He was lecturing on the cranial nerves, which do pretty much everything for the head. They control all 5 senses and motor output as well as some other non relevant stuff.
Professor: So class does everyone remember what the senses are?
Class: touch/pain, sight, hearing/equilibrium, smell, taste.
Professor: Does anyone know what the sixth sense is?
Class: (thinking hes serious) Guess random shit like magnetoreception.
Professor: The sixth sense is the ability to see dead people.
Class: combination of groans and laughter