IUBio

Size of brain and size of skull

Ian at dragoncon.netdeletethis Ian at dragoncon.netdeletethis
Mon May 31 00:04:35 EST 1999


On Sun, 30 May 1999 05:56:16 GMT, Ian at dragoncon.net deletethis wrote:

>On 28 May 1999 12:56:40 GMT, didier at Glue.umd.edu (Didier A. Depireux)
>wrote:
>
snipped
>>
>>I agree with that, but that's not what I meant. The mother's pelvis only
>>determines the brain size at birth, but you still have quite a ways to go
>>between the size of your head at birth and the adult size. What I find
>>surprising is that bone growth (the calvarium) should be _regulated_ in any
>>way by the growth of a soft tissue like the brain. 
>>
>>						Didier
>
I've had another thought or two on this matter. I think we have both
fallen for the trap of regarding bone as being rigid unchanging
material similar to stone or steel. But it isn't, it is a living,
always changing organ. Calcium, magnesium and phosporus is continually
being dissolved and redeposited.

While bone can easily withstand any pressure that a growing brain may
be able to exert at a given moment, withstanding a constant pressure
may be a different matter. On needs only think of the former Japanese
custom of binding feet to see that continual pressure can cause bone
growth to be altered by force.  I don't have any way of measuring the
amount of force you can apply with cloth bandages as they used, but I
imagine the force per square millimetre wouldn't be all that high and
possibly within the range that brain tissue can apply.

The other point that I've just thought of is regarding how the skull
grows. If I remember this correctly,  what happens is that calcium etc
is dissolved on the inside and re-deposited on the outside. Probably
after the first few years there is relatively little thickening. I may
be that this process is susceptible to being manipulated by pressure,
ie: as the brain grows pressure on the inside of the calvarium
increases and this causes bone to be dissolved more quickly.
Conversely when the pressure is reduced, dissolving slows or even
stops. 

Alternatively, the brain may release a chemical stimulus that triggers
an acceleration in the bone dissolving process.

This is all speculation on my part, but at least superficially,  it
makes some sense.  The folks at sci.med.orthopedics would be able to
give you more info. 

Ian





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