Cornea

“ The CORNEA is the transparent projecting layer, that forms the anterior fifth of the globe of the eye. In form, it is circular, convexo-concave, and resembles a watch-glass. It is received by its edge, which is sharp and thin, within the bevelled border of the sclerotic, to which it is firmly attached. The cornea is composed of several different layers; its blood-vessels are so small that they exclude the red particles altogether, and admit nothing but serum. ”

“ Care must be taken not to lacerate the anterior surface of the lens when the iris is drawn back by the pins.
Putting the pins into the cornea, and using them as levers with the point of entrance in the cornea as a fulcrum, will pucker the cornea considerably, and a good clear fundus cannot be obtained. This is easily overcome. Simply gather up all the tissues surrounding the eye, force them backward, and hold them firmly with the fingers of the left hand. ”

“ One particular layer of the cornea may be first attacked, but the whole structure soon becomes involved. The inflammation generally commences in the conjunctival covering. Vision is necessarily much obscured from even slight inflammatory affection of the cornea. Part only of the organ may be affected, but frequently the whole is involved. Sometimes only one or two vessels remain dilated; but still they, passing over the centre of the cornea, render vision indistinct. Opacity of the cornea, to a greater or less degree, always attends dilatation of its vessels. ”

“ The operators appear to rely largely on manual dexterity, and to aim at completing the procedure in a minimum of time. The needle or thorn is thrust suddenly through the cornea, and on through the pupil or iris, into or on to the periphery of the lens. The next movement, which appears to follow the first so rapidly as practically to melt into it, is that of depression or reclination. In this, the spot where the cornea grasps the shaft of the needle serves as a fulcrum. ”

“ If the light enter the cornea, as in the figure to the right, and if the light be moved, the image will be displaced in the same direction as the light, if the movement does not extend beyond the middle of the cornea, but in the opposite direction to the light when the latter is moved up and down. Thus, if a be moved to a′, d will be moved to d′, the shadow on the retina from c to c′, and the image b to b′. If, on the other hand, a be moved above the plane of the paper, d will move below, consequently c will move above, and b′ will appear to sink. ”

“ Each of these humours is enclosed in a delicate thin transparent membrane. The external coat of the eye is called the cornea, from its resemblance to horn, being a continuation of the dura mater and pia mater; and is every where opaque, except at the entrance of the light, which is called pupilla, and projects somewhat beyond the spherical form of the eye, similar to the crystal of a watch. Inside the cornea it is lined with a spreading of the optic nerves, called the retina, where objects are reflected; by which the ideas are immediately conveyed to the brain. ”

“ The compound eye is so delicate and wonderful, that great knowledge of anatomy or the science of optics is necessary before it can be really appreciated. Briefly, it is made up of a cluster of simple eyes, in each of which there are several parts. Beginning at the surface we have what is known as the facet, or cornea, which roughly corresponds to the surface of our own eyes. ”

“ Behind the horny covering, or cornea, which consists of two plano-convex lenses, is a layer of dark pigment, which gives the characteristic colour to these eyes. This is pointed like the neck of a vase, and serves the purpose of the iris in the higher animals. This is traversed by a minute aperture or pupil, through which the rays pass by a longer conical lens to the optic nerve. ”

“ The eyes are, in the nodular form, almost always affected; nodules are frequently present in the eyelids, the upper as well as the lower, and are usually situated close to their margin. The earliest affection of the eye itself, which we have observed, is a faint clouding of the upper part of the cornea, which often appears as a very fine dotting of the corneal surface, only noticeable when one can compare the upper part of the cornea with the black pupil, and often requiring for its recognition the use of a lens. ”

“ Operation.—A double-edged needle is introduced through the cornea near its margin; on arriving at the place where the pupil ought to be, one edge is drawn against the iris, and divides it transversely, if possible, without injuring the lens; the fibres of the iris start back, contract, so that a sufficiently large central pupil may be obtained.{169}
2. Excision.—In the far more frequent cases in which there exist adhesions between iris and cornea, or iris and anterior capsule, incision is not sufficient, and it is necessary to excise a portion of the iris. ”

