membrane itself is thick, in some places quite as thick as the skin of
the body. This thickness is caused by its being =crowded with glands=. In
the skin the sweat glands are generally some little distance apart, but
in the mucous membrane of the stomach and of the intestines they are
packed so close together, that the membrane seems to be wholly made up
of glands.
These glands vary in shape in different parts. Nowhere are they exactly
like the sweat glands, because none of them are long thin tubes coiled
up at the end in a knot, and none of them have a great thickness of
epidermis to pass through. Most of them are short, rather wide tubes;
some of them are branched at the deep end. They all, however, resemble
the sweat glands in being tubes or pouches closed at the bottom but open
at top, lined by a single layer of cells, and wrapped round with blood
capillaries. From these capillaries, a watery fluid passes into the
tubes, and from the tubes into the alimentary canal. This watery fluid
is, however, of a different nature from sweat, and is not the same in
all parts of the canal. The fluid which is, as we say, secreted by the
glands in the walls of the stomach is an =acid fluid=, and is called
=gastric juice=; that by the glands in the walls of the intestines is =an
alkaline fluid=, and is called =intestinal juice=.
=48.= But besides these glands in the mucous membrane of the mouth, the
stomach, and the intestines, there are other glands, which seem at first
sight to have nothing to do with the mucous membrane.
Beneath the skin, underneath each ear, just behind the jaw, is a soft
body, which ordinarily you cannot feel, but which, when inflamed by what
is called “the mumps,” swells up into a great lump. In a sheep’s head
you would find just the same body, and if you were to examine it you
would notice fastened to it a fleshy cord running underneath the skin
across the cheek towards the mouth. By cutting the cord across you would
discover that what seemed a cord was in reality a narrow tube coming
from the soft body we are speaking of and opening into the mouth. Just
close to the soft body this tube divides into two smaller tubes, these
divide again into still smaller ones, or give off small branches; all
these once more divide and branch like the boughs of a tiny tree; and so
they go on branching and dividing, getting smaller and smaller, until
they end in fine tubes with blind swollen ends. All the tubes, great and
small, are lined with epithelium and wrapped round with blood-vessels,
and being packed close together with connective tissue, make up the soft
body we are speaking of. This body is in fact a =gland=, and is called a
=salivary gland=; as you see it is not a simple gland like a sweat gland,
but is made up of a host of tube-like glands all joined together, and
hence is called a =compound gland=. Being placed far away from the mouth,
it has to be connected with the cavity of the mouth by a long tube,
which is called its =duct=. You cannot fail to notice how like such a
gland is, in its structure, to a lung. The lung is in fact a gland
secreting carbonic acid: and the duct of the two lungs is called the
trachea. The salivary gland beneath the ear is called the =parotid gland=;
there is another very similar one underneath the corner of the jaw on
either side, called the =submaxillary gland=. By each of them a watery
fluid is secreted, which, flowing along their ducts into the mouth and
being there mixed with the moisture secreted by the other glands in the
mouth, is called =saliva=.
In the cavity of the abdomen lying just below the stomach is a much
larger but altogether similar compound gland called =the pancreas=, which
pours its secretion called =pancreatic juice= into the alimentary canal
just where the small intestine begins (Fig. 17, _g._)
That large organ the liver, though the plan of its construction is not
quite the same as that of the pancreas or salivary glands, as you will
by and by learn, is nevertheless a huge gland, secreting from the blood
capillaries into which the portal vein (see p. 62) breaks up, a fluid
called =bile= or =gall=, which by a duct, the =gall duct=, is poured into the
top of the intestine (Fig. 17, _e_). When bile is not wanted, as when
we are fasting, it turns off by a side passage from the duct into the
gall-bladder (Fig. 17, _f_), to be stored up there till needed.
=49.= What are the uses of all these juices and secretions? To dissolve
the food we eat.
[Illustration: FIG. 17.--_The Stomach laid open behind._
_a_, the œsophagus or gullet; _b_, one end of the stomach; _d_,
the other end joining the intestine; _e_, gall duct; _f_, the
gall-bladder; _g_, the pancreatic duct; _h_, _i_, the small
intestine.
