plant. Every time we move a muscle, we manufacture soda water. Every
time we think, we turn a half dozen different chemicals into the
blood. We take a mouthful of cracker, which is mostly starch; and
straightway a ferment, or “enzyme,” in the mouth begins to turn the
starch into sugar. That is why a cracker or a potato tastes sweeter
and sweeter the longer one chews it. When the sugar gets into the
blood, another enzyme (the “y” is like “i,” and the word sounds like
en-zime) in the liver turns it back to starch again, so that it
stays there in the liver, and can’t get out. Then as you know, this
liver-starch slowly turns back to sugar again, and leaks out into
the blood to feed the muscles. In the muscles, still another enzyme
helps it to explode into carbon dioxid and water, only sometimes it
forms lactic acid instead, which is the acid of sour milk. Because
of these enzymes in the muscles, the gas engines which they are, can
run when only moderately warm, instead of needing to be too hot to
touch, like other engines when they are at work.
But if instead of making flour into crackers, or having the baker do
it, and then eating them, we had made the flour into bread with
yeast: then the yeast, which is a living and growing plant, would
have formed another enzyme, which in turn would have turned the
sugar in the bread into carbon dioxid and alcohol, and the carbon
dioxid would have puffed up the bread and made it light. That is
what yeast is for. The yeast of beer also, turns the sugar of the
grain into carbon dioxid and alcohol. The first of these gives us
the fizz, the second we use to poison ourselves with.
Even the color of our hair and eyes depends on an enzyme which
manufactures the coloring matter. People who have the enzyme, have
dark hair and brown eyes. People who haven’t it, have light hair and
blue eyes.
In fact, almost anything that any living part of the body has to do,
whether to take its food out of the blood stream and build it into
its own substance, or to do its work, or merely to grow, it
generally has to employ one or more of these enzymes to do it with.
These are, in fact, the tools of the living jelly, or protoplasm,
which makes our bodies and does our living for us. Without them we
could not live a single hour.
I suppose there must be hundreds, or perhaps even thousands, of
different chemicals and drugs and medicines and poisons and
antitoxins and enzymes forming all the time in different parts of
the body. The various organs put them to all sorts of various uses.
Among others, they employ certain of them as messengers, to carry
signals from one part of the body to another.
I have already explained that the head is one of the first organs in
the body to be formed. It gets a start, therefore, over the rest,
and a little baby’s head is something like four times as big as it
need be to fit its body. But the baby’s legs and arms, which started
late, are not nearly large enough to fit the rest of him. So the
limbs have to grow fast, and the head to grow slowly, in order to
come out right in the end. How do they know enough?
How fast little girls’ feet sometimes grow! At twelve they can wear
their mamma’s shoes. Then the feet stop growing and the rest of the
body catches up. Or when it is time for a boy’s voice to change, all
of a sudden, his Adam’s apple, where the voice comes from, starts
growing. In a few months it has increased from boy’s size to man’s,
and the voice has dropped to a deep, if uncertain, growl. Everywhere
throughout the body, the different parts start growing, and stop,
and keep along together, or get ahead of each other in the most
complicated fashion, but always right.
How they manage it, we do not altogether know. We do, however, know
that the different parts of the body do signal to one another by
means of these substances which they form within their cells, and
turn loose in the blood stream. In this way, each organ of the body
is able to send messages to the rest—“Start growing,” “Steady now,”
“Slow down,” “Stop”—as the case may be. You already know how a
working muscle signals to the heart and lungs, and how the blood
cells get wind of an invasion, by the altered smell or taste of the
blood, and rush to the point where the enemy has broken through the
defense, and the fight is on.
So in general, when a message has to go quickly from one part of the
body to another, it goes by way of the nerves. But when there is no
special hurry, or when the signal must go to some tissue or organ
which, like the blood corpuscles, has no nerve connection, then the
message has to go by way of the blood.
If then, our legs want to say: “We’re sitting on something hard and
sharp, please may we move,” they call us up over the nerves. If they
want to say: “We have been growing too fast, and it’s high time we
stopped and gave something else a chance,” they do it by turning
something into the blood. But if they want to say: “We’re so dead
tired that we simply can’t walk another step,” then as you know,
they use both methods—fatigue toxins and very dreadful aches.
