It's been a loooong time since I played this game, but I don't remember any limitation on the inventory. There are three main things you have to do, in this order:
1. On the wall to the far right, click on the leftmost carving.
2. Click the parchment in your inventory on the ?
3. On the stele beside the old woman, click your parchment on the cobra symbol on the eighth square from the left in the top row...
"Once you give up integrity, the rest is easy." Anonymous