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.
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 matrix (1) are 2 Ã 3 (read two by three), because there are two rows and three columns.