 This is my friend Dermatome Man. Dermatome Man is a map. He's a map of all the spinal nerves and the parts of spinal nerve that actually innervate skin. And you'll notice that he has stripes on him and the stripes represent the different spinal nerves sending some sort of sensory fiber or that are carrying some sort of sensory fiber in their spinal nerve highway to this area of skin. Now, I see that I have the unlabeled version of Dermatome Man here. I have one Dermatome memorized and that is the belly button. This is the skin of the belly button. That stripe of Dermatome Man, the sensory neurons that come from this stripe around your belly button travel through spinal nerve T10. Interestingly, the sensory information from your appendix also travels through T10. And if you think about that, it has clinical significance because there's a concept called referred pain where you actually sometimes perceive visceral pain, like pain in a structure like the appendix or your small intestine or your heart. You perceive it as pain in one of your dermatomes. Patients will sometimes come into a doctor's office and say, oh my God, my belly button. There's something wrong with my belly button. It really hurts. And the doctor will look at the belly button and be like, dog, pound. You don't have anything wrong with your belly button. It looks just fine. But knowing that the same spinal nerve is carrying fibers from the skin around your belly button as well as from your appendix, your brain sometimes gets the messages like, oh, mixed up and interprets the message as my belly button hurts, even though it's actually my appendix hurts. It's a really interesting phenomenon. You will not have to memorize dermatoman. I recommend throwing a labeled picture of dermatoman in your external brain, and that's so that you can actually have it to refer to. On an exam, I would definitely have a picture of dermatoman for you to refer to if I were going to ask you questions about that. A question could be something like, I'm going to have to use the T10 once again because I don't know any of the other ones, but look, I'm going to draw a little... Don't you know what that is? It's a spider. It's crawling on your belly button. And I could ask you, okay, there's a spider crawling on your belly button. What is the spinal nerve that is allowing you to feel that sensation? And then you would say, boom, I got dermatoman right here. It's T10. If you had a spider crawling on your shoulder, you could answer that question as well. I got dermatoman right here. That's C5. C5 is letting you feel the sensation right there. Does that make sense? It's pretty straightforward. Now, the other thing that happens with spinal nerves that's interesting is that they actually braid. Dermatome man is not braided. I mean, I guess you could kind of argue, like, really, this is all... It kind of looks like a stripy mess. But spinal nerves, when they becomes a named nerve, like the sciatic nerve or the musculocutaneous nerve or the axillary nerve, these nerves come from braided spinal nerves. And those are called plexuses. So we're going to talk about those next. What is a nerve plexus and why would we have them?