"database" is a very imprecise term here. SomePeople use that term to refer to a table, a file (which can have many tables) or a group of files.
In this case, it doesn't matter. Open up Manage | Value Lists and look for your value list of dentists. I suspect that you have a custom values value list where the names are typed into the value list instead of listed from the dental table. This is something you can change quite easily by setting up a value list that uses the "use values from a field" option.
But I sugest that you not match records in Dental to records in Patient by the dentist name. Names are neither unique nor unchanging and this will cause problems for your database. You will be much better off if you define an auto-entered serial number for each record in the dental table and then use a value list where this ID is specified as the first field and the dentist's name field ils specified for field 2. This allows you to select a dentist by name, but enter the ID number for that dentist when you make a selection from the value list.