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.
Matrix (mathematics) In mathematics, a matrix (plural matrices) is a rectangular array of numbers, symbols, or expressions, arranged in rows and columns. For example, the dimensions of the matrix below are 2 Ã 3 (read two by three), because there are two rows and three columns: [ 1 9 â 13 20 5 â 6 ].