ZBrushCentral

no inner sourface's geometry

Hello…Here is my trouble.I was cutting a part of a mesh and i was sculpting it.Then i realize that it missed the geometry of the inner sourface(i mean,if i rotate the object to the inner sourface i can’t see anything.
Can anyone explain me how to built the iner sourface geometry so that i can model also the inner sourface?
Thanks very much.Bye