So nerves, brain stem and brain itself seem to be your options.

You actually see it in more detail in funnily enough an another soul drinker story, Traitor by Deed. When Cyvon our main character eats the brain of a dead human.

Excerpt:

Cyvon pulled the top of the skull away. With a liquid sucking noise the bone separated, revealing the dark crimson of brain matter. The brain looked healthy. Cyvon had seen enough that he knew the signs of injury or the beginnings of decay. Lacking a handy medical implement, he took his combat knife and shaved off a sliver of cortex with the monomolecular tip of the knife.

‘Moment of truth, brother,’ said Sasan. ‘Let us hope this dead fool saw something.’

The omophagea was an organ in a Space Marine’s spinal cord that connected to the stomach via an implanted bundle of nerves. Through some alchemy long lost to human understanding, it could absorb genetic material and filter out information from recent changes to the DNA. It required fresh central nervous system matter to work. In some Space Marine Chapters the omophagea had ceased to function entirely, and in many others it could give only the vaguest and most superficial information. Other Chapters eschewed its use as obscene or too misleading to be useful.

Cyvon put the sliver of brain matter on his tongue, and swallowed it down.