]
We eat all manner of dishes, but in all of them that are worth eating we
find the same kind of things, which we call =food-stuffs=.
We eat various kinds of meat; but all meats are made up chiefly of two
things: the substance of the muscular fibre, which you have already
learnt is a =proteid= matter containing nitrogen, and the =fat= which wraps
round the lean muscular flesh. Now, proteids are, when cooked, insoluble
in water (see p. 49); and fat, you know, will not mingle with water.
Both these parts of meat, both these food-stuffs, must be acted upon
before they can pass from the inside of the alimentary canal, through
the epithelium of the mucous membrane, into the blood capillaries.
Besides meat we eat bread. Bread is chiefly composed of =starch=; but
besides starch we find in it a substance containing nitrogen,
exceedingly like the proteid matter of muscle or of blood.
Potatoes contain a very great deal of starch with a very small quantity
of proteid matter; and nearly all the vegetables we eat contain starch,
with more or less proteid matter.
Then we generally eat more or less sugar, either as such or in the form
of sweet fruits. We also take salt with our meals, and in almost
everything we eat, animal or vegetable, meat, bread, potatoes or fruit,
we swallow a quantity of mineral substances, that is, various kinds of
salts, such as potash, lime, magnesia, iron, with sulphuric,
hydrochloric, phosphoric, and other acids.
=In everything on which we live we find one or more of the following
food-stuffs:--Proteid matter, starch or sugar, and fat, together with
certain minerals and water. It is on these we live: any article which
contains either proteid matter, or starch, or fat, is useful for food.
Any article which contains none of them is useless for food, unless it
be for the sake of the minerals or water it holds.=
We are not obliged to eat all these food-stuffs. Proteid matter we must
have always. It is the only food-stuff which contains nitrogen. It is
the only substance which can renew the nitrogenous proteid matter of the
blood and so the nitrogenous proteid matter of the body.
We might indeed manage to live on proteid matter alone, for it contains
not only nitrogen but also carbon and hydrogen, and out of it, with the
help of a few minerals, we might renew the whole blood and build up any
and every part of the body. But, as you will learn hereafter, it would
be uneconomical and unwise to do so. Starch, sugar, and fats, contain
carbon and hydrogen without nitrogen; and hence, if we are to live on
these we must add some proteid matter to them.
=50.= Of these food-stuffs, putting on one side the minerals, sugar (of
which, as you know, there are several kinds, cane sugar, grape sugar,
and the like) is the only one which is really soluble, and will pass
readily by osmosis through thin membranes (see p. 84). If you take a
quantity of white of egg, or blood serum, or meat, or fibrin, or a
quantity of starch boiled or unboiled, or a quantity of oil or fat,
place it in a bladder, and immerse the bladder in pure water, you will
find that none of it passes through the bladder into the water outside,
as sugar or salt would do. In the same way a quantity of meat, or of
starch, or of fat, placed in your alimentary canal, would never get
through the membrane which separates the inside of the canal from the
inside of the capillaries, and so would remain perfectly useless as food
unless something were done to it. While the food is simply inside the
alimentary canal, it is really outside your body. It can only be said
to be inside your body when it gets into your blood.
In the things we eat, moreover, these food-stuffs are mixed up with a
great many things that are not food-stuffs at all; they are packed away
in all manner of little cases, which are for the most part no more good
for eating than the boxes or paper in which the sweetmeats you buy are
wrapped up. The food-stuffs have to be dissolved out of these boxes and
packing.
=The juices secreted by the glands= of which we have been speaking,
dissolve the food-stuffs out of their wrappings, act upon them so as to
make them fit to pass into the blood, and leave all the wrappings as
useless stuff which passes out of the alimentary canal without entering
into the blood, and therefore without really forming part of the body at
all.
This preparation and dissolving of food-stuffs is called digestion.
Different food-stuffs are acted upon in different parts of the
alimentary canal.
The saliva of the mouth has a wonderful power of =changing starch into
sugar=. If you take a mouthful of boiled starch, which is thick, sticky,
pasty, and tasteless, and hold it in your mouth for a few moments, it
will become thin and watery, and will taste quite sweet, because the
starch has been changed into sugar. Now sugar, as you know, will readily
pass through membranes, though starch will not.
