Also in Xephs character description, it does mention his implants are attached to the spinal column for maximum control. (Or just that it's the best place.)

[color=00aeef]EDIT:[/color]
I'm checking the organs.
The Catalapsean node (sleep control) is situated at the back of the brain. And if it is damaged it would require opening of the cranium. (It's also pea-size, teeny.)

The Omophagea is four nerve bundles that go from the brain, through the spine into the stomach. (It's the organ that makes marines learn by eating.)

Occulobe sits at the base of the brain. (Super vision.)

Sus-an membrane. It's the medical induced coma organ. Situated on top of the brain. Seeing Xephs problems it makes sense his sus-an membrane is damaged, since if he was mortally wounded it would try to induce a medical coma to halt the dying of the marine.
If this organ is damaged, this would not happen. Seeing Xephs dizziness and half awakehood, it seems the Sus-an membrane is trying to activate but unable.
(It puts the marine in suspended animation, can be done willingly or if trauma happens.)

It seems the apothecary might have to do brain surgery after all. And the sus-an membrane is something most space marines are trained to use willingly. So its functions and importance would be widely known to the other marines present.

[color=00aeef]EDIT:[/color] LOL, girl marines.