“ A light, preferably a candle, is placed in front of the eye and moved from side to side, upward and downward, so as to bring its images over all parts of the cornea and lens. In the normal eye there are reflected three images of the light, one large, clear, and upright from the anterior surface of the cornea, one, much smaller but still upright, from the anterior capsule of the lens, and one, small and inverted, from the posterior surface of the lens and capsule. ”

“ After it [190] has become completely established, the mesoblast around the edge of the cornea becomes divided into two strata; an inner one (fig. 290, cb) destined to form the mesoblastic tissue of the iris already described, and an outer one (fig. 290, cc) adjoining the epidermis. The outer stratum gives rise to the corneal corpuscles, which are the only constituents of the cornea not yet developed. The corneal corpuscles make their way through the structureless corneal layer, and divide it into two strata, one adjoining the epiblast, and the other adjoining the inner epithelium. ”

“ So, in the natural eye, if the cornea be single, and the humours clear, the light will act correctly; but if there be a film over the cornea, or a cataract, or a skin between any of the humours, the rays of light will never make any impression on the internal seat of sight, the retina. ”

“ This sign consists in the absence of contraction of the pupil, after puncture of the cornea and evacuation of the aqueous humor. When the pupil contracts, life is still present; when it remains immovable, it is a certain sign of death. The puncture of the cornea may be made with a cataract knife, or even an ordinary lancet. ”

“ That a change in the germ, which influences the formation of the retina, may affect at the same time also the formation of the cornea, the iris, the lens, the visual centres, etc., I admit, if necessary, although they are formations that differ much more from one another in their original nature than do probably hair and teeth. ”

“ The probable ætiology in this case was an overgrowth of granulation tissue closing the perforation in the cornea, which, owing to an unhealthy condition of the wound and eye, which was anæsthetic and atrophic, had become exuberant, simulating exactly the condition known as “proud flesh” elsewhere. By a process of accumulation and heaping up, the granulations gradually formed a cap over the cornea, whilst the external layers gradually became stratified and horny from the pressure of fresh growth from the central core and by the action of the air. ”

“ Continue cutting through the sclerotic and all underlying tissues, stopping at the corneo-scleral junction, but do not, during this procedure, [88] even touch the lens. After the eye has been thus partly separated into, as nearly as possible, two equal parts, lay it down upon the cornea, and, holding the razor blade in the forefingers and thumbs of both hands, cut the lens in two by forcing the blade down through it. (Fig. 42.) Partly open the cut eye to allow one jaw of the large scissors to enter, turn the eye over so the cornea will rest on that [89] jaw, and then cut through the cornea. ”

“ DISEASES OF THE EYES Albugo, or Spot in the Eye, Opacity of the Cornea In consequence of a blow, stroke of a whip, or similar injury, an inflammatory action is set up in the eye, the result of which is an effusion of serum between the lamina of the outer coating of the eye, which renders the cornea or clear part of the eye, whitish or opaque in spots, interfering materially with the sight and usefulness of the organ. ”

“ The astigmatism in the vast majority of all eyes shows that the cornea is curved more in its approximately vertical than in its horizontal meridian. The few cases in which this kind of curvature is not present are really 'exceptions which test the rule' The habitual placing of the upper lid at this precise line of the cornea is caused by the necessity of shading the cornea or protecting it from the light which would enter from above and dazzle or harm the delicate retina. Every one when facing a bright light has found how bad vision is for at least some minutes afterwards. ”

“ Similarly, then, on their emergence they are refracted chiefly by the lens and cornea, so as to form an image in the outer air, the emergent rays coinciding in their path with that which they took when entering, and the image formed in the air being conjugated with the retinal image; being formed, therefore, on the same side, varying with the position of the lens and object, and the accommodation of the eye. ”

