(a) Any of the following permanent disabilities shall be conclusively presumed to be total in character:
(1) Loss of both eyes or the sight thereof.
(2) Loss of both hands or the use thereof.
(3) An injury resulting in a practically total paralysis.
(4) An injury to the brain resulting in permanent mental incapacity.
(b) In all other cases, permanent total disability shall be determined in accordance with the fact.