"'I always ask leave, in the interests of science, to measure the crania of those going out there,' he said. 'And when they come back too?' I asked. 'Oh, I never see them,' he remarked, 'and, moreover the changes take place inside, you know.' He smiled as if at some quiet joke." 

As Marlow prepares to travel up the Congo River, his company requires him to see a doctor whom, among other things, is interested in observing the changes that Europeans experience after going to Africa. The doctor’s discussion of his phrenology study predisposes Marlow to thinking about the relationship between imperialism and madness, although more so with regards to the effects of the foreign terrain rather than the weight of moral consciousness. Phrenology itself, a popular pseudoscience of the 19th century, also perpetuates unfounded claims about individuals’ mental states based on physical characteristics, and its appearance here highlights the racial undertones of the doctor’s remarks.

"Everything belonged to him—but that was a trifle. The thing was to know what he belonged to, how many powers of darkness claimed him for their own. That was the reflection that made you creepy all over…He had taken a high seat amongst the devils of the land—I mean literally." 

This reflection on Kurtz’s darkness occurs as Marlow describes him to the group of listeners aboard the Nellie. Discussing the notion of ownership, both in terms of what Kurtz owns and what owns him, suggests that he does not have complete control over his circumstances or behavior. The fact that “everything belong[s] to him” becomes irrelevant the moment that “powers of darkness” such as greed and unchecked violence take over. Just thinking about this loss of self-control is enough to disturb Marlow, so it seems unsurprising that Kurtz becomes seriously ill as he succumbs to the influence of imperialism’s most evil qualities. 

"I tried to break the spell—the heavy, mute spell of the wilderness—that seemed to draw him to its pitiless breast by the awakening of forgotten and brutal instincts, by the memory of gratified and monstrous passions."

After tracking down an escaped Kurtz during the middle of the night, Marlow offers this observation as to why he may have been compelled to leave his cabin in favor of his devotees’ ritual. The “forgotten and brutal instincts” that Marlow mentions may appear to be a dehumanizing reference to the African’s cultural traditions, but an alternative reading suggests that the sense of lawlessness Kurtz feels as an actor in foreign territory allows him to satisfy   instincts of greed and violence. The possibility of unchecked power that imperialism allows ultimately creates the “spell,” or state of madness, under which Kurtz operates.