The tympanic membrane (eardrum) divides the outer and middle ear. It is inserted into the groove in the bony portion of the external auditory meatus and slanted so that the anteroinferior part is more medial. Most of it is taut (pars tensa), other than its upper part that is thin and loose (pars flaccida). The handle of the malleus is attached to its medial surface so that it bulges into the middle ear with a concave lateral (outer) surface – the umbo being the most depressed point on the membrane. Its outer surface is innervated by the auriculotemporal nerve (branch of Vb) while its inner surface is innervated by the glossopharyngeal nerve.