You may be missing an arm muscle and never notice

Put your left hand palm up, resting on the table. Touch your thumb to your little finger and bend your wrist slightly towards you. If a thin, taut tendon rises in the middle of your wrist, like a guitar string, you have a palmaris longus. If nothing rises, you don't have one, and nothing is wrong with you: it is missing in roughly one arm in seven.

It is a narrow muscle running from the elbow to the palm, and it does almost nothing for you. Measured in the laboratory, your grip strength is the same with it and without it. We inherited it from ancestors who hung from branches and genuinely used it; in us it is a loose end of evolution.

That is why surgeons go looking for it. When a thumb tendon has to be rebuilt or a ligament repaired, that thin tendon is ideal spare material: it is taken out, transplanted, and the arm goes on doing exactly what it did before. And having it in one arm is no guarantee of having it in the other: some people are missing it only on the left.