Which article to use and why?

Where do you keep them? Sorry, had to ask that :-)

I think in both cases it is better to use 'the'. The 'the' here does not refer to a particular limb but is used in a generic sense.

'The giraffe is the tallest of the animals'

'I can play the guitar'

Source: Raymond Murphy, English Grammar in Use, p. 150

You're looking at a snowclone.

• 'He has the X of a Y'

(where X and Y are suitable nouns/NPs, and 'he' can be a noun, 'she', 'it' or with verb adjustment 'I', 'you' or more rarely 'they').

The articles are respectively always 'the', and almost always 'a/an' though this may be replaced by the zero article if the second N/NP is plural.

'Mind' may be the original noun X, though 'body' is a close contender.

• "My brother has the mind of a scientist or a philosopher and yet he elects to be a detective." ['Sherlock'].
• 'This guy has the mind of an 11-year-old.'
• 'She has the body of an angel.'

Individual body parts for X in examples showing a closer relationship are rather unusual, though expressions like

• You have your mother's eyes, Harry.'

are of course common. But note that here 'inherited' rather than 'of a type / standard that is usually associated with' is the stronger semantic relation.

Used – let's say morefiguratively – this restriction is less binding:

• "I have the eyes of a hawk and the ears of a fox." [Gimli; The Fellowship of the Ring; Tolkien; film adaptation]

But 'I have the leg of a footballer' sounds unusual. 'He has the legs of a fellwalker' seems perhaps more suitable. The comparator is perhaps more likely, and most people have two legs – and while using a single example ('She has a good eye for a bargain') to refer to a plurality is common, novel examples ('the finger of a pianist'; 'the leg of a footballer') don't sound natural.