Because until we open the chest, we can't be sure our body interior looks properly realistic. So we tell everyone to take a break while we open the skin of the chest and "dress" the interior - adding any needed blood or details. Then we bring our actors back in and film them as they pantomine peeling the prepared skin with their cutting tools.
This leaves us with a "missing" scene between the original incision and the skin-peeling already in progress. But it's a minor omission - and it covers a multitude of possible sins.
After our skin-peeling scene, we can arrange our organs as needed before we roll camera again.
Ewwwwww!
Our shaky, soft focus cinematography should help hide the fact that we're looking at a random pile of disconnected organs. Now we can get loads of film of our actors as they remove these "organs" one by one.
We cheat just a bit and skip the moment where the skull is first exposed to allow for any needed touchup work, then let our actors peel the scalp back. We give our actor a saw and let him grind away on the underskull for a while.
Skipping the actual removal of the skull cap, we shoot the removal of the brain from a low angle where the skull can't be seen. We throw one of our organs in there and roll camera as it oozes out.
And that's our big finish.
The Truly Dangerous Company