tures, answering to their necessities and ways of living.

iii. As in man and most other creatures the eyes are situated in the head, because, among other reasons, it is the most convenient place for their defence and security, being composed of hard bones, wherein are formed two large strong sinuses, or sockets, commonly called orbits, for the convenient lodging of these tender organs, and securing them against external injuries; so in those creatures whose head, like their eyes and the rest of their body, is soft and without bones, nature has provided for this necessary and tender organ a wonderful kind of guard, by enduing the creature with a faculty of withdrawing his eyes into his head, and lodging them in the same safety within his body. We have a very beautiful example of this in snails, whose eyes are lodged in four horns, like atramentous spots, one at the end of each horn, which they can retract at pleasure when in any danger. Here it may be also observed, that the cornea in all animals that want eyelids, as fishes, exactly resembles in hardness the horn of a lantern; and therefore is not hurt by such particles as their eyes are commonly exposed to. And in the mole, because this animal lives under ground, it was necessary its eyes should be well guarded and defended against the many dangers and inconveniences to which its manner of living exposes it: this is the reason why its eyes are so small, and that they are situated so far in the head, and covered so strongly with hair; and besides, they can protrude and retract them at pleasure. The eyes of insects are more varied than in any other class of animals: for their form and distribution see ENTOMOLOGY.

iii. The third and last reflection we shall make upon the external motion of our eyes, is what regards a problem which has very much perplexed both physicians and philosophers, viz. What is the cause of the uniform motion of both eyes? In some creatures, such as fishes, birds, and among quadrupeds the hare, camelion, &c., the eyes are moved differently; the one towards one object, and the other towards another. But in man, sheep, oxen, and dogs, the motions are so uniform that they never fail to turn both to wards the same place; hence in operations upon the eye that require it to be kept immoveable, sometimes it is necessary to tie up the sound eye with a compress, by which means the other is easier kept fixed and immoveable. The final cause of this uniform motion is, (1). That the sight may be thence rendered more strong and perfect for since each eye apart impresses the mind with an idea of the same object, the impression must be more strong and lively when both eyes concur; and that both may concur it is necessary that they move uniformly; for though the retina, or immediate organ of vision, is expanded upon the whole bottom of the eye, as far as the ligamentum ciliare, yet nothing is clearly and dis tinctly seen but what the eye is directed to. (2.) Another advantage we reap from the uniform motion of the eyes, which is more considerable than the former, consists in our being thereby enabled to judge with more certainty of the distance of objects. (3.) There is yet another advantage,

full as considerable as any of the former, that is thought to arise from the uniform motion of our eyes, and that is, the single appearance of objects 'seen with both our eyes; which, though at first view it does not appear probable, is true: for if in looking at an object you turn one of your eyes aside with your finger, and alter its direction, every thing will be seen double.

By the internal motions of the eye we understand those motions which only happen to some of its internal parts, such as the crystalline and iris; or to the whole eye when it changes its spherical figure and becomes oblong or flat. The internal motions of our eyes are either such as respect the change of conformation that is necessary for seeing distinctly at different distances, or such as only respect the dilatation and contraction of the pupil. That our eyes change their conformation, and accommodate themselves to the various distances of objects, will be evident to any person who but reflects on the manner and most obvious phenomena of vision. See VISION.

PART II.

OF THE DISEASES OF THE EYE.

The principal diseases of the eye are ophthalmia; opacities and ulcers in the cornea; pterygium and encanthis; staphyloma; cataract; hypopium, dropsy, and cancer. For GUTTA SERENA, see that article. We shall exhibit the chief symptoms of these diseases in the foregoing order: referring to the articles MEDICINE and SURGERY, for a more particular description of the modes of treating them.

