OPHTHALMOLOGY (see EYE, ANATOMY OF; EYE, DIS EASES OF). The science of ophthalmology deals with the processes by means of which the images of external objects are brought to our consciousness. It is therefore concerned with: (a) the eye itself ; (b) the nerve paths and tracts which convey visual impulses originating in the eye, through the different parts of the brain to the brain cortex, where these impulses are con verted into conscious impressions; (c) the eyelids which cover and protect the eyes; (d) the tear glands and ducts; (e) the muscles that bring about the movements of the eyes and keep them trained in the desired direction ; (f) the nerves and their complicated cerebral connections which supply these muscles; (g) the bony walls of the orbit ; (h) the blood-vessels and lymph paths which maintain the nutrition of all these structures.
It treats of disease in these parts and derives importance from the fact that many diseases of the central nervous system and many general diseases manifest themselves by some derangement of function or structure which can be detected by the ophthalmic • surgeon.
The eye is unique in the body in that its retina, which is avail able to minute examination by means of the ophthalmoscope, is the only portion of the brain available to inspection during life. Similarly, the arteries and veins which supply the retina can also be minutely examined during life, the diseases of them observed and followed in all their changes. The value of these observations is enhanced by the fact that the eye itself acts as a low-power microscope, providing a magnification of about 15 diameters for the examination of these structures. The eyes are the subject of a number of hereditary diseases, and form one of the most con venient media for the study of transmission of such diseases.
Affections of the Eye.—The function of the eyeball is to provide that a clear image of external objects shall be formed upon the retina, but in certain cases it departs from the normal and the acuity of sight is lowered. It may be too long, so that the retina lies behind the point at which the images of external objects are formed; this is myopia or short-sightedness, and may be compensated by the wearing of concave lenses in the form of spectacles. In other cases, the eyeball is too short and the retina lies in front of the point at which the image of external objects is formed. This is hypermetropia or long-sightedness; it can be
compensated by the focusing muscle of the eye, making the lens more convex; or, preferably, by the wearing of appropriate convex lenses in the form of spectacles. In astigmatism the refractive power of the eye varies in different axes, so that, for instance, in an extreme case the vertical axis may be myopic whilst the hori zontal axis is hypermetropic.
Normally the visual axes of the two eyes are parallel and images of external objects are formed upon corresponding points of each retina. This arrangement is largely responsible for stereo scopic vision, which enables us to judge the position of objects in space with accuracy. Should it be upset e.g. by paralysis of some of the muscles, stereoscopic vision is lost and double vision usually arises.
The lens is biconvex, its back surface having the greater curva ture. It is perfectly transparent, and the focusing of the eye for near or distant objects is brought about by alteration in its curva tures by contraction of the ciliary muscle. Here is an essential difference between the eye and the photographic camera, whose focus is adjusted by shifting the position of the lens. As age pro ceeds, opacities frequently develop in the lens; almost everyone at. the age of 6o or over may be said to have the beginnings of cataract, though the sight is quite unaffected thereby. When, how ever, these opacities involve the centre of the lens, and render sight very imperfect, the term of cataract becomes applicable; in such a case the cataract can be removed by operation and the sight restored.