chest is air-tight, so that no air can get in between the outside of the
lungs and the inside of the chest wall. You know from your Physics
Primer (Art. 29, p. 34) that the atmosphere is always pressing on
everything. It is pressing on all parts of the rabbit; it presses on the
inside of the windpipe and on the inside of the lungs. It presses on
the outside of the abdomen, and so presses on the under surface of the
diaphragm, and drives it up into the chest as far as it will go. But it
will not go very far, because its edges are fastened to the firm walls
of the chest. The air also presses on the outside of the chest, but
cannot squeeze that much, because its walls are stout.
If the walls of the chest were soft and flabby, the atmosphere would
squeeze them right up, and so through them press on the outside of the
lungs; since they are firm it cannot. The chest walls keep the pressure
of the atmosphere off the outside of the lungs.
The lungs then are pressed by the atmosphere on their insides and not on
their outsides; and it is this inside pressure which keeps them on the
stretch or expanded. When you blow into a bladder, you put it on the
stretch and expand it because the pressure of your breath inside the
bladder is greater than the pressure of the atmosphere outside the
bladder. If, instead of making the pressure inside _greater_ than that
outside, you were to make the pressure outside _less_ than that inside,
as by putting the bladder under an air-pump, you would get just the same
effect; you would expand the bladder. That is just what the chest walls
do; they keep the pressure outside the lungs less than that inside the
lungs, and that is why the lungs, as long as the chest walls are sound,
are always expanded and on the stretch.
When you make a hole into the chest, and let the air in between the
outside of the lungs and the chest wall, the pressure of the atmosphere
gets at the outside of the lungs; there is then the same atmospheric
pressure outside as inside the lungs; there is nothing to keep them on
the stretch, and so they shrink up to their natural size, just as does
the bladder when you leave off blowing into it, or when you take it out
of the air-pump.
When before you made the hole in the diaphragm you pulled the diaphragm
down, you =still further lessened the pressure on the outside of the
lungs=; hence the pressure inside the lungs caused them to swell up and
follow the diaphragm. But this put the lungs still more on the stretch,
so that when you let go the diaphragm and ceased to pull on it, the
lungs went back again to their former size, emptying themselves of part
of their air and pulling the diaphragm up with them. When there is a
hole in the chest wall, pulling the diaphragm down does not make any
difference to the pressure outside the lungs. They are then always
pressed upon by the same atmospheric pressure inside and outside, and so
remain perfectly quiet.
=When in an air-tight chest the diaphragm is pulled down, the pressure of
the atmosphere drives air into the lungs through the windpipe and swells
them up. When the diaphragm is let go, the stretched lungs return to
their former size, emptying themselves of the extra quantity of air
which they had received.=
Suppose now the diaphragm were pulled down and let go again regularly
every few seconds: what would happen? Why, every time the diaphragm went
down a certain quantity of air would enter into the lungs, and every
time it was let go that quantity of air would come out of the lungs
again.
[Illustration: FIG. 12.--_The Diaphragm of a Dog viewed from the Lower
or Abdominal Side._
_V.C.I._ the vena cava inferior; _O._ the œsophagus; _Ao._ the
aorta; the broad white tendinous middle (_B_) is easily
distinguished from the radiating muscular fibres (_A_) which pass
down to the ribs and into the pillars (_C_ _D_) in front of the
vertebræ.
]
This is what does take place in breathing or respiration. Every few
seconds, about seventeen times a minute, the diaphragm does descend, and
a quantity of air rushes into the lungs through the windpipe. This is
called =inspiration=. As soon as that has taken place, the diaphragm
ceases to pull downwards, the stretched lungs return to their former
size, carrying the diaphragm up with them, and squeeze out the extra
quantity of air. This is called =expiration=.
As the diaphragm descends it presses down on the abdomen; when it ceases
to descend, the contents of the abdomen help to press it up. If you
place your hand on your stomach, you can feel the abdomen bulging out
each time the diaphragm descends in inspiration, and going in again each
time the diaphragm returns to its place in expiration.
=41.= But what causes the diaphragm to descend?
