MD-11 is right in saying that there is a centre tank, the only thing to add is that all B767 that l know have a centre tank. The size of the centre tank varies to from , i.e 200 to 200ER or 300ER for that matter. Also the makeup off the centre tank is different to each model. On the the 200 the centre tank is the first three rib placement of the wing structure, and is conected by 3 crossover tubes. On the 200ER, the same three rib stations of the wing structure still apply, but now half of the centre wing structure is a tank with only two cross over tubes. But on the 300ER, the bulk of the centre wing structure is all fuel tank as well as the small part of the inner wing. All B767's each left and right wing tanks each hold 18500KG of fuel, but the centre as you can see varies alot. The 200 holds 13800KG, the 200ER holds 24800KG, and the 300ER holds 36550KG.
Hope this was some help.