Two sheaths envelop the tendons as they pass beneath the transverse carpal ligament, one for the Flexores digitorum sublimis and profundus, the other for the Flexor pollicis longus.
They extend into the forearm for about 2.5 cm. above the transverse carpal ligament, and occasionally communicate with each other under the ligament.
The sheath which surrounds the Flexores digitorum extends downward about half-way along the metacarpal bones, where it ends in blind diverticula around the tendons to the index, middle, and ring fingers. It is prolonged on the tendons to the little finger and usually communicates with the mucoussheath of these tendons.
The sheath of the tendon of the Flexor pollicis longus is continued along the thumb as far as the insertion of the tendon.