The carpus is deeply concave on its flexor surface. This bony gutter is converted into ‘the carpal tunnel’ by the flexor retinaculum. The long flexors of the fingers and thumb run through the tunnel. The four tendons of the superficial flexor are separate from each other and pass through in two rows, middle and ring finger tendons lying in front of index and little finger tendons. The tendons of flexor digitorum profundus, on the other hand, lie all on the same plane deep on the carpal bones; but here only the tendon to the index finger is yet separated, the other three being adherent to each other as they run in the tunnel, and not splitting free until they gain the palm. All eight tendons of the superficial and deep flexors share a common synovial sheath. It does not invest them completely, but is reflected from their radial sides, where arteries of supply gain access to the tendons. It is as though the tendons had been invaginated into the sheath from the radial side. The tendon of flexor pollicis longus lies in its own synovial sheath as it passes through the radial side of the flexor tunnel. The median nerve passes beneath the flexor retinaculum on the lateral side of flexor digitorum superficialis (middle finger tendon), between it and flexor carpi radialis.