Yes, MEDULLAS is a Scrabble word!

Letter values in Scrabble US: M 3, E 1, D 2, U 1, L 1, L 1, A 1, S 1.

Is it a valid word?

MEDULLAS is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a white fatty substance that forms a medullary sheath around the axis cylinder of some nerve fibers
lower or hindmost part of the brain; continuous with spinal cord; (`bulb' is an old term for medulla oblongata)
the inner part of an organ or structure in plant or animal

Also: medulla oblongata.