Control sphere
From Tardis Wiki, the free Doctor Who reference
A control sphere was a device created by the Great Intelligence to control its Robot Yetis. They were glass spheres that were fitted into the chest of a Yeti and served as the brain.
The Intelligence was able to control the Yeti and provide its motive power. Should the sphere be removed from the chest, it will move by itself to locate a deactivated Yeti and install itself into its chest. If the chest is blocked by something like a rock, the sphere will no longer think it is needed.
The Great Intelligence was also able to take the form of a three sided pyramid of control spheres. (DW: The Abominable Snowmen/DW: The Web of Fear)