1. Of Ophthalmia.—Inflammation is the most frequent of all the disorders of the eyes, and there are few other diseases of these organs, on which it is not attendant. Of many, it is a necessary symptom or consequence. Pain and redness are its chief diagnostic symptoms; the latter one is particularly so, the healthy eye being free from all redness. But this appearance is not essential to ophthalmy; for many eyes, undergoing vehement inflammation, are little, or not at all, red. The internal ophthalmy is of this description. The red appearance, when present, is most evident in the white of the eye; but, when the inflammation is severe, red vessels are frequently seen ramifying over the cornea. Little vesicles, containing extravasated blood, also sometimes form on this transparent membrane. The dark red swelling of the conjunctiva, termed chemosis, arises from an effusion of blood into the loose cellular texture, which connects this membrane with the sclerotica. The eyelids also commonly partake of the redness attendant on ophthalmy. In mild cases, the pain may be compared with a sense of heat in the part affected, or with a sensation, seeming to arise from the lodgment of sand or dust, underneath the eyelids. In more severe instances, there is a violent, burning, spasmodic, darting pain.

This disease is sometimes attended with a diminution, or total loss of vision; and this unpleasant event may depend on opacity of the cornea, a closure of the pupil, or a paralytic state

of the retina. Tumefaction, which accompanies inflammation in general, seems principally to affect, in these cases, the conjunctiva forming the white of the eye. In very violent ophthalmies, this membrane swells, in such a manner, that it covers the whole cornea, protruding like a thick fold between the eyelids, which cannot be shut. The whole eye-ball seems, on this occasion, like a portion of red flesh. The cornea is also liable to become considerably thickened, so as to come into contact with the iris, and adhere to it, or to form an opaque prominence forward, termed staphyloma. In ophthalmy, the eyelids also are not unfrequently very much swollen. Acute inflammations of the eyes are usually accompanied with the common symptoms of inflammatory fever, the constitutional disturbance being proportioned to the vehemence of the local affection.

In severe ophthalmy, two distinct stages are commonly observable; the first is attended with a great deal of heat and pain in the eye, and considerable febrile disorder; the second is, comparatively, a chronic affection, without pain and fever. The eye is merely weakened, moister than in the healthy state, and more or less red. The second stage is frequently very obstinate, and much more difficult to cure than the first.

Mr. James Wardrop has recommended puncturing the cornea, and letting out the aqueous humor, in certain cases of ophthalmy. This gentleman states, that in the living body the transparency of the cornea varies according to the degree of its distention, and that, in cases of opacity of the cornea, attended with fulness of the eyeball, the transparency of the above membrane may be restored by the evacuation of the aqueous humors. Mr. Wardrop first tried the experiment in a case, in which there was a considerable degree of milkiness and opacity of the cornea, and in which the eyeball appeared distended and prominent, attended with acute inflammatory symptoms. He discharged the aqueous humor by a small incision, and found that the operation produced, not only an alteration in the degree of the transparency of the cornea; but an abatement of the pain, and a sudden check of all the inflammatory symptoms. Hence, Mr. Wardrop was afterwards led to perform the operation, for the purpose of relieving the ophthalmy, as well as preventing opacity of the cornea, when the inflammation is attended with a fulness, prominence, and sense of distention in the eye, and cloudiness of the cornea. He says also, that in all cases in which there is the smallest quantity of pus in the anterior chamber, accompanied with inflammation, he would let out the contents of this part of the eye. Mr. Wardrop recommends the operation to be done with such a knife as is employed for the extraction of the cataract. The instrument is to be introduced into the cornea, so as to make an incision as broad as the blade, at the place where the knife is usually introduced in operating to extract the cataract. Then the aqueous humor flows out along the edges of the blade, as soon as this is turned a little on its own axis.-See Edin. Med. and Surgical Journal, Jan. 1807. Purulent Ophthalmy in Adult Subjects.-The VOL. VIIL

purulent eyes of infants are remarkably subject to ophthalmy. But there is another kind of purulent ophthalmy, to which adults are subject, and which is so generally represented by the best modern authors as arising from two very peculiar causes, viz. the suppression of a gonorrhoea, and the inadvertent application of gonorrhoeal matter to the eyes, that the mention of the case cannot be prudently omitted.

The disease produces rather a swelling of the conjunctiva than of the eyelids. This tumefaction is followed by a discharge of a yellow, greenish matter, similar to what issues from the urethra in cases of clap. Heat and pain in the eyes; great aversion to light; and, in some instances, an appearance of hypopium in the anterior chamber, attend the malady. When the complaint proceeds from the second cause, it is said to be less severe.