The gastric juice in the stomach does not act much on starch, but it
=rapidly dissolves all proteid matters=.
If you take a piece of boiled meat, put it in some gastric juice and
keep the mixture warm, in a very short time the meat will gradually
disappear. All the proteid matter will be dissolved, and only the
wrappings of the muscular fibre and the fat be left. You will have a
solution of meat--a solution, moreover, which, strange to say, will
easily pass through membranes, and is therefore ready to get into the
blood.
The pancreatic juice and the juice secreted by the intestine act both on
starch as saliva does, and on proteids very much as gastric juice does.
=51.= The bile and the pancreatic juice together act upon all fats in a
very curious way.
You know that if you shake up oil and water together, though by violent
shaking you may mix them a good deal, directly you leave off they
separate again, and all the oil is seen floating on the top of the
water. If, however, you shake up oil with pancreatic juice and bile, the
oil does not separate. You get a sort of creamy mixture, and will have
to wait a very long time before the oil floats to the top. Milk, you
know, contains fat, the fat which is generally called butter. If you
examine milk under the microscope, you will find that the fat is all
separated into the tiniest possible drops. So also, when you shake up
oil or butter, or any other fat, with bile and pancreatic juice, you
will find on examination that the fat or oil is all separated into the
tiniest possible drops. What is the purpose of this?
If you look at the inside of the small intestine of any animal, you will
find that it is not smooth and shiny like the outside of the intestine,
but shaggy, or, rather, velvety. This is because the mucous membrane is
crowded all over with little tags, like very little tongues, hanging
down into the inside of
[Illustration: FIG. 18.--_Semi-diagrammatic View of Two Villi of the
Small Intestines._ (Magnified about 50 diameters.)
_a_, substance of the villus; _b_, its epithelium, of which some
cells are seen detached at _b_^{2}; _c d_, the artery and vein,
with their connecting capillary network, which envelopes and hides
_e_, the lacteal which occupies the centre of the villus and opens
into a network of lacteal vessels at its base.
]
the intestine. These are called =villi=; they are not unlike the papillæ
of the skin (Fig. 15), if you suppose all the epidermis stripped except
the bottom row of cells (_d_), and the papilla itself pulled out a good
deal. Fig. 18 is a sketch to illustrate the structure of a =villus=. The
epithelium (_b_), you see, is made up of a single row of cells. Beneath
the epithelium, just as in the papilla of the skin, is a network of
blood capillaries, shown, for convenience, in the right-hand villus
only. But besides the blood capillaries, there is in each villus, what
there is not in a papilla of the skin, another capillary (shown, for
convenience, in the left-hand villus only) which does not contain blood,
which is not connected with any artery or with any vein, but which
begins in the villus. This is a =lacteal=. I have said nothing of these at
present. In most parts of the body we find, besides blood capillaries,
fine passages very much like capillaries, except that they contain a
colourless fluid instead of blood, and do not branch off from any larger
vessels like arteries. They seem to start out of the part in which they
are found, like the roots of a plant in the soil. But though unlike
blood capillaries in not branching off from larger trunks, they resemble
capillaries in joining together to form larger trunks corresponding to
veins, and the colourless fluid flows from the fine capillary channels
towards these larger trunks. This colourless fluid is called =lymph=; it
is very much like blood without the red corpuscles, and the channels in
which it flows are called =lymphatics=.
The lymphatics from nearly all parts of the body join at last into a
great trunk called the =thoracic duct=, which empties itself into the
great veins of the neck, as is shown in the diagram, Fig. 6, _Lct._,
_Ly._, _Th. D._
Now, many of the lymphatics start from the innumerable villi of the
intestine, and are there called =lacteals= (Fig. 6, _Lct._); so that
lacteals may be said to be those lymphatics which have their roots in
the villi of the intestine.
