tints.

MRS. B.

This variety and harmonious graduation of colours, proceed, not so much

from the skin itself, as from the internal organs which transmit their

several colours through it, these being only softened and blended by the

colour of the skin, which is uniformly of a yellowish white.

Thus modified, the darkness of the veins appears of a pale blue colour,

and the floridness of the arteries is changed to a delicate pink. In the

most transparent parts, the skin exhibits the bloom of the rose, whilst

where it is more opake its own colour predominates; and 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 grace; and this not merely by producing a

pleasing variety of tints, but by a peculiar 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 firm and dignified deportment. The muscles delineate

the form, and stamp it with energy and grace; and the soft substance

which is spread over them smooths their ruggedness, and gives to the

contours the gentle undulations of the line of beauty. Every organ of

sense is a peculiar and separate ornament; and the skin, which polishes

the surface, and gives it that charm of colouring so inimitable by art,

finally conspires to render the whole the fairest work of the creation.

But now that we have seen in what manner the animal frame is formed, let

us observe how it provides for its support, and how the several organs,

which form so complete a whole, are nourished and maintained.

This will lead us to a more particular explanation of the internal

organs: here we shall not meet with so much apparent beauty, because

these parts were not intended by nature to be exhibited to view; but the

beauty of design, in the internal organisation of the animal frame, is,

if possible, still more remarkable than that of the external parts.

We shall defer this subject till our next interview.

CONVERSATION XXV.

ON ANIMALISATION, NUTRITION, AND RESPIRATION.

MRS. B.

We have now learnt of what materials the animal system is composed, and

have formed some idea of the nature of its organisation. 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 various

substances, either in their elementary state, or in a very simple state

of combination; as carbon, water, and salts, which they pump up from the

soil; and carbonic acid and oxygen, which they absorb from the

atmosphere.

Animals, on the contrary, feed on substances of the most complicated

kind; for they derive their sustenance, some 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 rendering 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, so that the four quarters of the globe are often

obliged to contribute to the preparation of our simplest dishes.

CAROLINE.

But the very complicated substances which constitute the nourishment of

animals, do not, I suppose, enter into their system in their actual

state of combination?

MRS. B.

So far from it, that they not only undergo a new arrangement of their

parts, but a selection is made of such as are most proper for the

nourishment of the body, and those only enter into the system, and are

animalised.

EMILY.

And by what organs is this process performed?

MRS. B.

Chiefly by the stomach, which is the organ of digestion, and the prime

regulator of the animal frame.

_Digestion_ is the first step towards nutrition. It consists in reducing

into one homogeneous mass the various substances that are taken as

nourishment; 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 into the stomach, where it is more completely dissolved

by the _gastric juice_.

This fluid (which is secreted into 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.

MRS. B.

They are probably not subject to its action; as long, at least, as life

continues. But it appears, that when the gastric juice has no foreign

substance to act upon, it is capable of occasioning a degree of

irritation in the coats of the stomach, which produces the sensation of

hunger. The gastric juice, together with the heat and muscular action of

the stomach, converts the aliment into an uniform pulpy mass called

chyme. This passes into the intestines, where it meets with the bile and

some other fluids, by the agency of which, and by the operation of other

causes hitherto unknown, the chyme is changed into chyle, a much thinner

substance, somewhat resembling milk, which is pumped by immense numbers

of small absorbent vessels spread over the internal surface of the

intestines. These, after many circumvolutions, gradually meet and unite

into large 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 the blood, thus formed,

nourishes the body and supplies all the secretions?

MRS. B.

Before this can be explained to you, you must first allow me to complete

the formation of the blood. The chyle may, indeed, be considered as

forming the chief ingredient of blood; but this fluid is not perfect

until it has passed through the lungs, and undergone (together with the

blood that has already circulated) certain necessary changes that are

effected by RESPIRATION.

CAROLINE.

I am very glad that you are going to explain the nature of respiration:

I have often longed to understand it, for though we talk incessantly of

_breathing_, I never knew precisely what purpose it answered.

MRS. B.

It is indeed one of the most interesting processes imaginable; 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?

EMILY.

Respiration, I conceive, consists simply in alternately _inspiring_ air

into the lungs, and _expiring_ it from them.

MRS. B.

Your answer will do very well as a general definition. But, in order to

form a tolerably clear notion of the various phenomena of respiration,

there are many circumstances to be taken into consideration.

In the first place, there are two things to be distinguished in

respiration, the _mechanical_ and the _chemical_ part of the process.

The mechanism of breathing depends on the alternate expansions and

contractions of the chest, in which the lungs are contained. When the

chest dilates, the cavity is enlarged, and the air rushes in at the

mouth, to fill up the vacuum formed by this dilatation; when it

contracts, the cavity is diminished, and the air forced out again.

