Schenkerian Analysis: "fundamental Structure" Outside Voices Only?

Hello everyone

Just a simple question about basic Schenekerian analysis, because I need something clarified.

The fundamental structure; the two voices that are left when you reduce any texture to its most simple possible form, do these have to be the outside voices i.e Bass and Soprano? I'm reading a few books on the theory, and they start with a simple two voice structure and then elabrorate it into a more comlex multi voices texture to show how it works, but the two voices that were started with always end up being the two outside voices of the new texture (though elaborated on with passing and embelishing motions etc...). Is it possible for the upper of the two voices, the most reduced form of a texture, to not represent the soprano, so that the alto for example is responsible for the basic structure?

Thanks

I could be possible that the fundamental structure is not in the bass and soprano but in other voices. However, it would take a lot to argue this as by their very nature the soprano and bass are fundamental points of a given piece. The only way I could think if it not being in the soprano or bass is if two things were to happen; the alto is the fundamental bass line and the surface bass line functions more as an embellishment to the alto and/or the soprano line is treated as a descant or ornamentation to the melody found in the alto or tenor line-- but theses are BIG maybes and would require a ton of evidence and persuasive arguing. And then even after that, your fundamental would still like like a soprano and bass line anyway once everything is reduced. 

One should also remember in major works, like a movement in a symphony, the fundamentals lines would be shared among many parts. Like in Beethoven's 3rd symphony, one would have to take into account the melody starting in the cellos before its handed off to the first violins and other parts. 

I would take a look at other pieces, particularly longer and bigger pieces, to see if it always just the outer voices and if not how is it analyzed in Schenkerian terms. I would also start at the surface level first and work your way down, and not the other way around. And take a look at journals and articles that analyze music through Schenkerian theory. 

Thanks for your help.