Latin musculus, little mouse, came from the fancied resemblance of a flexing muscle to a mouse moving under a sheet. Greek used mys for both mouse and muscle, the same root giving us myology.
Some note that anatomy is a zoo: muscles are mice, the inner ear has a snail (cochlea, Greek kokhlias), the knee cap is a little pan (patella), the pelvis a basin. A speculative reading holds these names keep an animist worldview hidden inside clinical language.
Picture where the word was born: the Greek gymnasion, where athletes trained naked and oiled, and physicians stood close enough to watch tendons roll under the skin like small animals darting. Greek made the image official, mys served for mouse and muscle alike. Rome shrank it to musculus, little mouse, and named the upper arm lacertus, the lizard. When Arabic scholars translated Galen, the mouse kept running in their tongue as well. Three civilizations, one shared hallucination: something alive beneath the skin, doing the pulling.
Medicine is built on metaphors frozen into names. Which of them still shape how we picture our own bodies?