XLIV
What Becomes Of The Tadpole’s Tail
The lymph cells, blood cells, white blood corpuscles, as they are
variously called, are a sort of standing army, always on duty to
repel invasion by any living germs of disease. In times of peace,
however, when there is no enemy to be set upon and devoured, they
have to work. All soldiers have to work when there is no fighting to
be done; and sailors on a battle ship have to keep it clean and in
order, row the captain about in his boat, and take turns paring
potatoes for the cook. So the cells in our blood are in this like
other fighting men.
Their particular work is to keep the body cleaned up, especially the
blood. Besides this, they take down and cart away any tissue or
organ or part of the body that is no longer any use.
There, for example, is the polywog’s tail, after he becomes a
frog—also his gills. The polywog, of course, is pretty nearly a
fish, with gills and tail, who breathes water. But Brer Frog is
pretty nearly a land animal. He breathes with lungs, tho not to be
sure especially good ones, so that he has to breathe thru his skin
also to help out. At any rate, he has lost his fishy gills, and no
longer has any tail.
Where has the tail gone? The white blood corpuscles have eaten it
up; and with it, a lot of other things, gills among the number,
which the tadpole has to have, but the frog has no use for.
The tadpole, as you know, has no legs. When they start growing (you
can see them under the skin, almost the first sign of the coming
change) these growing legs turn something into the blood which is a
signal to the blood cells to get to work eating up the tail. So the
blood cells do get to work. They devour the flesh, cutting into it
much as certain insects gnaw into wood, or a woodchuck digs a hole
in the ground. Then as fast as the blood cells take down the flesh
that is no longer useful, it is used again to build up the new legs.
So really the tadpole’s tail is used over to make the frog’s legs.
Much the same thing happens when a caterpillar turns to a butterfly.
The caterpillar, as you have no doubt often seen, shuts himself up
in a hard shell. Inside this coffin, or chrysalis, the body of the
caterpillar is taken to pieces, and changed over into a sort of
thick milk. Then out of this thick milk, the butterfly is formed.
[Illustration: The caterpillar changes into a moth.]
So the caterpillar does not become a butterfly simply by growing
wings. It is really almost a new creature, built up out of the
material of the old, as one might rip up an old coat and have it
woven into a rug; and we may be very sure that no butterfly ever has
the slightest recollection of the time when it crawled on a dozen
legs, and chewed leaves for its food.
We, too, grow and change like frog and butterfly. When we were, say,
four or five years old, we had twenty strong little white teeth,
each set firmly in the jaw bone on a stout root. By and by, these
teeth, one at a time, in pretty much the order in which they first
appeared, began to get loose. First we could wiggle the tooth a
little; then a good deal. By and by, it hung only by the thin skin
of the mouth, so that one good yank, or perhaps only the push of the
tongue against it, tore it out. Or possibly we bit into an apple,
and left the tooth behind.
Anyhow, the tooth hadn’t any root. The hard bone had gone, and with
it the soft nerve and blood vessel that used to be inside. There was
only the hollow cup that once was the crown of the tooth. The rest
had been eaten up, bone, nerve and all, by our blood corpuscles. We
needed larger teeth. Teeth, as I told you, once formed, cannot grow
any more. So the blood corpuscles chewed up the tooth, as much as
they could get at, and used the old stuff to build the new one. No
doubt they would consume the entire tooth, crown and all, and not
leave a shred behind, if only there were some way of holding the
last bit in place while they finished it off. That on the whole
would be pleasanter than having to tie a string round the tooth and
yank it out.
They do act that way with growing bones. The bones, as I have
already explained, grow much as a tree does, by adding on new stuff
to the outside. Meantime, since all bones are hollow, the hollow
also has to grow. The hollow grows by the taking down of the bone on
the inside, just as a tree might decay out at the heart, about as
fast as it grew under the bark. This, also, is the work of the white
blood corpuscles.
Then, too, the soft gristle of little children changes into the
solid bone of grown men. But the gristle never turns to bone. As
before, the gristle is taken out, eaten into, devoured, by these
living cells of the blood and lymph; and as fast as the gristle
disappears, the bone grows into its room.
