What's meaning of this saying regarding the word taraib

في قوله : يَخْرُجُ مِنْ بَين الصّلْب والتّرَائِبِ قال : الترائب : الأضلاع التي أسفل الصلب This roughly translates in English to:(taraib "ترائب") are the ribs that are below the backbone (sulb"صلب"). What does ribs below backbones means? Did any scholars ever explained the meaning of this? Please help me understand? Is this referring last ribs like floating ribs 11 or 12?