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.
Compact Bone. The diagram above shows a transverse view of an osteon (Haversian system) - the basic unit of compact bone. The diagram above shows a longitudinal view of an osteon. Some, mostly older, compact bone is remodelled to form these Haversian systems (or osteons).The osteocytes sit in their lacunae in concentric rings around a central Haversian canal (which runs longitudinally).ompact Bone. The diagram above shows a transverse view of an osteon (Haversian system) - the basic unit of compact bone. The diagram above shows a longitudinal view of an osteon. Some, mostly older, compact bone is remodelled to form these Haversian systems (or osteons).