fontanelle

The fontanelles ("soft spot") are two* areas on the head of an infant where the skull bones do not completely cover the brain. There is a larger, diamond-shaped one on the top of the head, slightly towards the front (anterior fontanelle). This is the "soft spot" people speak of. There is another smaller, triangular (posterior) fontanelle at the upper rear portion of the head. It is difficult to find after a month or two. Lying just beneath the anterior, or larger, more forward diamond-shaped fontanelle is a large vein. The pulsations of the heart can usually easily be seen or felt when the baby is quiet.

The posterior fontanelle is no longer palpable by the fourth month, but the larger anterior fontanelle can be felt until 9-16 months of age (although it can be seen on xray for a few more months).

Fontanelle facts and fallacies:

That they are delicate and can't be touched. Actually, the membrane underlying the "soft spot" is pretty tough.
The soft spot can "fall" and become somehow stuck. No, if the baby is healthy and not dehydrated, the fontanelle will indeed sink into the head a bit when the baby is upright since blood is not filling the central vein underneath it as fully. No big deal. If it bothers you, just lay the baby back in the horizontal position - voila - the fontanelle is full again.
A sunken fontanelle can be a sign of serious dehydration in a child known to be ill with fever, vomitng and diarrhea - gastroenteritis. Sunken eyes, dry mouth, and decreased or absent tears would likewise be noticible.
A bulging fontanelle could be benign intracranial hypertension (pseudotumor cerebri) or a sign of meningitis.

* OK, there are two more tiny lateral fontanelles on the side of the head that I have never felt; they are really only visible on xray.

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