If you look at the diaphragm of the rabbit (or of any other animal) a
little carefully, you will see that it is in reality a flat thin muscle,
rather curiously arranged; for the red fleshy muscular fibres are on the
outside all round the edge (Fig. 12, _A_ and _C_), while the centre _B_
is composed of a whitish transparent tendon. These muscular fibres, like
all other muscular fibres, have the power of contracting. What must
happen when they contract and become shortened?
When these muscular fibres are at rest, as in the dead rabbit, the whole
diaphragm is arched up, as we have seen, towards the thorax, somewhat as
is shown in Fig. 13, _B_. It is partly pushed up by all the contents of
the abdomen (for the cavity of the abdomen, you will remember, is quite
filled by the liver, stomach, intestines, and other organs), partly
pulled up by the lungs, which, as we know, are always on the stretch.
When the muscular fibres contract, they pull at the central tendon (just
as the biceps pulls at its lower tendon), =and pull the diaphragm flat=;
and some of the fibres, such as those at _C_, Fig. 12, also pull it
=down=. =The diaphragm during its contraction is flattened and descends=,
somewhat as is shown in Fig. 13, _A_.
[Illustration: FIG. 13.--_Diagrammatic Sections of the Body in_
_A._ inspiration; _B._ expiration. _Tr._ trachea; _St._ sternum;
_D._ diaphragm; _Ab._ abdominal walls. The shading roughly
indicates the stationary air. The unshaded portion at the top of
_A_ is the tidal air.
]
=The descent of the diaphragm in inspiration is caused by a contraction
of its muscular fibres. During expiration the diaphragm is at rest; its
muscular fibres relax; and it goes up because it is partly drawn up by
the lungs, partly pushed up by the contents of the abdomen.=
=42.= Other structures besides the diaphragm assist in pumping air in and
out of the lungs. By the action of the diaphragm the chest is
alternately lengthened and shortened. But if you watch anyone, and
especially a woman, breathing, you will notice that with every breath
the chest rises and falls; the front of the chest, the sternum, as you
have learnt to call it, comes forward and goes back; and a little
attention will convince you that it comes forward during inspiration,
_i.e._ while the diaphragm is descending, and falls back during
expiration. But this coming forward of the sternum means a widening of
the chest from back to front, and the falling back of the sternum means
a corresponding narrowing. So that while the chest is being lengthened
by the descent of the diaphragm, it is also being widened by the coming
forward of the sternum. In inspiration the lungs are expanded not only
downwards, by the movement of the diaphragm, but also outwards, by the
movement of the walls of the chest.
What thrusts forward the sternum? If you were to watch closely the sides
of the chest of a very thin person, you would be able to notice that at
every breathing in, at every inspiration, the ribs are pulled up a
little way. Now, each rib is connected with the backbone behind by a
joint, and is firmly fastened to the sternum in front by cartilage (see
Frontispiece). If you were to fasten a piece of string to the middle of
one of the ribs and to pull it, you would find you were working on a
lever, with the fulcrum at the backbone, with the weight acting at the
sternum, and the power at the point where your string was tied. Every
time you pulled the string the rib would move on its fulcrum at the
backbone, in such a way that the front end of the rib would rise up, and
the sternum would be thrust out a little. When you left off pulling, the
sternum, which in being thrust forward had been put on the stretch,
would sink back, and the rib would fall down to its previous position.
[Illustration: FIG. 14.--_View of Four Ribs of the Dog with the
Intercostal Muscles._
_a._ The bony rib; _b_, the cartilage; _c_, the junction of bone
and cartilage; _d_, unossified; _e_, ossified, portions of the
sternum. _A._ External intercostal muscle. _B._ Internal
intercostal muscle. In the middle interspace, the external
intercostal has been removed to show the internal intercostal
beneath it.
]
Between the ribs are certain muscles called =intercostal muscles= (Fig.
14). The exact action of these you will learn at some future time.