But what has all this to do with the digestion of fat? Lacteal means
=milky=, and the lymphatics coming from the villi are called lacteals
because, when digestion is going on, the fluid in them, instead of being
transparent as in the rest of the lymphatics, =is white and milky=. Why is
it thus white and milky? Because it is crowded with minute particles of
fat, and those minute particles of fat come from the inside of the
intestine. They are the same minute particles into which the bile and
pancreatic juice have divided the fat taken as food. We know this
because when no fat is eaten the lacteals do not get milky; and when for
any reason bile and pancreatic juice are prevented from getting into the
intestine, though ever so much fat be eaten, it does not get into the
lacteals at all, it remains in the intestine in great pieces, and is
finally cast out as useless.
=52.= This, then, is what becomes of the food-stuffs:--
The fats are broken up by the bile and pancreatic juice into minute
particles. These minute particles, we do not exactly know how, pass
through the epithelium of the villus into the lacteal vessels, from the
lacteals into the thoracic duct, and from the thoracic duct into the
vena cava. Thus the fats we eat get into the blood.
The starch is changed into sugar in the mouth by saliva, and in the
intestine by the pancreatic juice; but sugar passes readily through
membranes, and so slips into the blood capillaries of the walls of the
alimentary canal. Thus all the sugar we eat, and all the goodness of the
starch we eat, pass into the blood.
The proteids are dissolved in the stomach by the gastric juice, and what
passes the stomach is dissolved in the intestine, dissolved in such a
way that it can pass through membranes; and thus proteids pass into the
blood.
Probably some of the sugar and proteids pass into the lacteals as well.
The minerals are dissolved either in the mouth, or in the stomach, or in
the intestine, and pass into the blood.
And water passes into the blood everywhere along the whole length of the
canal.
When we eat a piece of bread, while we are chewing it in our mouth it is
getting moistened and mixed with saliva. Part of its starch is thereby
changed into sugar, and all of it is softened and loosened. Passing into
the stomach, some of the proteids are dissolved out by the gastric
juice, and pass into the blood, and all the rest of the bread breaks up
into a pulpy mass. Passing then into the intestine, what is left of the
starch is changed by the pancreatic juice into sugar, and is at once
drained off either into the lacteals or straight into the blood. In the
intestine what remains of the proteids is dissolved, till nothing is
left but the shells of the tiny chambers in which the starch and
proteids were stored up by the wheat-plant as it grew.
When we eat a piece of meat, it is torn into morsels by the teeth and
well moistened by saliva, but suffers else little change in the mouth.
In the stomach, however, the proteids rapidly vanish under the action of
the gastric juice. The morsels soften, the fibres of the muscle break
short off and come asunder; the fat is set free from the chambers in
which it was stored up by the living ox or sheep, and, melted by the
warmth of the stomach, floats in great drops on the top of the softened
pulpy mass of the half-digested food. Rolled about in the stomach for
some time by the contraction of the muscles which help to form the
stomach walls, losing much of its proteids all the while to the hungry
blood, the much-changed meat is squeezed into the intestine. Here the
bile and the pancreatic juice, breaking up the fat into tiny particles,
mix fat, and broken meat, and empty wrappings, and salts, and water, all
together into a thick, dirty, yellowish cream. Squeezed along the
intestine by the contraction of the muscular walls, the goodness of this
cream is little by little sucked up. The fat goes drop by drop, particle
by particle, into the lacteals, and so away into the blood. The
proteids, more and more dissolved the further they travel along the
canal, soak away into blood-vessel or into lacteal. The salts and the
water go the same way, until at last the digested meat, with all its
goodness gone, with nothing left but indigestible wrappings, or perhaps
as well some broken bits of fibre or of fat, is cast aside as no longer
of any use.
=Thus all food-stuffs, not much altered, with all their goodness
unchanged, pass either at once into the blood, or first into the
lacteals and then into the blood, and the useless wrappings of the
food-stuffs are cast away.=
While we are digesting, the blood is for ever rushing along the branches
of the aorta, through the small arteries and capillaries of the stomach
and intestine, along the branches of the portal vein, and so through the
liver back to the heart; and during the few seconds it tarries in the
intestine, it loads itself with food-stuffs from the alimentary canal,
becoming richer and richer at every round. While we are digesting, the
thoracic duct is pouring, drop by drop, into the great veins of the neck
the rich milky fluid brought to it by the lacteals from the intestine,
and as the blood sweeps by the opening of the thoracic duct on its way
down from the neck to the heart, it carries that rich milky fluid with
it, and the heart scatters it again all over the body.