As to scrofulous ophthalmy, no specific for scrofula being known, the treatment consists rather in preventing an aggravation, than attempting to effect the radical cure of the complaint.

2. Of Opacities and Ulcers of the Cornea.Opacity of the cornea is one of the worst consequences of obstinate chronic ophthalmy. Scarpa distinguishes the superficial and recent state of the disease, from the albugo and leucoma, which are usually accompanied with inflammation, assume a clear pearl color, and affect the very substance of the cornea. The nebula, or slight opacity, now to be considered, is preceded by, and attended with, chronic ophthalmy. The iris and pupil can be seen through a kind of dimness; and, of course, the patient is not entirely bereft of vision. The veins of the conjunctiva, greatly relaxed by the protracted duration of chronic ophthalmy, become preternaturally turgid, irregular, and knotty; first in their trunks, then in their ramifications, near the union of the cornea with the sclerotica; and, ultimately, in their most minute branches, returning from the delicate lamina of the conjunctiva, spread over the anterior surface of the cornea. When this happens, a milky albuminous secretion begins to be superficially effused in the interspaces between the red streaks. The opaque specks thus produced may cover only a part, or the whole of the cornea.

The opacity of the cornea, sometimes occurring in violent ophthalmies, is essentially different from the nebula, and arises from a deep extravasation of coagulating lymph into the cellular texture of the cornea, or from an abscess between the layers of this membrane.

Albugo and Leucoma are also effects of severe acute ophthalmy, and consist of an extravasation of dense lymph into the very substance of the cornea. The disease is sometimes the consequence of an ulcer, or wound of this membrane. The first species, arising from the extravasation of lymph, is named albugo; while the term leucoma is particularly applied to the other form of the complaint. The recent albugo may sometimes be dispersed by the same treatment as is applicable to violent ophthalmy.

An ulcer of the cornea is commonly the consequence of the rupture of a small abscess, which not unfrequently forms beneath

3 D

the delicate layer of the conjunctiva, continued over the cornea, or in the very substance of the cornea itself, in consequence of violent ophthalmy. At other times, the ulcer is produced by the irritation of extraneous substances in contact with the eye, such as quick-lime, pieces of glass, &c. Little abscesses of the cornea ought never to be punctured, although they are slow in bursting. The matter which they contain is so viscid, that not a particle of it ever issues from an opening, and the wound exasperates the disease, increases the danger of opacity of the cornea, and often occasions another small abscess in the vicinity of that which has been punctured. The safest plan is to temporise, until the pustule spontaneously bursts; promoting this event by means of frequent fomentations, by bathing the eye with warm milk and water, and by applying to it emollient poultices. The ulcer of the cornea is of a pale ash-color; its edges are high and irregular; it creates acute pain; discharges a serous matter, and has a tendency to spread widely and deeply.

Scarpa has observed, that this character is not peculiar to ulcers of the cornea, it is common to all those sores which are situated where the skin is delicate, tense, and exquisitely sensible; as on the nipples of the mamma, the glans penis, lips, apex of the tongue, &c. When ulcers of the cornea spread superficially, the transparency of the membrane becomes destroyed; when they proceed deeply, and penetrate the anterior chamber of the aqueous humor, this fluid escapes, and a fistulous opening may remain, or a prolapsus of a portion of the iris take place. If the ulcer be large, even the crystalline lens and vitreous humor may fall out; and, in short, a *otal destruction of the whole organ of sight be the result. The cicatrix of a large ulcer impairs the texture of the cornea so much, that the injury is irremediable.

3. Of Pterygium and Encanthis.-Pterygium is a term applied to the little, preternatural, reddish, ash-colored, triangular membrane, usually growing from the internal angle of the eye, about the caruncula lachrymalis, and extending over the cornea to the great impairment of vision. Although this kind of membrane generally proceeds from the inner canthus, it occasionally arises from the outer one, and, in some instances, from the superior and inferior hemisphere of the eye itself. Wheresoever it originates, it is a remarkable fact, that it is invariably of a triangular shape, the base of the triangle being towards the white of the eye, the apex towards the cornea, sometimes at a greater, sometimes at a less distance, from the axis of sight. In a few uncommon cases, two or three pterygia, of various sizes, are met with in the same eye, arranged with different interspaces around the circumference of the organ. Sometimes their points meet, and coalesce on the centre of the cornea, so as to completely abolish the functions of the eye.

