Universal Bald Spot.

[Jim B.]

[Dear Cecil:]
What is that little bald spot in the back of the head we all seem to have? Women have it. Even little babies have it. Does it have something to do with pre-natal scalp development? And what is it called?
[Jim B.]

There. That is for Mr. Adams if he takes my question. As for the rest of you, well?

[Ximenean]

I have always thought that "crown" was the term for that point on the back of one's head from which hair appears to originate. Although I see that, for some people, "crown" merely means the top of the head.

[KinkiNipponTourist]

For some reason I knew the word in Japanese (つむじ）but not in English. I looked it up, and it seems to be referred to as a hair whorl or crown, as Ximenean says. But the wiki article doesn't mention the one that everyone seems to have; they just define the characteristic itself.

[Wendell Wagner]

There's a mathematical theorem that says that it's impossible to comb all the hair flat on a sphere unless there's a point without hair:

http://en.wikipedia.org/wiki/Hairy_ball_theorem

Yeah, I know that that's not precisely what it says, but it's as close as I'm going to get to explaining it without going into a lot of detail. Knowing the SDMB, someone is going to berate me for not going into a lot of detail.

[aerodave]

I'm not sure the Hairy Ball Theorem (which is a fucking hilarious double entendre, btw) really covers this issue. That's because head hair doesn't have to cover a whole sphere...and the point of missing hair could be accounted for by the face. It's easy to imagine a non-vanishing continuous vector field being possible within the confines of one's hairline: just imagine hair that grows back away from the face and towards the back at every point. Like naturally slicked back. Hair doesn't grow that way, of course, but it certainly could.

So whatever the reason for the whorl, it's not because it's not mathematically possible to do otherwise.

[Mangetout]

The hairy ball theorem does apply - because it's not natural for straight hair to grow in a slicked-back arrangement. Because of gravity if nothing else.

[Mangetout]

Or to put it another way; if the natural growth habit for straight hair was for it to slick back, hairy ball theorem would not apply and nobody would be askingbabout the rosette at the crown.

[rhubarbarin]

Bald spot? You mean the part of a cowlick where there is a part in the hair because it is all growing strongly in the same direction?

[obbn]

Are you talking about the part of the head where hair doesn't seem to grow? Like my entire head?