Meanwhile it will be enough to say that they act like the piece of
string we are speaking of. =When they contract, they pull up the ribs and
thrust out the sternum; when they leave off contracting, the ribs and
sternum fall back to their previous position.=
There are many other muscles which help in breathing, especially in hard
or deep breathing, but it will be sufficient for you to remember that in
ordinary breathing there are two chief movements taking place exactly at
the same time, by means of which air is drawn into the chest, both
movements being caused by the contraction of muscles. First, the
diaphragm contracts and flattens itself, making the chest deeper or
longer; secondly, at the same time the ribs are raised and the sternum
thrust out by the contraction of the intercostal muscles, making the
chest wider. But as the chest becomes wider and longer, the lungs become
wider and longer too. In order to fill up the extra room thus made in
the lungs, air enters into them through the windpipe. This is
=inspiration=. But soon the diaphragm and the intercostal muscles cease to
contract; the diaphragm returns to its arched condition, the ribs sink
down, the sternum falls back, and the extra air rushes back again out of
the lungs through the windpipe. This is =expiration=. An inspiration and
an expiration make up a whole breath; and thus we breathe some seventeen
times in every minute of our lives.
=43.= But what makes the diaphragm and intercostal muscles contract and
rest in so beautifully regular a fashion? The biceps of the arm, we saw,
was made to contract by our will. It is not our will, however, which
makes us breathe. We breathe often without knowing it; we breathe in our
sleep when our will is dead; we breathe whether we will or no, because
we cannot help it. We can quicken our breathing, we can take a short or
deep breath as we please, we can change our breathing by the force of
our will; but the breathing itself goes on without, and in spite of, our
will. It is =an involuntary act.=
Though breathing is not an effort of the will, it is an effort of the
brain; an effort, too, of one particular part of the brain, that part
where the brain joins on to the spinal cord. Nerves run from the
diaphragm and the intercostal and other muscles through the spinal cord,
to this part of the brain. And seventeen times a minute a message comes
down along these nerves, from the brain, bidding them contract; they
obey, and you breathe. Why and how that message comes, you will learn at
some future time. When your head is cut off, or when that part of the
brain which joins on to the spinal cord is injured by accident or made
powerless by disease, the message ceases to be sent, and you cease to
breathe.
=44.= At every breath, then, a certain quantity of air goes in and out of
the chest; but only a small quantity. =You must not think the lungs are
quite emptied and quite filled at each breath.= On the contrary, you only
take in each time a mere handful of air, which reaches about as far as
the large branches of the windpipe, and does not itself go into the
air-cells at all. This is often called =tidal= air; and the rest of the
air in the lungs, which does not move, is often called the =stationary=
air (see Fig. 13).
How then does the carbonic acid at the bottom of the lungs get out? How
do the capillaries in the air-cells get their fresh oxygen?
The stationary air mingles with the tidal air at every breath. If you
want to ventilate a room, you are not obliged to take a pair of bellows
and drive out every bit of the old air in the room, and supply its place
with new air: it will be enough if you open a window or a door and let
in a draught of pure air across one corner, say, of the room. That
current of pure air flowing across the corner will mingle with all the
rest of the air until the whole air in the room becomes pure; and the
mingling will take place very quickly. So it is in the lungs. The tidal
air comes in with each inspiration as pure air from without; but before
it comes out at the next expiration it gives up some of its oxygen to
the stationary air, and robs the stationary air of some of its carbonic
acid. For each breath of tidal air the stationary air is so much the
better, having lost some of its carbonic acid and gained some fresh
oxygen. The tidal air rapidly purifies the stationary air, and the
stationary air purifies the blood.
Thus it comes to pass that the tidal air, which at each pull of the
diaphragm and push of the sternum goes into the chest as pure air with
twenty-one parts oxygen to seventy-nine parts nitrogen in every hundred
parts, comes out, when the diaphragm goes up and the sternum falls back,
as impure air with only sixteen parts oxygen, but with five parts
carbonic acid to seventy-nine of nitrogen. That lost oxygen is carried
through the stationary air to the blood in the capillaries, and the
gained carbonic acid came through the stationary air from the blood in
the capillaries. So each breath helps to purify the blood, and the
pumping of air in and out of the chest changes the impure, hurtful,
venous, to pure, refreshing, arterial blood; the blood breathes air in
the lungs, that all the body may in turn breathe blood.
HOW THE BLOOD IS CHANGED BY FOOD: DIGESTION. § VII.
=45.= The blood is not only purified by air, it is also renewed and made
good by food. The food we eat becomes blood. But our food, though
frequently moist, is for the most part solid. We cut it into small
pieces on the plate, and with our teeth we crush and tear it into still
smaller morsels in our mouth. Still, however well chewed, a great deal
of it, most of it in fact, is swallowed solid. In order to become blood
it must first be dissolved. It is dissolved in the alimentary canal, and
we call the dissolving =digestion=. Let us see how digestion is carried
on.
