The Atari Game Brain (Model C-700) was Atari's first cartridge based  video game console.    The Game Brain actually was nothing more then an empty box with no major circuitry at all.   Basically at had internal wiring run for input from built-in Paddles, built-in button controls (pre-joystick design), fire buttons, game selection button, reset button and power button.  

These connections all ran to a cartridge connector underneath the center cover of the console.

There are also several jacks are on the rear of the unit that allowed the connection of the older Atari paddle controllers previously used on the Atari Pong line of dedicated consoles.

        The "REAL" Brains of the Game Brain were the actual cartridges.    Atari had yet to finish a design for a multipurpose programmable console system, that wouldn't happen until the Atari Video Computer System (VCS) model 2600 was to be released.    (It was the decision to place all focus on the Atari VCS that brought the immediate demise of the Game Brain.)   

All of the circuitry to allow a game to run were in the cartridges.   The cartridges were all based on already existing stand-alone game consoles such as Super Pong, Video Pinball, Stunt Cycle, and Video Music.    The top of the Game Brain opened by pushing the Open/On/Off switch to the far left to the Open position and gently pushing the back of the middle cover (nope, no springs to pop the top open).

        You would then take the cartridge and set its Position Switch to "Disconnect", then place the cartidge face-up into the slot in the center of the open console.   Once in place you would slide the Position Switch towards you to the "Connect" position.   What this in effect did was slide the actual PCB forward into the cartridge connector.    You would then close the door and turn the system on.   Another nice feature was the instruction holder on the top of the console.   It held small plastic insert cards which told you basic information on which controllers performed which functions.