OPTIC NERVE AND RETINA, DIS EASES OF THE.
The optic nerve and the retina, form ing, together, an offshoot of the central nervous system, show the closest associ ation in their diseases, which are largely dependent on general disease and dis ease of other organs. Being open to in spection and minute investigation, they furnish valuable diagnostic and prog nostic indications regarding the condi tions with which they are associated.
Retinitis.—Retinitis is an inflamma tion of low grade and extremely chronic. Heat and pain are absent, redness is often confined to doubtful changes in the retinal vessels, and swelling is evi dent chiefly through the opacity of the exudate.
Symptoms.—Impairment of vision is the only constant rational symptom; and it is not characteristic, and tells little of the cause or gravity of the dis ease. In the early stages it appears as a diffuse clouding of the field of vision or some part of it. Later it may be a distortion of objects (metamorphopsia) or an annoying quivering of the thing looked at. In some forms impairment of vision is greatest by a bright light: day-blindness. In others it is greater by feeble illumination: -night-blind-ness. Flashes of light sometimes occur, but they may not be noticed at all. The important symptoms are wholly ophthal moscopical. They include opacity of the retina, alteration of the retinal vessels, haemorrhage, and pigment deposits and alterations.
Opacity prevents the seeing of the retinal pigment-layer, and the color or details of the choroid, which are visible through the normal transparent retina. The opacity may appear as a gray veil, faintly blurring or entirely obscuring the deeper structures, or it may have the form of definite dirty-white or clear, glistening-white masses. Its effect on the retinal vessels varies with their depth in the retina. A vessel running on the surface of the retina is more dis tinctly seen than normal, because of the contrast furnished by the gray or white opacity of the retina behind it. But a portion of a retinal vessel im bedded in the retina will be partly or entirely hidden by the opacity.
The retinal vessels may be distended uniformly or irregularly. Distension renders the visible vessels larger, and more of the small vessels visible; and it also makes the vessels more tortuous. The tortuosity is shown both by the wavy outline in the plane of the retina and by more decided differences of level in different parts of the vessels; so that some parts stand out with greater dis tinctness, while others are comparatively buried in the depths of the retina. Ir regular distension of the retinal veins occurs in retinitis, indicating disease of the retinal walls.