CAPT OF SCHEDULED AIR CARRIER DISORIENTED WHILE TAXIING AND INADVERTENTLY CROSSED ACTIVE RWY WITHOUT CLRNC.

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WE DEPARTED GATE 6 AND GOT INSTRUCTIONS TO TAXI TO RWY 21. I TURNED R ONTO TXWY 'A' FROM THE RAMP AREA. I FAILED TO TURN R AGAIN AT 'B' AND INSTEAD CONTINUED STRAIGHT AHEAD ALONG 'A' AND CROSSED RWY 3-21 WITHOUT A CLRNC. APCHING THE END OF 'A' THE COPLT ASKED ME WHERE I WAS GOING. I SAID I WAS GOING OUT TO RWY 21 AND LOOKED AT THE DIRECTIONAL GYRO EXPECTING TO SEE 030 DEGS (I ASSUMED THE TXWY 'A' WAS PARALLEL TO RWY 21). THE DIRECTIONAL GYROSCOPE SAID 330 DEGS. MY FIRST REACTION WAS THAT THE TXWY WAS NOT PARALLEL TO RWY 21; BUT IT BEGAN TO DAWN ON ME THAT ALTHOUGH THIS WAS TRUE THE LARGER PROB WAS THAT I HAD BEEN DISORIENTED WITHOUT REALIZING IT. THE READER HAS BY NOW FIGURED OUT THAT I WAS NOT A NATIVE OF FOSS FIELD. A FEW OTHER FACTORS CONTRIBUTED TO THIS INCIDENT. THERE WAS NO TFC; SURFACE OR AIRBORNE; TO PROVIDE CUES OR A FOCAL POINT FOR ONE'S ATTN DURING OUR TAXI OUT AND TKOF. THE NIGHT WAS CLR BUT DAYTIME VISIBILITY MAY HAVE PREVENTED NOT ONLY TUNNEL VISION BUT ALSO TUNNEL CONSCIOUSNESS. THERE HAD BEEN A PROB IN THE CABIN THAT TOOK UP MOST OF TIME AVAILABLE TO ME BEFORE GATE DEP THAT WAS NOT SPENT ON THE NECESSITIES OF CHKLISTS AND COORD WITH GND HANDLING CREW. THE MAJOR CONTRIBUTING FACTOR WAS THE FACT THAT A LOSS OF ORIENTATION LIKE THIS HAD NEVER HAPPENED TO ME BEFORE WITHOUT MY BEING AWARE OF IT. THIS IS AT ONCE THE MOST EXCULPATING -- AND THE MOST DAMNING - - FACT THAT CAN BE BROUGHT TO LIGHT CONCERNING MY AERONAUTICAL BEHAVIOR NOW AND IN THE FUTURE. WE HAD ASKED FOR MORE SPECIFIC INSTRUCTIONS THAN WERE INITIALLY GIVEN TAXIING IN AND THE GND CTLR EXPRESS REGRET THAT MORE SPECIFIC INSTRUCTIONS WERE NOT PROVIDED TAXIING OUT. BUT THE FAULT FOR THE BLUNDER WAS MINE. I AM GRATEFUL THE IMMEDIATE RESULTS WERE INCONSEQUENTIAL. IN THE FUTURE I WILL ADJUST SOME PERSONAL BEHAVIOR PATTERNS WHEN TAXIING AT UNFAMILIAR ARPTS AND CONTINUE IN GENERAL TO BE AS CAREFUL AS I CAN.