Medical school. It's as hard as everyone says. Many mnemonics exist for learning anatomy. This famous phrase deals with the nervous system: "Sympathetic, fight or flight, parasympathetic, rest and digest." These two divisions are part of the autonomic or visceral system, involuntary parts that regulate your internal organs, sweat glands, and arrector pili muscles which make your hair stand on end. Regarding the mnemonic, how do you make sense of erections? Erections are handled by the parasympathetic system, and ejaculation by the sympathetic system. I believe this means that if you had an erection, and a bear charged at you from the forest, you would ejaculate, but as I have no experience with this situation, I may be wrong.
If this bear happened upon you, he may ask if you would like tea. And let's keep our forests and hiking trails erection-free and do more of the rest and digest thing rather than test crass musings.