Or sometimes it happens that two little boys find themselves not in
entire harmony, with the result that one little boy goes home with
one eye shut up; and the next day, and all the next week, that
particular eye is a most remarkable study in black and blue. That is
because, during the lack of harmony, something happened to hit one
little boy in the eye, and smashed up some of the little blood
vessels around it, so that the blood leaked out into the flesh.
Then the white blood corpuscles, which are themselves always leaking
out of the lymph spaces and blood vessels, and wandering round thru
the tissues, have to set to work to clean up that spilled blood. So
they actually eat up the flecks of blood; until by and by, the flesh
is all clean and white again, and the black eye is black and blue no
more. In general, whenever we meet with an injury or a wound of any
sort, these white blood corpuscles take a hand in the healing
process, eating away the damaged tissue, and allowing chance for
fresh growth.
The white blood corpuscles, then, live to eat. Like certain other
creatures who are always hungry, they sometimes eat things that they
had better have left alone. Sometimes, people’s hair turns white
suddenly—corpuscles have actually eaten up the color. Some people
even go so far as to say that the reason why we grow old (since, as
I have already told you, growing old is mostly growing hard) is
because these white blood corpuscles have gone crazy and eaten up
the softer parts of us, and left us only the hard ones to grow old
with. I don’t know whether this last is so or not. But at any rate,
these white blood corpuscles, which let go their hold on the places
where they grow, and go wandering off all over the body eating
everything in sight, would be likely to make this sort of trouble if
anything would; and the man who especially says it is so, probably
knows more about these matters than anybody in the world.
XLV
Nature’s Repair Shop
Of course, we get hurt in all sorts of ways—cuts, bruises, barked
shins, black eyes, once in a while a bad sprain or a broken bone.
Then the white corpuscles that are in our blood, and the growing
life-stuff or protoplasm which is in our flesh and bones, and all
this wonderful and mysterious life that is in us, take us in hand to
mend us up. The same power that made us, and that keeps us alive,
heals also our hurts. By and by, if we take care of ourselves, we
are once more as good as new.
The small and weak and lowly creatures, which cannot take care of
themselves as we can, and so are all the time getting into all sorts
of trouble, are able to repair damages even better than we. The
world is full of timid animals, which have neither teeth and claws
to fight with, nor armor for their defence, nor speed of foot or
cunning of brain with which to escape their enemies. But to make up
in part for this lack, many of these simple beings seem not to mind
at all such injuries as would cripple us for life; while they
recover completely, and that within a few days, from accidents which
would mean instant death to us.
A tiny lizard, for example, may at any moment have to scamper for
his life in search of an equally tiny crack in the rock, where it
may take refuge from some larger animal which wants it for
breakfast. Naturally, oftentimes, the lizard so hardly escapes being
“it,” that just as he whisks into safety, his pursuer snaps off his
entire tail.
A loss like this would kill most larger animals, but not the lizard.
He simply waits round for a week or two while a new tail grows, just
as good as the old one, so that he is as well off as before. The
same lizard has been known to lose his tail a dozen or twenty times,
and each time to grow a new one. Since, therefore, a lizard’s tail
is longer than his body, and nearly as large round, the animal must
have grown enough new flesh to make at least five or six whole new
lizards. Curiously however, the new tails, tho they look exactly
like the old one, always have a rod of gristle or cartilage inside,
in place of the regular backbone.
So too, with legs. A lizard that has had a leg bitten off,
straightway grows a new one. He will even grow a whole new eye, when
something happens to the one with which he happened to be born.
Yet oddly enough, the lizard, when he grows new legs and tails is
continually liable to make the strangest mistakes. He will grow a
new tail, when he hasn’t lost the old one, but only had a bite taken
out of it. Then he has two tails. Sometimes he makes even a worse
break than this, and grows out two new tails to replace the single
one which after all he didn’t quite lose. Then he has three tails,
which is at least one more than any proper lizard ought to possess.
