In The Tomb of the Cybermen (1967), veins and brains were visible through the domed head of the Cyberman Controller and similarly, in Attack of the Cybermen (1985) and "The Age of Steel" (2006), the Cyber-Controller's brain is visible through the dome.
In The Tomb of the Cybermen (1967), veins and brains were visible through the dome of the Cyberman Controller, and likewise, in Attack of the Cybermen (1985) and The Age of Steel (2006), the brain of the Cyber-Controller is visible thru the dome.