Scarpa observes, with his usual accuracy, that between chronic varicose ophthalmy, with relaxation of the conjunctiva, the superficial opacity termed nebula, and the pterygium, the only difference is in the degree of the disease. All three

consist of a varicose state of the vessels of the conjunctiva over a certain extent of this membrane, together with a degree of relaxation of the tunic itself. In the chronic varicose ophthalmy the preternatural magnitude and knottiness of the veins, and the relaxation of the conjunctiva, are confined to the white of the eye; in the nebula the vessels of the conjunctiva are dilated and knotty, even over some part of the delicate layer of this membrane, covering the transparent portion of the eye: in the pterygium, to the varicose state of the vessels on the surface of the cornea is added a thickening of the lamina of the conjunctiva, spread over this membrane.

A very peculiar feature of the pterygium is the facility with which it may be taken hold of with a pair of forceps, and raised in a fold over the cornea. It is worthy of notice, however, that sometimes the pterygium assumes a malignant cancerous nature, and then it has a bright red color like sealing-wax, easily bleeds when touched, is firmly adherent to the cornea, and occasions lancinating pains, which extend over the whole eye and temple. Scarpa recommend this case to be treated on the palliative plan; but it seems questionable whether it might not be proper to attempt the total excision of the malady; if the trial should fail, we might then extirpate the eye, an operation truly horrible; but one which would certainly become inevitable, if the malignant pterygium were allowed to increase, and one which can be avoided by no other means than an effort to extirpate the recent disease.

The encanthis, in the incipient state, is a small, soft, red, and sometimes livid excrescence, growing from the caruncula lachrymalis, and neighbouring semilunar fold of the conjunctiva. In the inveterate state its magnitude is considerable; and its roots extend beyond the caruncula lachrymalis, along the lining of one or both eyelids. The complaint excites chronic ophthalmy, prevents the complete closure of the eye, and by compressing and displacing the puncta lachrymalia, obstructs the free passage of tears into the nose. The surface of the excrescence is at first granulated like a mulberry; but, after the tumor has become large, only a part of its outside has the above appearance, while the rest seems like a smooth, whitish, ash-colored substance. In this advanced state, the body of the encanthis divides (to use Scarpa's figurative expression), like a swallow's tail, so as to form two elongations, one of which extends along the inner surface of the upper eyelid, the other along the inside of the lower one. As of pterygium, so there is a malignant species of encanthis, denoted by its dull red color, bleeding tendency, lancinating pain, excessive hardness, and very fetid discharge. The same practical observations apply to this case, as to the cancerous pterygium,

4. Of Staphyloma and Prolapsus of the Iris.Staphyloma is the name given to that disease of the eye, in which the cornea loses its natural transparency, rises above its proper level, and even projects between the eyelids, in the form of a whitish, pearl-colored tumor, which is attended with total loss of sight. The malady commonly results from some violent species of ophthalmy

particularly that which is termed purulent, and affects children, and that which is consequent to the small-pox. As Scarpa observes, the staphyloma is one of the most serious diseases to which the eyeball is subject; for, to the total and irremediable loss of sight, are added all the evils which necessarily result from the protuberance of the cornea in advanced cases. The inability of closing the eyelids; the exposure of the eyeball to the contact of the air, and extraneous matter suspended in it; the friction of the eyelashes against the tumor; the incessant flux of tears down the subjacent cheek; render the eye painful and inflamed; sympathetically induce ophthalmy in the sound one; and cause ulceration both on the diseased part of the eye, and on the lower eyelid and cheek.