CAROLINE.

I thought that it was the lungs that contracted and expanded in

breathing?

MRS. B.

They do likewise; but their action is only the consequence of that of

the chest. The lungs, together with the heart and largest blood vessels,

in a manner fill up the cavity of the chest; they could not, therefore,

dilate if the chest did not previously expand; and, on the other hand,

when the chest contracts, it compresses 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.

MRS. B.

Precisely so. Here is a curious little figure (PLATE XV. Fig. 5.), that

will assist me in explaining the mechanism of breathing.

CAROLINE.

What a droll figure! a little head fixed upon a glass bell, with a

bladder tied over the bottom of it!

MRS. B.

You must observe that there is another bladder within the glass, the

neck of which communicates with the mouth of the figure--this represents

the lungs contained within the chest; the other bladder, which you see

is tied loose, represents a muscular membrane, called the _diaphragm_,

which separates the chest from the lower part of the body. By the chest,

therefore, I mean that large cavity in the upper part of the body

contained within the ribs, the neck, and the diaphragm; this membrane is

muscular, and capable of contraction and dilatation. The contraction may

be imitated by drawing the bladder tight over the bottom of the

receiver, when the air in the bladder, which represents the lungs, will

be forced out through the mouth of the figure--

EMILY.

See, Caroline, how it blows the flame of the candle in breathing!

MRS. B.

By letting the bladder loose again, we imitate the dilatation of the

diaphragm, and the cavity of the chest being enlarged, the lungs expand,

and the air rushes in to fill them.

EMILY.

This figure, I think, gives a very clear idea of the process of

breathing.

MRS. B.

It illustrates tolerably well the action of the lungs and diaphragm; but

those are not the only powers that are concerned in enlarging or

diminishing the cavity of the chest; the ribs are also possessed of a

muscular motion for the same purpose; they are alternately drawn in,

edgeways, to assist the contraction, and stretched out, like the hoops

of a barrel, to contribute to the dilatation of the chest.

EMILY.

I always supposed that the elevation and depression of the ribs were the

consequence, not the cause of breathing.

MRS. B.

It is exactly the reverse. The muscular action of the diaphragm,

together with that of the ribs, are the _causes_ of the contraction and

expansion of the chest; and the air rushing into, and being expelled

from the lungs, are only _consequences_ of those actions.

CAROLINE.

I 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 dilatations and

contractions of the lungs and chest.

MRS. B.

Try the experiment of merely opening your mouth; the air will not rush

in, till by an interior muscular action you produce a vacuum--yes, just

so, your diaphragm is now dilated, and the ribs expanded. But you will

not be able to keep them long in that state. 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 general, a person at rest and in health will breathe between

fifteen and twenty-five times in a minute.

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 _circulation_ of the blood. Tell me, Caroline, what do you

understand by the circulation of the blood?

CAROLINE.

I am delighted that you come to that subject, for it is one that has

long excited my curiosity. But I cannot conceive how it is connected

with respiration. The idea I have of the circulation is, that the blood

runs from the heart through the veins all over the body, and back again

to the heart.

MRS. B.

I could hardly have expected a better definition from you; it is,

however, not quite correct, for you do not distinguish the _arteries_

from the _veins_, which, as we have already observed, are two distinct

sets of vessels, each having its own peculiar functions. The arteries

convey the blood from the heart to the extremities of the body; and the

veins bring it back into the heart.

This sketch will give you an idea of the manner in which some of the

principal veins and arteries of the human body branch out of the heart,

which may be considered as a common centre to both sets of vessels. The

heart is a kind of strong elastic bag, or muscular cavity, which

possesses a power of dilating and contracting itself, for the purposes

of alternately receiving and expelling the blood, in order to carry on

the process of circulation.

EMILY.

Why are the arteries in this drawing painted red, and the veins purple?

MRS. B.

It is to point out the difference of the colour of the blood in these

two sets of vessels.

CAROLINE.

But if it is the same blood that flows from the arteries into the veins,

how can its colour be changed?

MRS. B.

This change arises from various circumstances. In the first place,

during its passage through the arteries, the blood undergoes a

considerable alteration, some of its constituent parts being gradually

separated from it for the purpose of nourishing the body, and of

supplying the various secretions. The consequence of this is, that the

florid arterial 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. In

consequence of 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 body, or other purposes of circulation.

EMILY.

And in this state does it mix in the heart with the pure florid blood

that runs into the arteries?

MRS. B.

No. The heart is divided into two cavities or compartitions, called the

_right_ and _left ventricles_. The left ventricle is the receptacle for

the pure arterial blood previous to its circulation; 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 presently explain.

