How do I open the safe?

In case you haven't found it, the safe is the item that you can take out of
the freezer in the room with the severed heads.

Put the safe in the box on the left side of the machine next to the safe. 
Then, when you go for a close-up view of the safe, you'll be able to look at
its top.  That's where you'll find the dial.

So now you need to look for the combination.

Part of the machine is an x-ray device.

If you pull the lever to the right of the little box, you'll turn the x-ray
on.  Go to another close-up of the safe, and you'll see two numbers.

Those two numbers are part of the combination.

Someone else can give you the rest of the combination.  WARNING: the sequence
of puzzles that you need to solve to get the combination is somewhat
gruesome.  If you want to avoid this sequence, read to the end of these hints
for the answer.  You do not need to solve any of the intermediate puzzles --
all you need is the combination itself.

There aren't any live people around here, but that's okay.

Maybe a dead one could help you.

Go down to the morgue in the basement.  If you look through the drawers,
you'll eventually find one that contains a body.

In case you hadn't noticed, Sartorius seems to have an obsession with severed
heads.

So maybe you need to decapitate this body.

There's a machine in the basement made to do just that.

All you need to do is put the body in the machine's box (along the wall
directly opposite from the elevator), and pull the levers on the control
panel in the middle of the room.  The game only lets you pull one lever at
any given time -- click on the levers in order from left to right.

Then, go back to the machine itself, and you'll find the severed head in a
bucket.

Now, it's time to make the head talk.