Tendon · Hand

Extensor expansions of the fingers

The flat triangular hood of tendon spread over the back of each finger. The long extensor tendon arriving from the forearm does not simply insert somewhere; it flattens out over the knuckle into this sheet, which then divides into a central slip attaching to the base of the middle bone and two lateral bands that run on and rejoin to reach the base of the fingertip. What makes it remarkable is who else joins it: the small muscles inside the hand — the interossei and the lumbricals — insert into the sides of this hood rather than pulling on the finger bones directly, the lumbricals reaching bone not at all and the interossei only partly, by a slip to the base of the first phalanx. That is how a muscle sitting in the palm ends up straightening the tip of a finger. It is the reason finger extension is a shared arrangement between forearm and hand rather than one muscle's job, and the reason a hand can bend at the knuckles while keeping the fingers straight.