the heart

أَلَا وَإِنَّ فِي الْجَسَدِ مُضْغَةً، إِذَا صَلَحَتْ صَلَحَ الْجَسَدُ كُلُّهُ، وَإِذَا فَسَدَتْ فَسَدَ الْجَسَدُ كُلُّهُ، أَلَا وَهِيَ الْقَلْبُ

"Truly, in the body there is a piece of flesh which, if it is sound, the whole body is sound; and if it is corrupt, the whole body is corrupt. Truly, it is the heart."