It is the same, too, with legs. The lizard seems to get an idea that
his leg has been bitten off, when it has only had a piece taken out
of it. So he goes ahead to grow a new leg, and as a result, has
five. In fact, it is quite possible, to manufacture a lizard,
perfectly healthy and apparently happy, with eight legs and three or
four tails.
Perhaps you have heard that “the early bird catches the worm.” If
so, did you ever consider the transaction from the side of the worm?
If the worm happens to be retiring into his hole just as the hungry
robin catches sight of him, there is likely to be a tug of war
between the eater and the breakfast. The worm swells out the front
end of his body, and gets a grip on the sides of its hole, while the
bird digs its claws into the ground. Sometimes the worm lets go, and
gets eaten up. Sometimes he gets pulled in halves, and only the rear
end goes down the robin’s red lane.
The worm minds being pulled in halves just about as much as a train
of cars minds getting uncoupled. The front end calmly crawls away
into its hole, goes on eating dirt as usual, and pretty soon grows
itself a new tail as before.
But what becomes of the severed tail end, if by any chance the robin
fails to eat it up? That perhaps is the queerest thing of all. If
the front part that crawled away is short, say a third of the animal
or less, so that two-thirds of the worm at least is left in the tail
piece, then this severed tail grows a new head. So the one worm
becomes two. But if the tail piece is shorter, no more say than a
half the whole animal, then that tail, instead of growing a head,
grows another tail. In that case, the final result is one entire
worm, old head end and new tail; and two tails, one old, one new,
growing end to end. But of course, two tail ends, and nothing else
cannot very well make a living, and, the worm soon dies of
starvation.
There is, however, a smaller creature, the fresh water hydra, which
quite outdoes the earthworm, when it comes to growing on new parts.
The hydra is a very small sea-anemone, about as long as a pin is
thick, with a long slender stalk and a circle of long fingers or
tentacles round their mouths. One finds them growing on sticks and
water plants almost anywhere in ponds.
Cut off one of these fingers, and it grows again. Cut the entire
animal in halves as one chops thru the trunk of a growing tree, and
the root end grows a new head, and the head end grows a new; root,
and there are two new hydras in place of the one old one. Cut the
animal into three pieces, head end, root end, and a small bit out of
the middle of the stalk. The head end forms an entire new creature.
The root end forms an entire new creature. The middle piece, which
is neither head end nor root end, but just a little drum-shaped
snipped out of the stalk between the two, even that grows a new root
end and a new head end, and becomes an entire new creature, which in
time grows to be as large as the one out of which it was made.
It is, in short, as if when one ripped off the sleeve of a coat, the
coat grew a new sleeve; or when one pulled off the tail of the coat,
the coat grew a new tail; while the severed sleeve and tail each
grew new coats—pockets, buttons, collars, tails, sleeves, and all.
Our common star-fish, that we find in the salt water pools at the sea
shore after the tide goes out, also isn’t at all bad at mending
itself up after an accident. The five rays which grow out from its
center are continually getting broken off. So one finds often in the
water, star-fish with only four rays, or with four large ones, and
one smaller one just growing out to take the place of one which has
been lost.
But the lost ray, if something doesn’t eat it up, will grow a new
star-fish. From the broken end, four more little arms bud out like
those of a tiny star just getting its start in the world. The single
arm or ray, not only grows out the other five, but in addition forms
a new center for them to grow to, a new mouth, a new stomach, a new
nerve ring round the mouth which is the creature’s brain; and so one
thing with another forms a whole new star-fish with all its parts
complete. And all the while, of course, the severed arm cannot eat
anything, because it hasn’t any stomach. So it has to make the new
parts out of the old, as the tadpole builds its new legs out of its
old tail.
Still, I don’t know why any of these things are any more remarkable
than that the lizard and hydra and earthworm and star-fish should
have formed in the egg in the first place. That after all, is about
the most remarkable thing there is in this world.
Not a few animals, moreover, in addition to having this power of
mending themselves up after they have been injured, have also an
arrangement for getting themselves hurt in a convenient place. The
ray of a star-fish, for example, almost never breaks in halves.
Whenever the ray gets caught so that it has to come off, it breaks
close to the center, so that the whole arm comes off at once.
