rendering a subregion of a 3D texture?

How do I render only a certain region within a 3D texture. I mean when I map the 3D texture, I want only a subvolume on it to be rendered. I’ve read in the redbook that glpiexelstorei(GL_UNPACK_SKIP_IMAGES,n) will increment the opengl pointer to a certain region within the 3D Texture, but it didn’t work. I got an access violation message, I’m using opengl 2.0 on Geforce 7800 GTX 256MB.