To begin creating the inside of the mouth, first draw a radial loop just outside the edge that defines the center of the mouth. Make sure you split the edges at the corner of the mouth instead of taking the edge to a single central point. If you split the edges you will be left with quads and this is what we want.
Next, select all of the inside faces you just created.
Then, press the delete key on your keyboard to remove them. This will create an opening in the mouth if you have split the corner correctly, you will have nothing but quads left over. If you have done it wrong, you will have triangles at the corner. If you do... fix them up by drawing in new edges so all you have is quads before proceeding to the next step
Next, go to your side viewport and view your model in wireframe. This will allow us to see where we are going to place the inside of the mouth.
Using your ep curve tool (go to create>ep curve tool) trace out 3 curves that define the inside of your mouth. One from the upper lip back to the throat, another from the lower lip back to the throat and a third beginning at the corner of the mouth, going half-way through the other two curves, all the way back to the throat.
Next, go into your perspective viewport and move the center curve in the X direction to the corner of the mouth.
Now that we have our curves in place, we need to create a surface out of them. I could simply loft them but this would
give me unpredictable results and put isoparms wherever Maya feels like. I want to have more control and be able to direct the iso's where I want them to go. So next we are going to force the Iso's to go the way we want them to.
The next illustration shows the direction I want the Iso's to go.
So... with that direction in mind... I right click over my curves and select curve point from the marking menu. I select the points where the red lines in the above illustration intersect the curves. This will leave a little yellow dot that indicates where we are going to cut our curve later. If you hold down shift while selecting the other curves you can perform this action on multiple curves at once.
Notice how the yellow dots line up directly with the red lines in the above illustration.
Once you have selected all your curve points, you should cut your curves. You can do this by simply going to edit curves>detach curves. Now wherever you had a yellow dot, the curve should be cut, leaving you with a bunch of little curves.
These curves are still not ready to loft yet. We must do one more thing first... rebuild our curves so that we have an even amount of isoparms on each surface when we loft them.
To do this... select all your curves and go to edit curve>rebuild curve and open your options dialog box. Set the number of spans to 3 and hit the "rebuild" button in the lower left of the box to apply.
Now we are ready to loft our surfaces. You might want to go to your viewport settings and turn off your polygon display to
make this step easier (show>polygons).
Start by selecting the top curve in front (upper lip), then holding down shift, select the next curve in the middle, then last but not least, select the bottom curve (lower lip). Then to loft them, go to surfaces>loft. Repeat this pocess for the rest of the curves.