"We replace the rods and cones in our retina every 48 hours, so we create a lot of "sewage" in our eyes. Therefore we need a special system capable of handling all this sewage. The inside of the eye is hollow so it has the room for all this sewage. To keep it flowing, some fluid is added by the lining of the focusing muscle (ciliary body) secreting what is known as "aqueous humor". Thus aqueous humor is like extracellular fluid in the rest of the body."