A and B, Lateral aspect of brainstem and posterior fossa showing the brainstem origin of the cranial nerves, which form the margins of the cavernous sinus and middle fossa triangles. The tentorial edge was preserved in A and removed in B. There are four cavernous sinus triangles, four middle fossa triangles, and two paraclival triangles. The cavernous sinus triangles are the clinoidal, oculomotor, supratrochlear, and infratrochlear triangle. The clinoidal triangle, exposed by removing the anterior clinoid process, is situated in the interval between the optic and oculomotor nerves. The optic strut is in the anterior part, the clinoid segment is in the midportion and the thin roof of the cavernous sinus is in the posterior part of this triangle. The oculomotor triangle is the triangular patch of dura through which the oculomotor nerve enters the roof of the cavernous sinus. The posterior margin of this triangle is formed by the posterior petroclinoid dural fold, which extends from the petrous apex to the posterior clinoid process. The lateral margin is formed by the anterior petroclinoid dural fold, which extends from the petrous apex to the anterior clinoid process. The medial margin is formed by the intraclinoid dural fold, which extends from the anterior to the posterior clinoid. The supratrochlear triangle is situated between the lower surface of the oculomotor nerve and the upper surface of the trochlear nerve, and has a line joining the points of entrance of these nerves into the dura as its third margin. This triangle is very narrow. The infratrochlear triangle (Parkinson’s triangle) is located between the lower margin of the trochlear nerve and the upper margin of the ophthalmic nerve, and has a third margin formed by a line connecting the point of entry of the trochlear nerve into the dura to the site where the trigeminal nerve enters Meckel’s cave. The posterior bend of the carotid artery and the origin of the meningohypophyseal trunk are located in this triangle. The middle fossa triangles are the anteromedial, anterolateral, posterolateral, and the posteromedial triangles. The anteromedial triangle is situated between the lower margin of the ophthalmic and the upper margin of the maxillary nerves, and has a third edge formed by a line connecting the point where the ophthalmic nerve passes through the superior orbital fissure and the maxillary nerve passes through the foramen rotundum. Removing bone in the medial wall of this triangle will create an opening into the sphenoid sinus. The anterolateral triangle is located between the lower surface of the maxillary nerve, the upper surface of the mandibular nerve, and a line connecting the foramen ovale and rotundum. Opening the bone in the medial wall of this triangle exposes the sphenoid sinus. The posterolateral triangle (Glasscock’s triangle) is formed on the anterolateral side by the lateral surface of the mandibular nerve distal to the point at which the greater petrosal nerve crosses below the lateral surface of the trigeminal nerve, and on the posterolateral side is formed by the anterior margin of the greater petrosal nerve. This triangle encompasses the floor of the middle cranial fossa between these two structures. The middle meningeal artery passes through the foramen spinosum in this triangle. Opening the floor of the middle fossa in this triangle exposes the infratemporal fossa. The posteromedial triangle (Kawase’s triangle) is located between the greater petrosal nerve and the lateral edge of the trigeminal nerve behind the point where the greater petrosal nerve passes below the lateral edge of the trigeminal nerve, and a line along the connecting hiatus falopi to the dural ostium of Meckel’s cave. The petrous carotid crosses the anterior margin of this triangle. The cochlea is located below the floor of the middle fossa in the lateral apex of the triangle. Drilling the bony floor of the triangle in the area behind the internal carotid artery and medial to the cochlea exposes the lateral edge of the clivus. The paraclival triangles are the inferomedial and inferolateral triangles. The inferolateral paraclival triangle is located on the posterior surface of the clivus and temporal bone. The medial margin is formed by a line connecting the dural entry sites of the trochlear and abducens nerves; the upper margin extends from the dural entrance of the trochlear nerve to the point at which the first petrosal vein lateral to Meckel’s cave joins the superior petrosal sinus (removed); and the lower margin is formed by a line connecting the point at which the abducens nerve enters the dura to the site at which the first petrosal vein, lateral to the trigeminal nerve, joins the superior petrosal sinus. The porus, through which the posterior trigeminal root enters Meckel’s cave, is situated in the center of the inferolateral paraclival triangle. The inferomedial paraclival triangle is formed above by a line extending from the posterior clinoid process to the dural entrance of the trochlear nerve, laterally by a line connecting the dural entrances of the trochlear and abducens nerves, and medially by a line extending from the dural entrance of the abducens nerve to the posterior clinoid process. The dura in this triangle forms the posterior wall of cavernous sinus. (Images courtesy of AL Rhoton, Jr.)