Split reins are a piece of equipment used for communicating with a horse while it is being ridden. Split reins are actually two separate, individual reins that are used together, with one rein on each side of the horse's neck.
As discussed above, mitochondria are the sites of respiration, and generate chemical energy in the form of ATP by metabolizing sugars, fats, and other chemical fuels with the assistance of molecular oxygen.
Mitochondria (singular mitochondrion) are the 'electric generators' of cells. They take oxygen and produce carbon dioxide together with ATP, the fuel of most of the activities of the cell. Because they take oxygen and release CO2, it is easy to think they 'breathe'.