During early pregnancy, both male and female human embryos develop something called milk lines, ridges that eventually recede into nipples. Why have some species lost this unisex trait, and can human men actually produce milk?
WHAT'S INSIDE: HUMAN BODY
- Body Parts
- Body Systems
You remember a little ditty about the thigh bone being connected to the hip bone, which was in turn connected to the backbone. But what else do you remember about human anatomy? The answers to this quiz are all inside of you -- literally.
The heart muscle is required to keep a steady beat. Learn all about the heart and how it functions in these videos on HowStuffWorks.