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.
Osteons, also known as the haversian system, are the building blocks of cortical. bone. They form two-third of the cortical bone volume. An osteon is in the form of. a cylinder having concentric layers of lamellae. These lamellae are composed of. collagen fibers which are reinforced by hydroxyapatite crystals. These concentric. lamella layers surround the cylindrical cavity known as haversian canal.