The thighbone is the single long bone in the upper part of your leg, running from your hip down to your knee. It is the longest and strongest bone in the human body, which is why doctors talk about it so often: a broken thighbone is a serious injury, common in older people who fall. The useful thing for a student is that English has two words for it. 'Thighbone' is the everyday one, built from parts you already know — thigh + bone — and any ordinary person will understand it. 'Femur' is the medical word, from Latin, and that is what a doctor writes on your chart or says to another doctor. Both mean exactly the same bone; only the room changes. English does this a lot: house words from Old English, hospital words from Latin. Spelling: modern writing joins it up as one word, thighbone, though you will still see 'thigh bone' in two words, and both are accepted. It is countable — you have two of them, one in each leg.

大腿骨、股骨。醫院、病歷上一律寫 femur（股骨），日常對話講 thighbone 就可以。