Engine Bay Thingy

Excuse the dumb question .. I'm still learning. While gazing around the engine bay of my 328 (1988) I spied this small cable emerging from a rectangular opening in the rear aluminum firewall and disappearing under the lower portion heading toward the muffler. It is small (like 1/8 inch dia.), looks like it is stainless steel, and does not look like a wire but rather a braided cable with ??? inside (or not). I checked out two other 328's and they have it too. So the musical question is: what is heck is this thing and where does it come from and go? Thanks.

Robert