Your skin, though sometimes quite moist with perspiration, is as
frequently quite dry. The inside of your mouth is always moist--very
frequently quite filled with fluid; and even when you speak of it as
being dry, it is still very moist. Why is this? The inside of your mouth
is also very much redder than your skin. The redness and the moisture go
together.
In speaking of the capillaries, I said that almost all parts of the body
were completely riddled with them, but =not quite all=. A certain part of
the skin, for instance, has no capillaries or blood-vessels at all. You
know that where your skin is thick, you can shave off pieces of skin
without “fetching blood;” if your
[Illustration: FIG. 15.--_Section of Skin, highly magnified._
_a_, horny epidermis; _b_, softer layer; _c_, dermis; _d_,
lowermost vertical layer of epidermic cells; _e_, cells lining the
sweat duct continuous with epidermic cells; _h_, corkscrew canal of
sweat duct. To the right of the sweat duct the dermis is raised
into a papilla, in which the small artery, _f_, breaks up into
capillaries, ultimately forming the veins, _g_.
]
knife were very sharp and you very skilful, you might do the same in
every part of your skin. If you were to put some of the skin you had
thus cut off under the microscope, you would find that it was made up of
little scales. And if you were to take a very thin upright slice running
through the whole thickness of the skin, and examine that under a high
power of the microscope, you would find that the skin was made up of two
quite different parts or layers, as shown in Fig. 15. The upper layer,
_a_, _b_, is nothing but a mass of little bodies packed closely
together. At the top they are pressed flat into scales, but lower down
they are round or oval, and at the same time soft. They are called
=cells=. As you advance in your study of Physiology you will hear more and
more about cells. This layer of cells, either soft and round, or
flattened and dried into scales, is called the =epidermis=. No
blood-vessel is ever found in the epidermis, and hence, when you cut it,
it never bleeds. As long as you live it is always growing. The top
scales are always being rubbed off. Whenever you wash your hands,
especially with soap, you wash off some of the top scales; and you would
soon wash your skin away, were it not that new round cells are always
being formed at the bottom of the epidermis, along the line at _d_ (Fig.
15), and always moving up to the top, where they become dried into
scales. Thus the skin, or more strictly the epidermis, is always being
renewed. Sometimes, as after scarlet fever, the new skin grows quickly,
and the old skin comes away in great flakes or patches.
The lower layer below the epidermis is what is called the =dermis=, or
=true skin=. This is full of capillaries and blood-vessels, and when the
knife or razor gets down to this, you bleed. It is not made up of cells
like the epidermis, but of that fibrous substance which you early learnt
to call connective tissue (see p. 9). Its top is rarely level, but
generally raised into little hillocks, called =papillæ=, as in the figure;
the epidermis forming a thick cap over each papillæ, and filling up the
hollows between them. Most of the papillæ are full of blood-vessels.
Now, then, I think you will understand why your skin is not red, but
flesh-coloured, and why it is generally dry. The true skin under the
epidermis is always moist, because of the blood-vessels there; the waste
and fluid parts of the blood pass readily through the walls of the
capillaries, as you have learnt, by osmosis, and so keep everything
round them moist. But this moisture is not enough to soak through the
thick coating of epidermis, and so the top part of the epidermis remains
dry and scaly.
The true skin underneath the epidermis is always red; you know that if
you shave off the surface of your skin anywhere, it gets redder and
redder the deeper you go down, even though you do not fetch blood. It is
red because of the immense number of capillaries, all full of red blood,
which are crowded into it. When you look at these capillaries through a
great thickness of epidermis, the redness is partly hidden from you, as
when you put a sheet of thin white paper over a red cloth, and the skin
seems pink or flesh-coloured; and where the epidermis is very thick, as
at the heel, the skin is not even pink, but white or yellow, more or
less dirty according to circumstances.
=46.= But if the moist true skin is thus everywhere covered by a thick
coat of epidermis, which keeps the moisture in, how is it that the skin
is nevertheless sometimes quite moist, as when we perspire?
