Jon: more disturbed by revealing his ignorance to his dream girl, or by coming face to face with a depiction of his best friend's guts? My guess: just happy to be so grossly violating Liz's personal space.
Liz: obsessively looking under Garfield's eyelids. She just fooled with his eye yesterday. Perhaps she is just excited to see a cat with human-sized eyes. Or maybe something in the ear-exam made her double-check the eye.
Garfield: thinks digestive tracts can talk, sure. Sadder is that he thinks all cats are as obese and food-addicted as himself.
Jim Davis: I hope Jim did intensive, cat-dissecting research to draw that diagram. He does deserve a ribbon for what is surely the most detailed depiction of innards in American comics page history.