Figure 2b. Carpal and Guyon tunnels. (a, b) Drawing (a) and corresponding transverse 5-12-MHz US scan (b) show the proximal level of the carpal tunnel delimited by the pisiform (P) and the scaphoid (S). (c, d) Drawing (c) and corresponding transverse 5-12-MHz US scan (d) show the distal level of the carpal tunnel delimited by the hook of the hamate (H) and the tubercle of the trapezium (T). The flexor retinaculum (medium gray region in a and c; open arrowheads in b and d) forms the roof of the carpal tunnel and the floor of the Guyon tunnel. The palmar carpal ligament (dark gray region in a) forms the volar boundary of the Guyon tunnel. The flexor tendons and median nerve (MN) extend through the carpal tunnel, with the nerve lying palmar and radial. At the level of the pisiform, the ulnar nerve (U) courses medial to the ulnar artery (solid arrowhead) within the Guyon tunnel. At the level of the hamate, the ulnar nerve divides into two terminal branches, a deep motor branch (curved arrow) and a superficial sensory branch (straight arrow). * = flexor pollicis longus tendon, * = flexor carpi radialis tendon.