This is a costotome! It has two lever-action jaws: one jaw grasps the rib, while the other cuts it!
This is used to cut the cartilage between the ribs and sternum. This is often used in lung surgery.
For open-heart surgery, a sternal saw is more often used.
The sternum (breastbone) is split using this saw in order to access the full thoracic cavity, after which the lungs and ribs are retracted, and the surgery is performed. The sternum is then re-joined.
For anyone who hasn’t seen a heart transplant (which also usually splits the sternum), it’s viewable at this link.
Obviously, not for the squeamish.