THis is a follow-up question to 248-organs.
The Mishnah in Ohalot (1:8) states that there are 248 organs and lists all of them:
"מָאתַיִם וְאַרְבָּעִים וּשְׁמֹנָה אֵבָרִים בָּאָדָם, שְׁלשִׁים בְּפִסַּת הָרֶגֶל, ..., אַחַת עֶשְׂרֵה צְלָעוֹת, שְׁלשִׁים בְּפִסַּת הַיָּד, ... וּשְׁמֹנֶה עֶשְׂרֵה חֻלְיוֹת בַּשִּׁדְרָה, תִּשְׁעָה בָרֹאשׁ, וכו' :
There are two hundred and forty-eight limbs in the body: Thirty in the foot,..., eleven ribs, thirty in the palm... And eighteen vertebrates in the spinal chord: nine in the head, eight in the neck, etc.
The Mishnah counts 11 pairs of ribs, whereas it is anatomically 12.
Tifferes Israel tries to explain it by the fact they didn't count the first pair but it does not sound so convincing - why not count the first pair?
That brings us back to the question: did the Sages count the bones (organs) empirically or theoretically, based on verses or the tradition? Either way, how did they get to only 11 pairs of ribs?