This SMAS layer is present in the face immediately deep to the subcutaneous fat. If you follow it anteromedially (toward the mouth and nose) it will invest (or wrap) around the musculature of the face. If you follow it inferiorly (down into the neck) it will merge with the platysma, the superficial neck muscle that can produce tents in the skin when contracted. If followed superiorly (up into the forehead) the SMAS will become contiguous (one) with the frontalis muscle, the muscle responsible for lifting the eyebrows. The significance of this layer is that most surgeons agree that mobilizing it, in some form, and using it to anchor the overlying skin in some fashion, will yield more durable results, and a heartier tissue flap more resistant to ischemia (oxygen deprivation). While this makes sense it has never been shown unequivocally to produce superior result to skin-only face lifts. It is however safer to do for smokers in whom tissue viability with skin only flaps would be questionable.