Words that have 2 syllables, whose second vowel is ı/i/u/ü are prone to losing these vowels when inflected in accusative, dative, possessive and genitive.
A lot of the body parts behave this way: ağız, burun, koyun, göğüs, karın, omuz…, but not only the body parts:
şehir (city), ufuk (horizon), vakit (time), kasıt (intention)…
Unfortunately, there is no rule.
Thanks, you are fast to answer. Between you and Deorme I have now a list of 8 irregular body parts. Only question remains where in your body you have the 'koyun' ? Except if it was for the inside of your stomach you should move it down to the second list. :D Thanks again.
Ha! Koyun is also your "chest". It's a poetic word that you can hear in songs. (Here's an example: https://www.youtube.com/watch?v=aiMB4HgA1EY go to 1:16)
I think that's all the body parts that undergo such a change! :-] Unless we also count "gönül", which kind of means "heart", but it's not really a body part. It's a literary reference to love.
So yeah, that's: 8+1 !