Ok, this is a bit of a tangent question, but it came up yesterday and I didn't know the answer: How are arms and legs defined physiologically? For example, we say humans have two arms and two legs, while cats have four legs, and some monkeys (appear) to have four arms (although I guess they could be legs too). It's really unclear to me how we make these distinctions.
What is the physiological definition of both "arm" and "leg"? How are they different?