For an example, the professor pulled back the sheet, stuck his finger into the butt of the cow, withdrew it, and stuck his finger in his mouth.
He then told his students, "Go ahead and do the same thing."
The students freaked out, hesitated for several minutes, but eventually took turns sticking a finger in the butt of the dead cow and then sticking their finger in their mouth, followed by assorted gagging, retching, etc.
When everyone had finished wiping their faces, the professor looked at them and said, "The second most important quality is observation. I stuck in my middle finger and then put my index finger in my mouth. Now learn to pay attention. Veterinary life is tough but it's even tougher if you're stupid."