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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.

So:

What is the physiological definition of both "arm" and "leg"? How are they different?

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    $\begingroup$ perhaps to do with whether or not they are walked on? $\endgroup$ – rg255 Sep 26 '14 at 16:59
  • $\begingroup$ Why do they have to be defined physiologically? It is a matter of common everyday language, and human language is generally inconsistent and full of "because we've always called it this way" and "because they look similar" reasons. Do you have any example of any scientific research which divides appendages in "arms" and "legs" and proves results which are true for the one and not for the other? Maybe if you got to stuff which is about gripping, or bearing weight, but then you are back at the informal language level. $\endgroup$ – rumtscho Sep 26 '14 at 22:49
  • $\begingroup$ I don't think "arm" or "leg" is a matter of everyday language, and it just seems really silly that I don't know a) why my cat has four legs instead of two arms or b) how many legs/arms a monkey has. $\endgroup$ – Danny W. Sep 27 '14 at 12:14
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I think that's more of a matter of linguistics rather than any scientific/physiological reason. For instance, in Portuguese, you wouldn't call legs to a cat or a dog's limbs. You would say they have 4 "patas" (roughly translated as "paws"). And calling them legs, in Portuguese at least, would be weird.

When veterinary doctors refer to animal limbs, they usually use anterior and posterior limbs. In fact, anatomically speaking, anterior limbs in cats and dogs (and probably in other animals too) have more in common with human arms (upper limbs) that with their own posterior limbs.


Also, as rg255 commented, maybe the reason behind the colloquial nomenclature is quite simple...

To (miss)quote Nancy Sinatra:

These "legs" are made for walking

And that's just what they'll do

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