Endoscopic anatomy of the clival and perimedullary membrane groups. (A) The mesencephalic leaf of the Liliequist’s membrane complex viewed through left supraorbital approach. Note the transition between the intact middle portion of the membrane and the trabecular part around the basilar artery trunk (blue arrow-head). (B) The lateral edge of the mesencephalic leaf on the left side. The endoscope was introduced through a contralateral supraorbital approach. The blue arrow-head points to the trabecular portion of the mesencephalic leaf. Through the holes on this portion the left posterior cerebral artery is also visible. (C) Junction of the lateral edge of the mesencephalic leaf and the anterior pontine membrane at the left side (blue arrow-head). The approach is the same as Figure 3B. (D) The anterior pontine membrane on the left side viewed from the same approach as Figure 3C. (E) Membranous relations of the left superior cerebellar artery. The artery passes through (blue arrow-head) at the three-point junction between the superior edge of the anterior pontine membrane, the lateral edge of the mesencephalic leaf and the medial edge of the superior cerebellar membrane (not visible, because the lateral edge of the mesencephalic leaf is intact). The same approach as Figure 3B. (F) View of the lateral surface of the right anterior pontine membrane through a right retrosigmoid approach. Note the extra connection between the trigeminal membrane and the anterior pontine membrane (blue arrow-head). (G) The pontomedullary membrane on the right side through right retrosigmoid approach. Note the dorsal attachment of the membrane along the pontomedullary sulcus (blue arrow-head). (H) The superior surface of the left pontomedullary membrane viewed through a left supraorbital approach. Note the junction between the medial edge of the pontomedullary membrane and the inferior edge of the anterior pontine membrane (blue arrow-head). In this case the abducent nerve is located below the level of the membrane in contrast to Figure 3G. (I) View of the lateral portion of the left pontomedullary membrane along the inferior limb of the cerebellopontine fissure below the flocculus. Note the connection between the glossopharyngeal nerve and the inferior surface of the membrane. The endoscope was introduced in the same manner as Figure 3H but the medial part of the membrane was removed. (J) The median medullary membrane from below visualized through a right retrosigmoid approach. Note the anterior spinal arteries running in the wall of the membrane. (K) The upper portion of the rhomboid membrane in the corner between the lateral surface of the medulla oblongata, right vertebral artery and the superior edge of the right denticulate ligament. Right retrosigmoid approach. (L) View of the right apex of the rhomboid membrane through a transnasal-transclival approach. The C1 spinal nerve perforates the membrane but the ventral root of the C2 spinal nerve runs below it (blue arrow-head). Directly under the rhomboid membrane is the denticulate ligament locted. Its second insertion is in the same level as the connection of the lateral apex of the rhomboid membrane to the outer arachnoid. The spinal roots of the accessory nerve are located below the denticulate ligament. The dorsal root of the C2 spinal nerve is the deepest structure. (M) The first insertion of the right denticulate ligament, located between the vertebral artery (ventral) and the spinal roots of the accessory nerve (dorsal). Medially, the ligament is attached to the lateral surface of the medulla oblongata. The rhomboid membrane was removed, therefore, the upper edge of the ligament is disconnected (blue arrow-head). Right retrosigmoid approach. Same approach as Figure 3K. (N) Second insertion of the right denticulate ligament. Same approach as Figure 3L. View after removal of the rhomboid membrane. Note the two macroscopically different tissue-types of the ligament: medially an arachnoid membrane-like transparent structure, laterally a white band-like strong construct. (O) View of the intermediate cervical septum from the right side. The approach is the same as Figure 3M. Above the level of the foramen magnum the membrane is only a dense network of arachnoid trabeculae.
a.=artery, AICA=anterior inferior cerebellar artery, APM=anterior pontine membrane, arachn.=arachnoid, ASA=anterior spinal artery, BA=basilar artery, Boch.=Bochdalek flower basket, C1=C1 spinal nerve, C2=C2 spinal nerve, cliv.=clival, CNIII=oculomotor nerve, CNV=trigeminal nerve, CNVI=abducent nerve, CNVII-VIII=vestibulocochlear and facial nerves, CNIX-X-XI=caudal cranial nerves, co.=communicating, dors.=dorsal, flocc.=flocculus, ICS=intermediate cervical septum, ins.=insertion, l. VA=left vertebral artery, LD=denticulate ligament, LM-ML=mesencephalic leaf of the Liliequist’s membrane complex, MMM=median medullary membrane, p.=posterior, PCA=posterior cerebral membrane, PICA=posterior inferior cerebellar artery, PM=pontomedullary membrane, r. VA=right vertebral artery, RM=rhomboid membrane, SCA=superior cerebellar artery, SCM=superior cerebellar membrane, ventr.=ventral.