Synovial flexor sheaths of hand
In the carpal tunnel the flexor tendons are invested with synovial sheaths that extend proximally into the lower part of the forearm and proceed distally to a varying extent. On the tendon of flexor pollicis longus the sheath extends from above the flexor retinaculum to the insertion of the tendon into the terminal phalanx of the thumb. The tendons of the superficial and deep flexors are together invested with a common synovial sheath that is incomplete on the radial side. This common sheath commences a short distance above the wrist and extends down into the palm. On the little finger, it is continued along the whole extent of the flexor tendons to the terminal phalanx. Next to the first, the fifth metacarpal is the most mobile bone of the palm. The common flexor sheath ends over the remaining three sets of tendons just distal to the flexor retinaculum. In the case of a very mobile fourth metacarpal (e.g. in violin players) the sheath may extend to the terminal phalanx of the ring finger, like that of the little finger in the average hand. The common flexor sheath communicates at the level of the wrist with the sheath of flexor pollicis longus in 50% of individuals. In the index, middle and ring fingers, where the common sheath ends beyond the flexor retinaculum, a separate synovial sheath lines the fibrous flexor sheath over the phalanges. There is thus a short distance of bare tendon for index, middle and ring fingers in the middle of the palm. It is from this situation that the lumbrical muscles arise. The fourth lumbrical obliterates the synovial sheath along its origin from the tendon to the little finger.