ACR WDB PIC BECAME INCAPACITED DURING NORTH ATLANTIC CROSSING.

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PRIOR TO DEP ON THE FLT FROM BOS TO PIK (PRESTWICK; SCOTLAND) THE CAPT SAID HE HAD A SLIGHT CRAMP IN HIS UPPER BACK BUT OTHERWISE FELT FINE. HE ASKED IF I WOULD FLY THE LEG OVER TO PIK AND HE WOULD FLY BACK THE FOLLOWING DAY. THE FIRST HALF OF THE FLT WAS NORMAL WITH ME FLYING AND THE CAPT HANDLING THE COM. ABOUT HALF WAY OVER (3 HOURS INTO THE FLT) THE CAPT BEGAN TO COMPLAIN ABOUT THE CRAMPING IN HIS BACK AND BEGAN TO SHAKE. HE SAID HE WAS NOT IN PAIN BUT JUST UNCOMFORTABLE. GRADUALLY; HE EXPERIENCED MORE DISCOMFORT AND SHAKING TO THE POINT WHERE HE NEEDED TO GET OUT OF HIS SEAT FREQ AND DID NOT WANT TO TALK ON THE RADIO. AT APPROX 2 HOURS OUT FROM PIK HE DECIDED THAT HE WOULD LIKE A DOCTOR TO EXAMINE HIM UPON ARR. AS THE FLT PROGRESSED HE BECAME MORE UNCOMFORTABLE AND APPREHENSIVE ABOUT HIS CONDITION. AT 1 HOUR OUT FROM PIK THIS S/O NOTIFIED THE COMPANY VIA PIK OPS TO ARRANGE FOR A DOCTOR TO MEET THE ACFT AND TO NOTIFY THE COMPANY THAT WE WOULD NEED A REPLACEMENT CAPT FOR THE RETURN FLT THE NEXT DAY. ON DSCNT WE DECIDED TO HAVE THE S/O HANDLE THE ATC COM WHILE I FLEW THE ACFT. AT APPROX 15000' MSL ON DSCNT THE CAPT AGAIN GOT OUT OF HIS SEAT AND DID NOT SEEM TO WANT TO BE NEAR THE CONTROLS. AT ABOUT 10-12000' HE WAS STILL OUT OF HIS SEAT AND I SUGGESTED THAT WE PUT THE S/O IN THE CAPT SEAT AND THE CAPT IN THE S/O SEAT. THE CAPT AGREED THAT THIS WAS DESIRABLE AND IT WAS DONE. THE S/O THEN PERFORMED THE DUTIES OF THE PLT NOT FLYING WHILE THE CAPT READ THE CHKLISTS AND MONITORED THE APCH FROM THE S/O SEAT. THE APCH AND LNDG WERE NORMAL. I SLOWED THE ACFT TO A VERY SLOW TAXI SPD ON THE RWY AND THE S/O TAXIED THE ACFT TO THE GATE. PIK HAS A WIDE OPEN RAMP AREA AND THE S/O HAD RECENTLY BEEN THERE WHEREAS IT WAS MY FIRST TIME IN PIK. HE ALSO WAS CURRENT AS A PLANE COMMANDER IN THE MIL SPC WITH RECENT TAXI EXPERIENCE. I BACKED HIM UP ON THE BRAKES AND HE DID AN OUTSTANDING JOB. HE COULD ALSO MONITOR THE S/O PANEL FROM THE CAPT SEAT. A DOCTOR EXAMINED THE CAPT UPON ARR AND PRESCRIBED SOME MUSCLE RELAXANT TYPE MEDICATION. THE CAPT IS UNDERGOING FURTHER MEDICAL TEST. SUPPLEMENTAL INFO FROM ACN 90752. THREE HOURS OUT OF BOS MY BACK AND NECK BEGAN TO TIGHTEN UP NOTICEABLY. WITHIN AN HOUR I WAS IN GREAT DISCOMFORT; HAD IN EFFECT LOST THE USE OF MY LEFT ARM DUE TO TREMORS AND WAS QUITE CONCERNED. AT ABOUT 15000' DURING THE DSCNT THE F/O SUGGESTED THAT THE S/O OCCUPY THE CAPT SEAT AND THAT I OCCUPY THE S/O SEAT. THAT SEEMED LIKE AN EXCELLENT SUGGESTION SO AFTER SETTING THE PANEL UP FOR APCH AND LNDG THE SWITCH WAS MADE. SINCE IT WAS THE F/O'S FIRST TIME INTO PIK I SUGGESTED A FULL ILS WHICH HE FLEW FLAWLESSLY AND THE S/O PARKED THE ACFT. BY SWITCHING SEATS WE HAD TWO ABLE BODIES IN FRONT AND I READ ALL THE CHKLISTS WHICH WAS THE ONLY POSITIVE CONTRIBUTION I COULD HAVE MADE AT THE TIME. SUPPLEMENTAL INFO FROM ACN 90686. I WAS NOT AWARE THAT THE CAPT; F/O; STATION MGR OR ANYONE ELSE HAD FAILED TO IMMEDIATELY NOTIFY THE NTSB OR FAA. AS IT TURNED OUT THEY HAD NOT NOTIFIED THE COMPANY EITHER UNTIL WE RETURNED TO BOSTON ON WED JULY 88; WHEN THE CAPT SUBMITTED A FULL WRITTEN RPT TO THE BOSTON CHIEF PLT'S OFFICE.