Please do not attempt to do this, it can be very dangerous!
Some folks have argued about the existance of a PTC in 123 batteries. I'd like to assure folks that at least Duracell, Energizer, and Surefire all use PTC devices in their Primary Lithium Batteries.
I also took the PTC and measured it's resistance with temperature, and once it hit a certain temperature, it's resistance rapidly increases, to remove the load to a cell, in order to protect the user from a battery explosion.
Next, the rest of the cell: