How do cats purr, by exhaling or inhaling?

Cats purr in the same way lions roar. It is really much like the way humans hum, requiring the breath to rattle through the throat. Although we think purring is an expression of pleasure or contentment, cats also purr when injured or in pain. The theory is that purring may be a form of self-hypnosis, a way a cat comforts itself or conveys a good mood.