A large Tiger shark rolls its eyes back in its head while taking a bite out of a Humpback Whale carcass. This process is referred to as ocular rotation where their eyes will roll back in their head while biting and a tough piece of cartilage called the nictating membrane protects the eye.  Sharks do this so they don't damage their precious eyes while hunting and feeding, as this is the mostly likely time their prey can inflict damage in an attempt to protect themselves.