A B737-300 CREW ENCOUNTERED SEVERE TURB 10 NM FROM TSTM CELLS AT FL340. A PAX IS INJURED DURING TURB ENCOUNTER; WHILE NOT WEARING SEATBELT. EMER DECLARED.

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AT APPROX FL340; RADAR INDICATED A LINE OF TSTMS 160 NM AHEAD RUNNING ABOUT 120 MI N AND S. I REQUESTED AND WAS CLRED FOR A 10-15 DEG L DEV FOR STORM AVOIDANCE. THERE WAS A 20 NM BREAK BTWN THE MAIN LINE OF RETURNS AND THE SOUTHERNMOST CELL; AND I DECIDED TO FLY THROUGH THE GAP. ABOUT 20-30 MINS PRIOR TO THE ENCOUNTER; I ILLUMINATED THE SEATBELT SIGNS AND ADVISED THE PAX OF POTENTIAL TURB. THE FLT ATTENDANTS WERE ADVISED VIA INTERCOM TO PREPARE THE CABIN FOR POTENTIAL MODERATE TURB AND TO BE SEATED. APPROX 2/3 THROUGH THE BREAK WE ENCOUNTERED APPROX 10 SECONDS OF SEVERE TURB WITH A R 30-40 DEG DSNDING ROLL (1000-1500 FPM). DURING THIS TIME THE AUTOPLT REVERTED TO CTL WHEEL STEERING AND THE FO MANUALLY RESUMED CTL WHILE I REQUESTED AND RECEIVED CLRNC TO FL320. A FEW SECONDS LATER WE WERE CLR OF CLOUDS IN SMOOTH AIR. FOLLOWING THE EVENT; I COMMUNICATED WITH ATC; FLT ATTENDANTS AND DISPATCH. FLT ATTENDANTS ADVISED THAT 1 PAX; NOT WEARING SEATBELT; WAS INJURED WITH BRUISED HEAD AND NECK AREA. INJURED PAX ATTENDED BY PAX RN AND EMT.