Human anatomy, including structure and development and practical considerations . f retinitis. The retina between the optic nerve and the ora serrata is held in apposition tothe choroid only by the support afforded by the vitreous body. It may be readilydetached from the choroid by such causes as injury, extravasation of blood orserum between the two layers, or by tumors of the choroid. In contusions of the eye the retina is sometimes torn alone, although this israre. The retina does not tear as easily as the choroid, as is shown by the fact thatin ruptures of the choroid the retina is general
Human anatomy, including structure and development and practical considerations . f retinitis. The retina between the optic nerve and the ora serrata is held in apposition tothe choroid only by the support afforded by the vitreous body. It may be readilydetached from the choroid by such causes as injury, extravasation of blood orserum between the two layers, or by tumors of the choroid. In contusions of the eye the retina is sometimes torn alone, although this israre. The retina does not tear as easily as the choroid, as is shown by the fact thatin ruptures of the choroid the retina is generally not lacerated. Glioma is the only tumor found in the retina, and occurs exclusively in children,usually under three years of age. A rare tumor arising from the pars ciliaris retinae has been described, to whichthe name terato-neiiroma has been applied by Verhoeff. The Optic Nerve.—The extraocular portion of the optic nerve has been de-scribed elsewhere (page I223_). Likewise, the three sheaths—thedural, the arachnoid Fig. 1226. Physiological excavation Lamina cribrosa /. Sclera — Dural sheath - Arachnoidal,sheath Pial sheath UfU •y ¥i kM r-ubarachnoid spaceSubdural space WU-M r /• Central retinal vessels within optic nerveSection of eyeball through entrance of optic ner\e. X 20. and the pial—which, with the subdural and the subarachnoid lymph-spaces, are con-tinued over the nerve as prolongations of the corresponding brain-membranes (page949). On reaching the eyeball, the dural sheath bends direcdy outward, its fibrescommingling with those of the outer third of the sclera (Fig. 1226) ; the arachnoidends abrupdy on the inner wall of the intervaginal space ; whilst the pia archesoutward to form part of the inner third of the sclera, but sends longitudinal fibres asfar as the choroid. As the nerAe-fibres enter the eyeball, for convenience assumingthat they are passing from the brain toward the retina, they traverse a fenestrated I470 HUMAN ANATOMY. Fig. 1227. Blood
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