How to split a patch element (struct) into two half

I have a 3D mesh represented in a patch object inside a struct of faces and vertices. i need to split this faces and vertices in two halfs. first from 0-11 and the second from 11-22. as shown in the figure attached. also the .mat file attached.

[madhan ravi]

What have we tried so far?

[sana3 sal]

If you opened the .mat file you would find a struct that is built as this https://en.wikipedia.org/wiki/Polygon_mesh#/media/File:Mesh_fv.jpg

I dont know how to access the elements inside the struct to extract only vertices that their z axis is from 0 to 11, and then extract the related faces.