In The Merchant of Venice, why does Shylock claim a pound of Antonio's flesh and not the heart itself?

Actually, if you look carefully at the text, Shylock is
very careful to say that the pound of flesh could be cut from wherever he chose.
Therefore, in theory, he could have chosen to cut out the heart of Antonio. If we look
back at Act I scene 3 which is an important scene for many reasons, we see the exact
terms of the "merry bond" that Shylock draws up with
Antonio:

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...let the
forfeit

Be nominated for an equal
pound

Of your fair flesh, to be cut off and
taken

In what part of your body pleaseth
me.

So therefore, whilst
Shylock does not specify which part of the body he will take as forfeit, he could have,
in theory, chosen to cut out the heart of Antonio as his pound of flesh. I suppose we
need to think about Shylock's motivation in answering this question. He is after revenge
for the wrongs that Antonio has committed against him and his race, and thus his desire
is to cause pain, to mutilate, and perhaps to kill.