 Hey everybody, Dr. O here, let's talk about the sacrum and the coccyx. So the sacrum is this triangular bone at the base of the lumbar spine, so it actually is going to be connected to the L5 vertebrae, your fifth lumbar vertebrae, by, you see, the facets of the superior articular processes, so that's going to be the joint that holds them together there. The sacral promontary is going to be there on the top, but the sacrum itself, it's as large triangle that tapers down at the end at what's called an apex. That is, it comes from the fusion of five sacral vertebrae, and you can kind of see the remnants of this fusion process. The transverse ridges would be the sites of vertebral fusion, then on the back, the median sacral crest would be like what would have been a spinous process of a normal vertebrae, and the lateral sacral crest would be what the transverse process would have been with the normal vertebrae there. Again, you know, not very, very important stuff there. The sacral promontory we already said is on the top, that's going to be that anterior lip at the base or top of the sacrum. Then we have to notice those rough edges on the side, the auricular surface, that's where they fuse with the ilium and what's called the sacroiliac or sacroiliac joint. All the openings and tunnels you see here, you have the sacral canal which runs down the center, which is going to be where nerves go, and then they're going to exit on the posterior sacroformina and anterior sacroformina, so that's how all the nerves are going to escape through that. The coccyx cells is your tailbone. This comes from the fusion of four very small coccygeal vertebrae, and they're all fused together. They're not involved in weight-bearing when you're standing, maybe a little bit when you're sitting, but that's going to be the tailbone there. You can actually see if someone has a fall on their hind end, you can see a dislocation of the coccyx there under the tailbone sometimes, but probably as often, if not more often, than actually breaking it. What about this fusion? This fusion happens pretty late, so it probably isn't done until the ages of six, maybe it doesn't even really begin until your teen years, so like 16 to 20 is probably when the sacrofusion actually begins, so these bones take a long time to get this way. But that is the sacrum in the coccyx, not a ton to know there. I hope this helps. Have a wonderful day. Be blessed.