OPHTHALMOLOGY

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: (a) with the eye itself; (6) with 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; (¢) with the eyelids which cover and protect the eyes; (d) with the tear glands and ducts; (e) with the muscles that bring about the movements of the eyes and keep them trained in the desired direction; ({) with the nerves and their complicated cerebral connections which supply these muscles; (¢) with the bony walls which comprise the boundaries of the orbit; (k) with the blood-vessels and lymph paths which maintain the nutrition of all these structures. it concerns itself with diseased conditions of these parts and derives particular importance from the fact that many diseases of the central nervous system and many general constitutional diseases of the body manifest themselves by some derangement of function or structure which is to be discovered by the various means of investigation which are available to the ophthalmic surgeon. many of these investigations are capable of an accuracy and a minuteness which is possible with no other structure 1n the body, and the eye, as an organ, is unique in the body in the important particular, that its retina is, in essence, a portion of the brain itself which is available to minute examination by means of the ophthalmoscope, and is the only portion of the brain which is available to inspection during life. 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, and they are the only arteries and veins in the body which can be seen clearly during life. the value of these observations is en- hanced by the fact that the eve itself acts as a sort of 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 cliseases, and form one of the most con- venient media for the study of transmission of such discases. affections of the evye-—the whole function of the eyeball itself is to provide that a clear image of external objects shall be formed upon the retina, but, considered as an optical instrument, in certain cases it departs from the normal, with the result that the acuity of sight is lowered. thus it may be too long, so that the retina comes to be behind the point at which the images of ex- ternal objects are formed; this condition is known as 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, so that the retina comes to be placed in front of the point at which the image of external objects is formed. this condition is known as hypermetropia or long sightedness; this can be compensated by the use of the focusing muscle of the eye, making the lens more convex; or, what is preferable, 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 eves are parallel, so that images of external objects are formed upon corresponding points of each retina. this arrangement is largely responsible for the phenomenon of stereoscopic vision, which enables us to judge the position of objects in space with great accuracy. should this par- allel arrangement of the two eyes become upset by any means, such for example as paralysis of some of the muscles, the power of stereoscopic vision is lost and double vision usually arises. eyeball and camera compared.—the eyeball may with con- siderable aptitude be likened to a photographic camera. roughly speaking it is globular in shape and is an inch long in all dimen- sions. the front part, or cornea is curved, is perfectly trans- parent and functions as a lens. behind it is a chamber filled by the aqueous humour—little more than water. further back is the iris—the coloured part of the eye—which in infants, up to about three months of age, is always of the same bluish-grey tiz1 colour, and afterwards attains the different tints and colours which produce the blue, the grey, the green or the brown eye. the hole in its centre forms the pupil, and by contraction or dilation of the tissues of the iris the pupil can be varied in size through a wide range; it may be compared to the stop in the camera, in bright lights the pupil is small, and it becomes jarge in dull illumination. the iris rests behind upon the lens. the lens is bi-convex, its back surface having a greater curva- ture than the front. it is perfectly transparent, and the focusing of the eye for near or distant objects is brought about by altera- tion in its curvatures 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 proceeds, opacities frequently develop in the lens, until at the age of 60 very few individuals have lenses which are entirely free of opacities; almost everyone, then, at the age of 60 or over may be said to have the beginnings of cataract, though the sight is quite unaffected thereby. when, however, these opacities have so increased as to involve markedly the central part of the lens, and so render sight very imperfect, the term of cataract becomes applicable to the condition; in such a case the cataract can be removed by operation and the sight restored. vitreous chamber and retina.—behind the jens is the jargest chamber of the eye, the vitreous chamber, which is occupied by a perfectly transparent, colourless substance, much like the white of an egg. clothing the back part of the eye, and extending for- ward some distance in front of its equator, is the retina. it rests upon a highly vascular membrane which is responsible far the nutrition of the greater part of it, namely the choroid; in man, however, the retina has blood vessels of its own, which are scen with great distinctness by means of the ophthalmoscope. these vessels supply the inner parts of the retina with their nutrition. the retina is the sensitive layer of the eye and may be likened to the sensitive plate of the camera for upon it images are formed which initiate impulses which, when conveyed to the cortex of the brain, give rise to the sensations of sight. its sensi- tive layer is placed posteriorly, and is composed of very delicate structures known as the rods and cones. at the central spot or yellow spot of the retina, the point of distinct vision, cones alone are present. the rods are believed to be concerned with lights of lower intensities, and in accordance with this they alone are present in night-flying birds (see colour vision). optic nerves.—the nerve fibres from all these tissues converge upon the optic disc, and leave the eye by means of the optic nerve, which traverses the orbit to enter the skull. inside the skull the two optic nerves meet and each is divided into two parts, one part continuing to the mid-brain on the same side, the other part crossing over to the opposite side. this crossing forms what is known as the chiasma; in many animals, the birds for instance, the whole of each optic nerve crosses over in this way. beyond the chiasma the nerve fibres are again collected into a compact bundle known as the optic tract, which terminates in the mid- brain. from here nervous impulses are relayed in two chief directions; some connect up with the nerves which control the movements of the eves and others, forming the so-called optic radiations, make a long sweep backwards to reach the cerebral cortex, where, as already stated, the impulses are transformed to sensations of sight. cerebral cortex.—theexact area of the cerebral cortex, in which these fibres end, is known with great accuracy and forms the visual cortex. it is placed at the extreme hind end of the brain, and the adjoining mesial surface of each hemisphere, in the region of the calcarine fissure (see 4.408). should a minute portion of this cortex be cut out as the result of injury or blocking of its blood supply, the precise area of the defect which will be found in the vision can be stated with certainty. this accurate localisa- tion was much advanced as a result of observations made during the war. amongst the general diseases of the body in which important manifestations occur in connection with the eye a few may be mentioned. ii22 clinical sympitoms.—in brain tumours or abscesses, swelling of the optic disc is seldom absent, and forms perhaps the most important sign in the diagnosis of the condition. in addition the ocular nerves may have their functions interrupted, so that the movements of the eyes are interfered with, they no longer move in unison and double vision occurs. if nothing can be done for the brain tumour the optic nerve atrophies and blindness ensues. in advanced renal disease changes in the retina develop and con- vey a particularly grave prognosis, for patients seldom live so long as two years after their discovery. in diabetes a similar change may also arise, which may greatly spoil the sight, and although of less serious import than in renal cases, it must be considered a bad omen. syphilis frequently attacks the eye, whether in the early or late stages, and is perhaps the most prolific single source of blindness. tuberculous disease occasion- ally occurs. in diseases of the blood in general, eye signs are very common, and there are many other diseases where the diag- nosis may be greatly assisted by the discovery of changes in some part of the ocular apparatus. the introduction of what is known as the “ slit lamp ” has macle many investigations possible which previously were quite outside our scope. bratiocrapny.—m. l. hepburn, the ophthalmology of general practice (1922); c. h. may and c. worth, afanual of the diseases of the eye, for students and practitioners, 4th ed. (1922); j. meller, ophthalmic surgery. edited by w. m. sweet, 3rd ed. (1923) and sir j. h. parsons, diseases of the eye, 4th ed. (1923). (r. f. m.)