when creating a complex nurbs object, you have to use patch modeling. Patch modeling involves aligning the edges of the seperate nurbs patches, so that visually, there are no seams. Only visually. It is IMPOSSIBLE to physically combine an entire nurbs head together (if you are going for realism, anyway) into one nurbs surface.

I would recomend creating the head with the mouth and jaw included, as is described in our Radial Head tutorial.