The Small Forearm Muscle Some People Never Develop
The missing structure is the palmaris longus, a slender muscle with a long tendon that runs down the forearm and crosses the wrist. In people who have it, the muscle can help bend the wrist and tighten the palm’s connective tissue. Its contribution is minor, though: other muscles do most of that work, so its absence usually causes no noticeable loss of strength or movement.
You can sometimes spot the tendon by touching your thumb to your little finger and gently bending your wrist. A cord may rise near the middle of the wrist. Not seeing one does not necessarily mean the muscle is absent on both sides; it can be missing from one arm, both arms, or difficult to see.
A Spare Part With a Practical Use
The palmaris longus is an example of anatomical variation: bodies do not all develop exactly the same set of structures. When the muscle is absent, the wrist’s other muscles and tendons compensate well enough that everyday tasks carry on normally. Its low importance helps explain why losing it has little consequence, though it does not prove precisely why the variation evolved.
Surgeons can also make use of the tendon when repairing other tendons. Because it is generally not essential for wrist function, it can be removed and used as a graft—a piece of tissue that helps replace or reinforce damaged tissue elsewhere. So this small evolutionary leftover is not quite useless: for some people, it is an unobtrusive spare part.