=Thus the blood feeds on the food we eat, and the body feeds on the
blood.=
HOW THE BLOOD GETS RID OF WASTE MATTERS. § VIII.
=53.= But if the blood is thus continually being made rich by things, it
must also as continually be getting rid of things. The things with which
it parts are not, however, the same as those which it takes. The blood,
as we have said, is fuel for the muscles, for the brain, and for other
parts of the body. These burn the blood, burn it with heat but without
light. But, as you have learnt from your Chemistry Primer, Art. 4,
burning is only change, not destruction; in burning nothing is lost. If
the muscle burns blood, it burns it into something; that something,
being already burnt, cannot be burnt again, and must be got rid of.
Into what things does the body burn itself while it is alive?
I have already said that if you were to take a piece of meat or some
blood, and dry it and burn it, you would find that it was turned into
four things--water, carbonic acid, ammonia, and ashes. The body is made
up of nitrogen, carbon, hydrogen, and oxygen, with sulphur, phosphorus,
and some other elements. The nitrogen and hydrogen go to form ammonia;
the hydrogen, with the oxygen of combustion, forms water; the carbon,
carbonic acid; the phosphorus, sulphur, and other elements go to form
phosphates, sulphates, and other salts.
=In whatever way the body be oxidized, whether it be rapidly burnt in a
furnace, whether it be slowly oxidized after death, as when it moulders
away either above ground or in the soil, whether it be quickly oxidized
by living arterial blood while still alive--in all these several ways
the things into which it is burnt, into which it is oxidized, are the
same. Whatever be the steps, the end is always water, carbonic acid,
ammonia, and salts.=
These are the things which are always being formed in the blood through
the oxidation of the body, these are the things of which the body has
always to be getting rid.
In addition to the water which comes from the oxidation of the solids of
the body, we are always taking in an immense quantity of water; partly
because it is absolutely necessary that our bodies within should be kept
continually moist, partly because food cannot pass into the blood except
when dissolved in water, and partly because we need washing inside quite
as much as outside; if we had not, so to speak, a stream of water
continually passing through our bodies to wash away all impurities, we
should soon be choked, just as an engine is choked with soot and ashes
if it be not properly cleaned. We have, then, to get rid daily of a
large quantity of washing water over and above that which comes from the
burning of the hydrogen of our food.
We have already seen that a great deal of the carbonic acid goes out by
the lungs at the same time that the oxygen comes in. A large quantity of
water escapes by the same channel. You very well know that however dry
the air you breathe, it comes out of your body quite wet with water.
We have also already seen how the blood secretes sweat into the
sweat-glands, and so on to the skin. Perspiration is little more than
water with a little salt in it. The skin, therefore, helps to purify the
blood through the sweat-glands, by getting rid of water with a little
salt. You must remember that a great deal of water passes away from your
skin without your knowing it. Instead of settling on the skin in drops
of sweat, it passes off at once as vapour or steam. Some carbonic acid
also makes its way from the blood through the skin.
=54.= It only remains for us to inquire, In what way does the blood get
rid of the ammonia and the rest of the saline matters that do not pass
through the skin?
These are secreted from the blood by the kidney, dissolved in a large
quantity of water in the form of =urine=.
What is the kidney? You will learn more about this organ by and by.
Meanwhile it will for our present purpose be sufficient to say that a
kidney is a bundle of long tubular glands, not so very unlike
sweat-glands, all bound together into the rounded mass whose appearance
is familiar to you. =Into these glands the blood secretes urine just as
it secretes sweat into the sweat-glands=. The glands themselves unite
into a common tube or duct which carries the urine into the receptacle
called the urinary bladder, from whence it is cast out when required.
What is urine? Urine is in reality water holding in solution several
salts, and in particular containing a quantity of ammonia. The ammonia
in urine is generally in a particular condition, being combined with a
little carbonic acid, in the form of what is called =urea=. If urea is not
actually ammonia, it is at least next door to it.