What were the actors actually doing during the surgery scenes?

In watching the show I'm always impressed with the attention to details all the actors put into the surgery scenes. Even the ones in the background "look" like they are performing surgery.

But I've always wondered, what were they actually doing? Was there a dummy on the table and they were pretending to operate on that? Was the table empty and they were just really good the performing the movement?

When they were pretending to sew, or retract, sponge, etc. Was there something actually on the table or was it just empty?