"So much of the interpretation and judgement of horoscopes in older forms of astrology is decided by planetary significators.  There are planetary significators for virtually every aspect of the individual's life, each with its own distinct rules for determination.  From a field of candidates, the planet chosen as the significator is the one that best fits all the selection criteria. 
     In Medical Astrology, the most famous and fabled of such planetary significators is the Hyleg, also called the Apheta, or the Giver of Life.  It signifies the native's potential for health and longevity. "