72-pin SIMM RAM: how do ground (Vss) and supply (Vcc) voltage pins work
The board is a multi-layer stackup, probably 4-layer. 

This means that there are more layers inside the PCB on which other connections are routed.

You can tell this from the seemingly disappearing routing, but also from the colour of the board. Notice how it is light around the edges (where light can shine through), but then suddenly gets dark. The dark region is where there is more copper inside the board.

The internal layers in this case are most likely just power planes - one of Vcc and one for Vss. All connections for power and ground will connect to one or other of the planes providing nice low impedance power routing.