CAROLINE.

For my part, I always thought that the same blood circulated again and

again through the body, without undergoing any change.

MRS. B.

Yet you must have supposed that the blood circulated for some purpose?

CAROLINE.

I knew that it was indispensable to life; but had no idea of its real

functions.

MRS. B.

But now that you understand that the blood conveys nourishment to every

part of the body, and supplies the various secretions, you must be

sensible that it cannot constantly answer these objects without being

proportionally renovated and purified.

CAROLINE.

But does not the chyle answer this purpose?

MRS. B.

Only in part. It renovates the nutritive principles of the blood, but

does not relieve it from the superabundance of water and carbon with

which it is encumbered.

EMILY.

How, then, is this effected?

MRS. B.

By RESPIRATION. This is one of the grand mysteries which modern

chemistry has disclosed. When the venous blood enters the right

ventricle of the heart, it contracts by its muscular power, and throws

the blood through a large vessel into the lungs, which are contiguous,

and through which it circulates by millions of small ramifications. Here

it comes in contact with the air which we breathe. The action of the air

on the blood in the lungs is, indeed, concealed, from our immediate

observation; but we are able to form a tolerably accurate judgment of it

from the changes which it effects not only in the blood, but also on the

air expired.

The 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 portion of watery vapour and of carbonic acid gas. Hence it

is inferred, that when the air comes in contact with the venous blood in

the lungs, the oxygen attracts from it the superabundant quantity of

carbon with which it has impregnated itself during the circulation, and

converts it into carbonic acid. This gaseous acid, together with the

redundant moisture from the lungs*, 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.

[Footnote *: The quantity of moisture discharged by the lungs in

24 hours, may be computed at eight or nine ounces.]

CAROLINE.

This is truly wonderful! Of all that we have yet learned, I do not

recollect any thing that has appeared to me so curious and interesting.

I almost believe that I should like to study anatomy now, though I have

hitherto had so disgusting an idea of it. Pray, to whom are we indebted

for these beautiful discoveries?

MRS. B.

Priestley and Crawford, in this country, and Lavoisier, in France, are

the principal inventors of the theory of respiration. Of late years the

subject has been farther illustrated and simplified by the accurate

experiments of Messrs. Allen and Pepys. But the still more important and

more admirable discovery of the circulation of the blood was made long

before by our immortal countryman Harvey.

EMILY.

Indeed I never heard any thing that delighted me so much as this theory

of respiration. But I hope, Mrs. B., that you will enter a little more

into particulars before you dismiss so interesting a subject. We left

the blood in the lungs to undergo the salutary change: but how does it

thence spread to all the parts of the body?

MRS. B.

After circulating through the lungs, the blood is collected into four

large vessels, by which it is conveyed into the left ventricle of the

heart, whence it is propelled to all the different parts of the body by

a large artery, which gradually ramifies into millions of small arteries

through the whole frame. From the extremities of these little

ramifications the blood is transmitted to the veins, which bring it back

to the heart and lungs, to go round again and again in the manner we

have just described. You see, therefore, that the blood actually

undergoes two circulations; the one, through the lungs, by which it is

converted into pure arterial blood; the other, or general circulation,

by which nourishment is conveyed to every part of the body; and these

are both equally indispensable to the support of animal life.

EMILY.

But whence proceeds the carbon with which the blood is impregnated when

it comes into the lungs?

MRS. B.

Carbon exists in a greater proportion in blood than in organised animal

matter. The blood, therefore, after supplying its various secretions,

becomes loaded with an excess of carbon, which is carried off by

respiration; and the formation of new chyle from the food affords a

constant supply of carbonaceous matter.

CAROLINE.

I wonder what quantity of carbon may be expelled from the blood by

respiration in the course of 24 hours?

MRS. B.

It appears by the experiments of Messrs. Allen and Pepys that about

40,000 cubic inches of carbonic acid gas are emitted from the lungs of a

healthy person, daily; which is equivalent to _eleven ounces_ of solid

carbon every 24 hours.

EMILY.

What an immense quantity! And pray how much of carbonic acid gas do we

expel from our lungs at each expiration?

MRS. B.

The quantity of air which we take into our lungs at each inspiration, is

about 40 cubic inches, which contain a little less than 10 cubic inches

of oxygen; and of those 10 inches, one-eighth is converted into carbonic

acid gas on passing once through the lungs*, a change which is

sufficient to prevent air which has only been breathed once from

suffering a taper to burn in it.

[Footnote *: The bulk of carbonic acid gas formed by respiration,

is exactly the same as that of the oxygen gas which disappears.]

CAROLINE.

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

MRS. B.