[Illustration: FIG. 16.--_Coiled end of a Sweat Gland, Epithelium not
shown._
_a_, the coil; _b_, the duct; _c_, network of capillaries, inside which
the duct gland lies.]
If you look at Fig. 15, you will see that the epidermis is at one point
pierced by a canal (_h_) running right through it. You will notice that
this canal is not closed at the bottom of the epidermis, but runs right
into the dermis or true skin, where the canal becomes a tube, with just
one layer (_e_) of cells, like the cells of the epidermis, for its
walls. There is no room in Fig. 15 to show what becomes of this tube,
but it runs some way down under the skin all among the blood-vessels,
and then twisting itself up into a knot, ends blindly, as is shown in
Fig. 16, where _b_ is a continuation on a smaller scale of the same
tube which is seen in Fig. 15. This knot is covered by a close network
of capillaries, which at _c_ are supposed to be unravelled and taken
away from the knotted tube in order to show them. The capillaries, you
will understand, though inside the knot, are always outside the tube. If
you were to drop a very diminutive marble in at _h_ (Fig. 15), it would
rattle down the corkscrew passage through the thick epidermis, shoot
down the straight tube _b_ (Fig. 16), and roll through the knot _a_,
until it came to rest at the blind end of the tube. Along its whole
course it would touch nothing but cells, like the cells of the
epidermis, a single layer of which forms the walls of the tube where it
runs below the epidermis. If it got lodged at _h_ (Fig. 15), or got
lodged in the knot at _a_ (Fig. 16), it would in both cases be touching
epidermic cells. But there would be this great difference. At _h_ it
would be ever so far removed from any blood capillary; at _a_ it would
only have to make its way through a thin layer of single cells, and it
would be touching a capillary directly. At _h_ it might remain dry for
some time; at _a_ it would get wet directly, for there is nothing to
prevent the fluid parts of the blood oozing out through the thin wall of
the capillaries, and so through the thin wall of the tube into the canal
of the tube, on to the marble.
In fact, the inside of the knot is always moist and filled with fluid.
When the capillaries round the knot get over-full of blood, as they
often do, a great deal of colourless watery fluid passes from them into
the tube. The tube gets full, the fluid wells up right into the
corkscrew portion in the thickness of the epidermis, and at last
overflows at the mouth of the tube over the skin. We call this fluid
sweat or =perspiration=. We call the tube with its knotted end =a gland=;
and we call the act by which the colourless fluid passes out of the
blood capillaries into the canal of the tube, =secretion=. We speak of the
=sweat gland secreting sweat out of the blood brought by the capillaries
which are wrapped round the gland=.
=47.= Now we can understand why the inside of the mouth is red and moist.
The mouth has a skin just like the skin of the hand. There is an outside
epidermis, made up of cells and free from capillaries, and beneath that
a dermis or true skin crowded with capillaries. Only the epidermis of
the mouth is ever so much thinner than that of the hand. The red
capillaries easily shine through it, and their moisture can make its way
through. Hence the mouth is red and moist. Besides there are many glands
in it, something like the sweat gland, but differing in shape; these
especially help to keep it moist.
Because it is always red and moist and soft, the skin of the inside of
the mouth is generally not called a skin at all, but =mucous membrane=,
and the upper layer is not called epidermis, but =epithelium=. You will
remember, however, that a mucous membrane is in reality a skin in which
the epidermis is thin and soft, and is called epithelium.
The mouth is the beginning of the alimentary canal. Throughout its whole
length the alimentary canal is lined by a skin or mucous membrane like
that of the mouth, only over the greater part of it the epithelium is
still thinner than in the mouth, and indeed is made up of a single
layer only of cells. The whole of the inside of the canal is therefore
red and moist, and whatever lies in the canal is separated by a very
thin partition only from the blood in the capillaries, which are found
in immense numbers in the walls of the canal. The alimentary canal is,
as you know, a long tube, wide at the stomach but narrow elsewhere. In
all parts of its length the tube is made up of mucous membrane on the
inside, and on the outside of muscles, differing somewhat from the
muscles of the body and of the heart, but having the same power of
contracting, and by contracting of squeezing the contents of the tube,
just as the muscles of the heart squeeze the blood in its cavities. The
muscles, and especially the mucous membrane, are crowded with
blood-vessels.