The ultimate transformation
of a fertilized blob of genetically directed, chemically driven cells into
the very person reading this--you--is an amazing process. It all starts
off, of course, with the eggs your mother was born with. (You are really
as old as your age + the age of your mother when she conceived, because
that is how old her eggs were at the time.) With fertilization,
as mentioned above, the genetics direct and the chemicals drive the cell
to split over and over according to a certain architecture. Eventually
there are arms and legs and eyes, a stomach, liver, and everything else,
hopefully where they all belong. Of course,
one structure, the brain, makes us what we are on the food chain. The superior
brain of Homo sapiens, with the help of an opposable thumb, has
allowed us to dominate and run the world. (Whether our brains are superior
enough to do that well is a whole other topic!)
The brain of the developing
fetus changes so much over the course of
gestation that it is even possible to date the stage of the pregnancy by
what the brain looks like. What becomes progressively more pronounced and
what makes us the thinking beings that we are is the development of neurons
(nerve cells). Nature has taken advantage of a property of geometry to
cram in as many neurons as possible into a relatively small space--that
property being surface area.
To illustrate by way
of example, if you were to drive the relatively straight interstate from
Los Angeles to San Francisco, you would travel a certain distance that
would be recorded on your car's odometer. But if you were to take Highway
1, which follows the curves, bulges, inlets, and other miscellaneous variations
of the coast, the odometer is going to record a larger number. Now I know
we're talking about a whole lot of tennis balls either way, but the theoretical
number of tennis balls that could be lined up along curvy Highway 1 will
dwarf the theoretical number that can be lined up along the interstate.
The mathemeticians will glibly point out that this is fractal, or "non-Euclidean"
geometry. But our DNA thought of it before it was ever described in a math
book, and this is the way we can cram into our brains so much more thinking
and reasoning power. For the brain is not just a chunk of connected nerve
cells, but bundles and tracts that interweave--up and down, over and under,
in and out--presenting as the famous convolutions that is the crowning
glory of our species.
If you were to look
at a human brain, you'll see these convolutions, serpentine, thick tissue
cylinders. If you were to measure along these structures, you would mark
real estate along the bulges and into the valleys (called "sulci") between
them. The more convoluted the brain is and the deeper the sulci, the more
surface area there is to pack extra neurons. It is during the second
trimester (around 20 weeks) that the sulci begin developing, and the
quantum leaps in further development begin after 28 weeks. By 40 weeks,
or term, the brain is a masterpiece of architecture which, because of convolutions,
takes the mental abilities of a brain that should be about 10 feet wide
and puts them into a package that will fit comfortably inside your typical
head.
The second aspect I would
like to touch upon is how does that head, with its precious masterpiece
of a brain, come out of the mother intact? Putting a sphere of bone around
it helps, certainly. But the bony capsule cannot be too rigid, or it would
have trouble passing through the maternal pelvis
at birth. Herein is a dilemma that once again our DNA has solved:
1) Brain needs protection
on the way out, and
2) the protection can't keep it
from getting out
The skull is not one
sealed container, but a collection of plates, attached but mobile, much
like the tectonic plates on the Earth. Because there are fault lines between
tectonic plates (probably running along Highway 1 I would suspect), the
land masses are able to move. On the skull, these faults, or separations,
are called by an old name, "sutures,"
and have nothing to do with the modern sense of stitching. The sutures
of the skull run between the bony plates that together make up a helmet
of protection for our quarterback, the brain. But because of these sutures,
the plates can move--there is flexibility, which is so necessary when trying
to cross all of the altering diameters of the maternal
pelvis.
The newborn's skull
is often first seen as being similar to the shape of his or her mother's
internal pelvis. This misshapen head is usually back to it's cute, round
shape withing a couple of days, but when particularly severe can raise
the eyebrows of the new parents, for sure.
At birth the brain doesn't
stop growing, as is true with the rest of the body. Therefore there's considerable
slack that the sutures provide so that the skull can balloon out with the
expansion of the brain as it grows. Just as the separations among the bones
of the skull allow the head to mold itself to a shape that can deliver,
they also allow spread for the brain to increase in size further.
There are areas
where there is considerable distance between the skull plates, where the
sutures are widest, and these are called "fontanelles."
There are four main fontanelles. The anterior one is the famous "soft spot"
of the baby's head. There is also a corresponding one at the back of the
head and one on either side (the temporal fontanelles). At birth, the sutures
and fontanelles allow the protective skull around the brain to be somewhat
flexible as the baby's head negotiates that passageway
to the outside world. Because of the way the skull is shaped, the displacement
of one skull plate results in a force against the brain that is distributed
somewhat evenly all around. Even though the skull is not anatomically an
intact ring structure (due to sutures and fontanelles), it is nevertheless
functionally a ring structure to protect the brain from the compression/decompression
exerted against it by the maternal pelvis, the vaginal walls, and the forces
of labor.
It is the single best case of having your
cake and eating it, too, from an anatomical standpoint.
After birth, the skull
plates continue growing toward each other, ultimately fusing together the
skull into a solid bony globe that continues to protect the brain, our
most cherished organ (which is pretty unusual for a man to state). Ultimately,
the sutures, fontanelles, and flexibility are no longer necessary, because
the protection that our skulls provided us at birth have allowed us to
be smart enough not to stick our heads into anything that tight again.
We hope.
