Sentence:  Certainly not in Orchestra hall where he has played countless recitals , and where Thursday night he celebrated his 20th season with the Chicago Symphony orchestra , playing the Brahms Concerto with his own slashing , demon-ridden cadenza melting into the high , pale , pure and lovely song with which a violinist unlocks the <WSD>heart</WSD> of the music , or forever finds it closed . 
Meaning of the word marked with <WSD> tags:  the choicest or most essential or most vital part of some idea or experience