First of all, an optical drive can be a CD, DVD, or BluRay drive. The button isn't invisible, it's just hidden in plain site by design. The previous pictures show its location, but here is one more. This is a picture of the top of a 660 showing 2 drives.
What is difference between an osteon and osteocyte? An osteon is the functional unit of cortical bone. Osteons contain osteocytes, which are cells that produce bone matrix.
Each osteon looks looks like tree rings. The large holes in the center of each osteon (tree) is a central canal. The ants are the lacuna (the holes that house the osteocytes) and the legs of the ants are canaliculi (singular canaliculus; that allow the osteocytes to touch each other).