. This means that in order to utilize all the rods, there must be pupillary dilatation (dilatation of pupil, typically occurs in the dim light or in d

rods but no cones. This means that in order to utilize all the rods, there must be pupillary dilatation (dilatation of pupil, typically occurs in the dim light or in darkness). Rods are required for vision in darkness. Senile macular degeneration (SMD). SMD is common in old persons. Blood vessels appear in the macula (recall macula is normally avascular), ultimately, this leads to blindness. Treatment consists of using laser beams to halt the progress of blood vessels. Optic disc The axons of the ganglion cells are collected together into a bundle and this bundle exits from the eye as the optic nerve. The beginning of the optic nerve in the retina is called the optic disc. Optic disc is also the blind spot in the retina as there is no rod or cone over the optic disc. The optic disc can be seen directly by an instrument called ophthalmoscope. In glaucoma, the optic disc, owing to high intraocular pressure, may become cup shaped ("glaucomatous cupping", a diagnostic sign of glaucoma), whereas in papilledema (edema of the optic disc; incidentally, optic disc is also called 'optic papilla', hence papilledema = edema of the optic disc) which may develop in severe hypertension, kidney disease and brain tumor (all due to edema of the brain) the optic disc is swollen and its outlines become blurred. Both glaucomatous cupping and papilledema can be seen by ophthalmoscope. APPLIED PHYSIOLOGY Retinal detachment A clevage may appear between the pigment layer and the rest of the layers of the retina. This is usually the result of contraction of vitreous humor. The detachment can be repaired and provided the repair is made within a short time (a few days) after the detachment, restoration of vision occurs satisfactorily. If however, the repair is done after a long delay, inspite of the anatomical repair, restoration of vision does not occur, because the retina has degenerated in the mean time. One method of treatment is to reattach the layers by photocoagulation. Oxygen poisoning There are many conditions where 02 inhalation is required for treatment purposes.