Inner sheath of optic nerve

The inner sheath (inner layer) of the optic nerve refers to the inner lamina of the bilaminar optic nerve sheath (dura mater). While the optic nerve sheath appears unilaminar in children, in adults it is composed of two distinct layers — an inner layer and an outer layer — each with different structural and biomechanical properties.

The inner layer inserts near the lamina cribrosa, where it is subject to the greatest tensile loading. This is particularly relevant during adduction tethering of the eye, when the optic nerve and its sheath transmit the reaction force of the medial rectus muscle to the globe. The greater stiffness and elastin density of the inner layer are thought to reflect its role in bearing these concentrated mechanical loads.

Anatomical relationships: Within the optic canal, fibers from the optic nerve sheath — including the inner layer — may intermingle with tendinous fibers of the extraocular muscles and attach to the periosteum of the sphenoid bone, rather than continuing purely as dura mater. The pia and arachnoid maters also merge within the canal, and a dense trabecular meshwork occupies the orbital portion, significantly reducing free communication of the subarachnoid space between the intracranial cavity and the orbital optic nerve sheath.