Basically, it's the inner portion of the adrenal glands. The outher portion is called the adrinal cortex. It produces epinephrine (a.k.a. adrenaline, a.k.a. kick-ass juice) and norepinephrine (a.k.a. noradrealine). Oddly, there is almost no instance of underfunction of the adrenal medulla, though an overfunction is normally caused by a tumor called a pheochromocytoma, common in the adrenal medualla and other parts of the blood system.