Handbook of ophthalmology . us envelope of the vitreous body. The place ofentrance of the optic nerve is, however, a weak point, because justhere the sclera is penetrated by the nervous fascicles, and it istherefore plain that just at this place it will first yield to theincreased pressure. The essential anatomical character of this excavation consistsin this, that the lamina cribrosa is compressed and forced back- * Arch. f. Ophth., B. ix. 2, pag. 21G; Klin. Monatsblatt fur Augenheil-kunde, 1864, pug. 434. t Arch. f. Ophtli., B. xiv. 3, pag. 219, und B. xv. 1, pag. 265. PEESSUEE EXCAVATION. 5
Handbook of ophthalmology . us envelope of the vitreous body. The place ofentrance of the optic nerve is, however, a weak point, because justhere the sclera is penetrated by the nervous fascicles, and it istherefore plain that just at this place it will first yield to theincreased pressure. The essential anatomical character of this excavation consistsin this, that the lamina cribrosa is compressed and forced back- * Arch. f. Ophth., B. ix. 2, pag. 21G; Klin. Monatsblatt fur Augenheil-kunde, 1864, pug. 434. t Arch. f. Ophtli., B. xiv. 3, pag. 219, und B. xv. 1, pag. 265. PEESSUEE EXCAVATION. 503 ward behind the general inner surface of the sclera, often so farthat the floor of the excavation lies posterior to the outer surfaceof the sclera. The place of entrance of the optic nerve is nowoccupied by a hollow, its lioor formed by the lamina cribrosaand- its walls by the sclera. It is filled partly by the vitreousbody, and partly by the degenerated tissue of the optic nerve,lying above the lamina cribrosa. Fig. 2?, retina; C7(, choroid ; Tc, central vessel*. The walls of the excavation ai-e covered with anill-defined tissue, consisting of the remains of the terminal fibres of the optic nerve and of thevitreous body. Its floor is funnel-shaped, due to the dilatation of the central canal of the retinal vessels. The excavation is often kettle-shaped, which may be explainedpartly by the anatomical structure of the lamina cribrosa. Thatpart of the optic nerve which penetrates the sclera is often not asimple cylinder, but is enlarged at certain places in such a waythat the greatest diameter of the lamina cribrosa is greater thanthe foramen choroidere, and lies posterior to it. When an opticnerve so formed becomes excavated, a kettle-shaped depressionmust necessarily result. Moreover, the same pressure which forcedthe lamina cribrosa backward acts upon the lateral walls of theexcavation itself, tending thus to bulge them outward. The retinal vessels, together with the re
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