Prolapsus of the iris is also denominated by some writers staphyloma. Sometimes, when the aqueous humor has escaped through an ulcer, or wound of the cornea, the iris is pressed forward by the humors situated behind it, until a portion of it protrudes from the eye, at the same opening through which the aqueous humor made its escape. The little tumor is of the same color as the iris, viz. brown, or grayish, and is surrounded at its base by an opaque circle of the cornea. If we reflect a little on the delicate structure of the iris, on the great quantity of blood-vessels which enter it, and the numerous nervous filaments which are distributed to it, we shall easily conceive the nature and severity of those symptoms which are wont to attend this disease, how small soever the portion of the iris projecting from the cornea may be, even when not larger than a pin's head. Pain, similar to what would arise from something pricking the eye; an oppressive sense of tightness in the whole eyeball; inflammation of the conjunctiva and eyelids; a copious effusion of tears; and an absolute inability to endure the light, are the symptoms which successively follow this complaint. The pupil deviates from the centre of the iris, towards the seat of the prolapsus, and assumes an oval shape. In very old cases the protruded portion of the iris seems frequently to become less sensible and irritable, so that patients do not experience inconvenience equal to the above account. 5. Of Cataract.--When opacity gradually affects the crystalline lens, or its capsule, the species of blindness thence resulting is denominated a cataract. The first effect of an incipient opacity is a mist before the eyes, surrounding every object, and afterwards gradually increasing so much in density as to render things quite invisible. The opacity behind the pupil increases in proportion as the cloudiness in vision augments. As the lens is thick at its centre, and thin at its edge, the incipient opacity, when viewed externally, always seems the greatest in the middle of the pupil; while the circumference of the lens appears like a black ring, surrounding the white nucleus of the crystalline. Some rays of light are capable of penetrating the thin margin of the lens in its most opaque state; and hence patients with cataracts are almost always able to distinguish light from darkness, and, in the early stage of the complaint, discern objects best when these are a little on one side of

the axis of vision, and not immediately opposite the eye. Hence, also, such patients see better in a moderate, than a brilliant light, which makes the pupil contract over the thin circumference of the lens. When the opaque lens is either more indurated than in the natural state, or retains a moderate degree of consistence, the case is termed a firm or nard cataract.

Mr. Pott has explained, that an opaque lens is very rarely firmer than, or even so firm as, a healthy one; and both this eminent surgeon and Richter make it appear probable, that the harder a cataract is, the thinner and smaller it becomes. The latter states, that a firm cataract usually presents either an ash-colored, a yellow, or a brownish appearance. The interspace between the cataract and pupil is very considerable. The patient very distinctly discerns the light, and can even plainly perceive large bright objects. the dilated state of the pupil, a black circle, surrounding the lens, is very perceptible. The motions of the pupil are free and prompt; and the anterior surface of the cataract appears flat, without the least degree of convexity. When the substance of the lens seems to be converted into a whitish, or other kind of fluid, lodged in the capsule, the case is denominated a milky, or fluid cataract. According to Richter, this species of the malady has usually a white appearance, while irregular spots and streaks, different in color from the rest of the cataract, are often observable on it. These are apt to change their figure and situation, when frequent and sudden motions of the eyes occur, or when these organs are rubbed or pressed. The lower half of the pupil seems more opaque than the upper one. The crystalline lens, as it loses its firmness, commonly acquires an augmented size. Hence the fluid cataract is thick, and the opacity close behind the pupil. Sometimes one can perceive no space between the cataract and margin of the lens. In advanced cases the pupil is usually very much dilated, and the iris moves very slowly and inertly, in consequence of the cataract touching this membrane, and impeding its action. Patients, who have milky cataracts, usually distinguish light from darkness very indistinctly, and sometimes not at all; partly be cause the cataract, when bulky, lies so close to the iris that few or no rays of light can enter between them into the eye; and partly because the fluid cataract always assumes a globular form, and consequently has no thin edge, through which the rays of light can penetrate.

When the opaque lens is of a middling consistence, neither hard, nor fluid, but about as consistent as a thick jelly, or curds, the case is termed a soft, or caseous cataract. As the lens softens in this manner, it commonly grows thicker and larger, even acquiring a much greater size than that of the fluid cataract. Hence, this species of the malady impedes the motion of the pupil, even more considerably than the latter sort of case.

The only other species of the disease, necessary to be noticed, is the secondary membranous cataract, which is an opacity of the anterior or posterior layer of the crystalline capsule, taking place after the lens itself has been removed from