In "Heart of Darkness," how does Conrad make Kurtz's head significant?
Phrenology was an acceptable science of the time which determined a person's character profile (and eventual criminal or regressive tendencies) by analysing the shape and irregularities of his skull. From enotes:
The doctor who measures Marlow's head prior to his departure for Africa warns him of changes to his personality that may be produced by a long stay in-country. Prolonged silence and solitude are seen to have damaging effects...."
Closely linked to the themes of order and disorder are those of sanity and insanity. Madness, given prolonged exposure to the isolation of the wilderness, seems an inevitable extension of chaos. The atmospheric influences at the heart of the African continent—the stifling heat, the incessant drums, the whispering bush, the mysterious light—play havoc with the unadapted European mind and reduce it either to the insanity of thinking anything is allowable in such an atmosphere or, as in Kurtz's case, to literal madness.... In the wilderness, he came to believe he was free to do whatever he liked, and the freedom drove him mad.