It was the beating of the old man's heart. It's not madness, says the narrator, "but over-acuteness" of his sense of hearing caused by his nervousness, as he says in the opening paragraph of the story. ... The narrator's obsession shifts from the old man's "Evil Eye" to the old man's loudly beating heart.
One way to prevent taeniasis is to cook meat to safe temperatures. A food thermometer should be used to measure the internal temperature of cooked meat. Do not sample meat until it is cooked. USDA recommends the following for meat preparation.
