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.
an osteoblast that has become embedded within the bone matrix, occupying a bone lacuna, and sending through canaliculi cytoplasmic processes that connect with other osteocytes in developing bone. osteocyte-osteoblast pump.