The sclera is the outer shell and principal load-bearing tissue of the eye, and consists primarily of avascular lamellae of collagen fibers. Ninety percent of the collagen fibers in the sclera are Type I, which provide the eye with necessary mechanical strength to withstand intraocular pressure (IOP). A small hole pierces the posterior sclera, known as the scleral canal, through which the retinal ganglion cell axons turn and pass out of the eye on their path to the brain. The scleral canal is spanned by a fenestrated connective tissue called the lamina cribrosa that provides structural and nutritional support to the axons as they leave the eye. This region, including the peripapillary sclera (the sclera closest to the canal), the lamina cribrosa, and the contained retinal ganglion cell axons, is collectively known as the optic nerve head or ONH.

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