"The whole room shows the four areas of human knowledge: philosophy, religion, poetry and law, with The Parnassus representing poetry. The fresco shows the mythological Mount Parnassus where Apollo dwells; he is in the centre playing an instrument (a contemporary lira da braccio rather than a classical lyre), surrounded by the nine muses, nine poets from antiquity, and nine contemporary poets. Apollo, along with Calliope, the muse of epic poetry, inspired poets.
"Raphael used the face of Laocoön from the classical sculpture Laocoön and His Sons, excavated in 1506 and also in the Vatican for his Homer (in dark blue robe to the left of centre), expressing blindness rather than pain. Two of the female figures in the fresco have been said to be reminiscent of Michelangelo's Creation of Adam, Euterpe and Sappho, who is named on a scroll she holds. Sappho is the only female poet shown, presumably identified so that she is not confused with a muse; she is a late addition who does not appear in the print by Marcantonio Raimondi that records a drawing for the fresco." -- Wikipedia