Original: 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.
Simplified: In The Tomb of the Cybermen (1967), veins were visible through the circular head of the Cyberman Controller and  in Attack of the Cybermen (1985) and "The Age of Steel" (2006), the Cyber-Controller's brain is visible through the dome.