Hjärta, Icelandic hjarta, English heart, German Herz, from a Germanic stem *hertan- is one of those words that are most easily followed all the way back to Indoeuropean *kerd- , *krd-, with relatives such as Latin cor, cordis and Greek kardia.
Close to hjärta in the dictionary is hjärna, ‘brain’ , Icelandic hjarne, German Hirn, from the Germanic stem *hirzan- , Indoeuropean *kerz-, Latin cerebrum and Greek kranion.
English brain, Old English brægen, has relatives only in West Germanic languages (Dutch brein). Origin is unclear, but it could be Germanic. *bragnam coming from Indoeuropean *bhragno 'something broken'. (Or, rather *mregh-m(n)o- - to compare with Greek brekhmos 'top of the head').
(The Christmas tree is not with us now.)