“ The cornea of the right eye is rapidly becoming transparent. The convexity of the left eye is still considerable, but the opacity is so far removed as to give evidence that a continuation of the medicine will render it again perfect, although it must take a considerable time to effect so desirable an object. ”

“ Others again, on the contrary, place the book or object they are looking at, at a greater distance from the eye than that named. Such people are called long-sighted, and the defect results from the too great flatness of the cornea and the crystalline lens. ”

“ The aqueous humour is either secreted in greater abundance than it usually is, or absorption is diminished. The cornea gradually accommodates itself to the increase of the fluid behind, and becomes wider and more prominent, but retains its transparency; in looking at the eye, the anterior chamber is seen evidently enlarged, and occasionally the aqueous humour is of a turbid appearance. There is little or no pain in the eyeball, but the patient complains of an annoying sense of fulness and tension in the part. ”

“ The increased tension is supposed to be due to an excess of fluid in the eyeball, and this is commonly attributed to an impeded outflow. The aqueous humor, which is secreted very rapidly, is supposed to escape at the angle formed by the junction of the iris with the cornea, and in glaucoma it is believed that the iris adheres to the cornea so that the angle is obstructed. ”

“ These white projecting spots are formed by the outer transparent membrane of a bag or ball filled with fluid, which ball or bag rests inside another cavity in the head. It resembles our own eye in having this external transparent tough membrane which corresponds to the cornea; which, like the cornea, is backed by the fluid in the ear-ball corresponding to our eyeball, and the back of this ear-ball appears to receive the outspreadings of a nerve, just as the back of our eye is lined with the outspread of the optic nerve forming the retina. ”

“ The muscles of the eye are six, and by the excellence of their arrangement it is enabled to move in all directions. Vision is performed by the rays of light falling on the pellucid and convex cornea of the eye, by the density and convexity of which they are united into a focus, which passes the aqueous humours, and pupil of the eye, to be more condensed by the crystalline lens. ”

“ Each of these little eyes has its own cornea, its lens, and a curious six-sided, transparent prism, at the back of which is a special retina spreading out from a branch of the main optic nerve, which, in the cockchafer and some other creatures, is half as large as the brain. ”

“ In cases of excessive prolapse of the iris after extraction of the lens, where the iris has formed adhesions to the wound, and still more frequently in cases where central opacities of the cornea have fairly occluded the natural pupil, the only chance for vision is to enlarge the old one, or make a new pupil by removal of the iris. ”

“ On cutting off with scissars the cornea of a calf's eye, and holding it in the palm of one's hand, so as to gain a proper light, the artery, which supplies nutriment to the crystalline humour, is easily and beautifully seen ”

“ This curious circumstance cannot be owing to the action of light on the inflamed vessels of the cornea; it therefore shews, that the extremity of the optic nerve or retina is also rendered more exquisitely sensible to light, by partaking of the inflammation; and I have been told, that red colours are in these cases sometimes painfully perceived even in perfect darkness. This shews that the retina is excited into motion by the stimulus of light; and that, when it is inflamed, these motions give great pain, like those of other inflamed parts, as the muscles, or membranes. ”

“ The way the cornea seems to fit into the sclerotic is not quite as one is led to believe when told that it fits into the sclerotic much the same way in which a watch crystal fits into a watch. [3] Holding this part of the eye up to a strong light one will see that the sclerotic seems to overlap the cornea in the vertical axis.
By using the tweezers the cornea may be split. Nothing in the way of locating its layers can be recognized, however, unless a section is made for microscopic examination. The epithelial may be scraped off when the cornea is a trifle dry. ”

“ This patient, who was under the care of one of us at the Philadelphia Hospital, was examined by O. E. Shakespeare, ophthalmologist to the hospital. At his first examination the bulbar conjunctivæ were slightly injected and the cornea clear. The sensibility of the cornea was possibly a little lowered. Ten days later, at a second examination, the central corneal epithelium of the left eye was found to be hazy and the whole bulbar conjuntivæ much congested. ”