The crabs, such as one finds at the sea shore in salt water, have a
special place in each leg, close up to the body, where the leg is
meant to break off. The hard outer shell is turned back into the
flesh and makes a round plate of shell with a hole in the middle,
cutting right across the leg. Thru the hole run the nerves and blood
vessels, but the muscles come to the plate on each side and there
stop.
So when a hungry fish “catches a crab” by one leg, the crab digs his
claws into sand or sea weed and pulls. The fish backs water with his
fins and he pulls. Off comes the leg. Away goes the crab in safety.
The leg has pulled off at this plate of shell. No muscle has been
torn. The end of the stump is nicely protected; and all the injury
that has been done is to the small thread of tissue no larger than a
pin, which ran thru the hole in the center of the plate.
More than this, the stump is all ready to grow out again; and
actually grows out quicker, after the leg has been taken off at this
“breaking joint,” than if only a small piece had been taken off
lower down. Such breaking joints, where the wound is all healed over
before it is made, are somewhat common among animals, especially
among those which have a jointed shell on their outside.
Even here, however, there are some queer doings. Sometimes, after a
five jointed leg has been pulled off, the one which grows in its
place, tho just as large and just as long, has only four joints.
Some crabs, and the like, which have the big claws unlike on the two
sides, if they lose either one, instead of growing one like it in
its place, make a new claw like the one that belongs on the other
side. Also, if they lose an eye, they grow a new one; but if they
lose both the eye and the eye stalk on which the eye is set, then
instead of an eye, they grow a small feeler. Really, one does not
know which most to marvel at—their strange power of growing new
parts, or their crazy way of growing them wrong.
But of all living repair shops the most uncanny are certain little
flat worms, called planarians. These are a half inch or less in
length, and fairly common in some places at the sea side, tho they
are likely to be mistaken for snails which they much resemble.
Cut off the head of one of these creatures, and inside a week he
will grow another—eyes, brain, mouth, and all as good as before. Cut
off both head and tail, and still the creature will straightway fit
himself out with new ones. Split him fairly in two lengthwise of the
body, and each half will grow out a new half; and there will be two
planarians, each complete, where only one was before.
Or if one splits the body in halves from the neck down, then each
half body grows a new half, and there is a single head with two
bodies. Or on the other hand, if one splits the head in two and
leaves the body, each half head will become a whole one. Then we
shall get a double headed planarian, a sort of Y-shaped creature.
Sometimes, after the planarian has been split in two from the neck
backwards, and formed a Y upside down with two bodies and one head,
another head, or occasionally two new heads, eyes, brain and all
complete, grow out facing backwards in the fork of the Y. Indeed the
animal seems to have a sort of mania for growing new heads and tails
in all sorts of unbecoming places. If he gets a wound or cut almost
anywhere in the body, and the wound happens to open on the whole
backward, out of that wound or cut will grow a new tail in addition
to the one he already has. But out of such wounds as face forward,
there grow out complete new heads. As many as seven different heads
have thus been made to grow on a single planarian—some on the middle
of the back, some underneath the body, some even on the sides of the
tail, and all, no doubt, greatly to the embarrassment of their
owner.
As for cut off heads and tails, one might expect them to grow new
bodies and become whole animals again. They do not, however, unless
the pieces are pretty large. A head, cut off close behind the eyes,
grows out, not a body, but another head. So all there is to the
creature is two heads, joined neck to neck and looking in opposite
directions.
All the time these things are going on, while the worms are making
new heads, or bodies, or the two halves of one body are filling out
again, the creatures can get nothing to eat. How then do they manage
to grow? They live on their own flesh. The old fragment is taken
down and used to build the new. An entire worm, made from a half
worm, is no larger than the half from which it came. The animal,
instead of growing large, grows small.
Whether it also grows young again, is by no means clear. Apparently
it does. At least, a planarian that does not get enough to eat,
proceeds at once to ungrow, becoming gradually smaller and smaller,
till after a few months, it is no more than a fifteenth part of its
former size. Whether it really becomes a baby worm again or not, it
certainly looks like one, for it is a pretty small human baby that
is not more than one fifteenth as large as a grown man.
XLVI
Little Monsters