From the metacarpal heads to the distal phalanges all five digits are provided with a strong unyielding fibrous sheath in which the flexor tendons lie. In the thumb the fibrous sheath is occupied by the tendon of flexor pollicis longus alone. In the four fingers the sheaths are occupied by the tendons of the superficial and deep flexors, the former splitting to spiral around the latter within the sheath. The proximal ends of the fibrous sheaths of the fingers receive the insertions of the four slips of the palmar aponeurosis. The sheaths are strong and dense over the phalanges, weak and lax over the joints.