THE Cataract, called by the Latines, Suffusio, is a Disease of the Crystalline Humour, rendering the whole Body of it opake, so that the Rays of Light, which in the natural State of its Transparency, were transmitted to the Tunica Retina, become now totally intercepted, and produce no Effect. This is pretty nearly the Account delivered down to us by Hippocrates and the ancient Greeks, who likewise knew it by the Name of Glaucoma. Galen was perhaps the first who specified any Difference, in defining the Cataract to be a Film situated behind the Iris; and the Glaucoma, a Disorder of the Crystalline Humour; which Opinion, with very little Alteration, has prevailed from his Time, down to the latter End of the seventeenth Century, when there arose a Dispute on this Distinction of Galen's, some of the Moderns asserting with Hippocrates, that the Cataract is always a Disease of the Crystalline Humour, and indeed with so much Reason, that there is now hardly any one who doubts it: However, during these last forty Years, this Subject has produced many Arguments on both Sides.

The Mathematicians having observed in those who have been couched, that the Defect of Sight remaining after the Operation, answers nearly to what in Optics, the removing the Crystalline Humour would occasion, have endeavoured to prove, that the Operation must in consequence be the depressing that Humour, and leaving the Eye to perform its Function afterwards with the Aqueous and Vitreous only; which wanting the Density of that Humour, will not refract the Rays sufficiently to re-unite them on the Retina; whence Patients after their Cure are obliged to use Convex Glasses, as Substitutes for the depressed Crystalline Humour.

Dr. Petit, a most accurate Anatomist of Paris, has from a critical Examination of the Figure of the Eye, argued against the Possibility of a Film's Existence in the Posterior Chamber, by reason of the Smallness of that Chamber, or Proximity of the Crystalline Humour to the back of the Iris; and again, from the Impracticability of dislodging such a Film, without offending the sound Crystalline Humour.

Lastly, and what is more certain, Anatomists have frequently dissected the Eyes of Persons under this Disorder after their Death, and have found it to be always an Opacity of the Crystalline Humour, agreeably to the Definition of a Glaucoma: so that by consequence, we must understand the Words Cataract and Glaucoma, as synonymous Terms, since they are in fact, but one and the same Disease.

I think it needless to state the Reasons on the other Side of the Question, as they are of little weight, and indeed almost universally exploded.

In describing the Nature of a Cataract, it has hitherto been a positive Maxim laid down by Oculists of every Nation, that there is one certain Stage of the Distemper, in which only the Operation is proper, and this State of the Disease, is said to be the Maturity of the Cataract: They have compared it to the Ripeness of Fruits, and have supposed a regular Change in the Consistence of the Crystalline Humour, from the Moment it is affected. They say the Disease upon its first Invasion gradually liquefies the Humour, and that after its Arrival to the utmost Period of Liquefaction, it then begins to acquire various Degrees of Tenacity, 'till at last it becomes perfectly hard, or as they stile it, horny: That the Skill of the Surgeon discovers itself, by fixing on that Time for the Operation, in which the Fluidity of the Cataract is no Obstacle to the Depression of it, from its want of Resistance to the Needle; nor its Hardness, from the Elasticity of its connecting Fibres, which immediately return it to its former Position.

This, in a few Words, is the general Doctrine; but I think the regular Alteration of the Density of the Crystalline Humour is very much to be doubted, and for my part I cannot help positively excepting to the Rule here laid down; having not only seen Cataracts of twenty or thirty Years growth, often upon the Touch of the Needle prove soft and milky, but also many Instances, in which a due Degree of Consistence occurred after four or five months, I may venture to say Days, when the Cataract was the Consequence of a Blow or Puncture; both which Cases so little correspond with this supposed Change, that they seem not only to overthrow it, but to imply, that the Cataract, after it has acquired its total Degree of Opacity, may frequently, if not generally, continue in the same state of Tenacity to the Life's end: And tho' I will not take upon me to affirm that Cataracts come always very early to their greatest Consistence, yet this we may safely deduce from these Observations; that Whenever they become intirely opake, we may properly undertake the Operation; which has been my Method of Practice hitherto, nor do I find any Reason to lay it aside.

I shall however observe in this Place, that contrary to the received Opinion, I have upon Examination, found Cataracts of a proper Consistence to be couched, long before they would have been opake; but this only confirms what I have already laid down, that there is not such a regular Change in them as has been suggested, and that we may always venture on the Operation when they are quite opake; since it might be successful as I have here intimated, even before that time; though I should never advise it, nor do I believe that Patients would submit to it, whilst they enjoyed a certain degree of Sight.

Since then the Glaucoma is no other Disease than the Cataract, we must at once discard the Distinction of these two Distempers as merely imaginary; and from what has been said with regard to the Consistence of a Cataract, that whatever it be, the Removal of the Humour is the sole End of the Operation, the Distinction of a true and false Cataract will appear equally frivolous; and consequently most of the Subdivisions comprised under this last; such as the Bag, the Milky, the Purulent, the Doubtful, the Membranous, the Fibrous, the Shaking, and many more, in the Books on this Disease; the greatest part of Which are Names that puzzle the Memory, without informing the Understanding, and indeed, have not a sufficient foundation in Nature, but owe their diversity of Character more to the Imagination of Writers, than any real Variety in the Disease.

The general Criterion of the Fitness of Cataracts for the Operation, is taken from their Colour; the Pearl-coloured, and those of the Colour of burnished Iron, are esteemed proper to endure the Needle; the White are supposed milky, the Green and Yellow horny and incurable: The black Cataract is described by most Authors, but I dare say, has been mistaken for a Gutta Serena, where no Disease appearing, the Pupil seems black as in a natural state of the Eye: And as to the Green one, I have not as I remember, in a great number of Cataracts, met with a single Instance of it, but possibly it may be in Nature; and one would indeed imagine the Describers of it could not be mistaken, in what must have been so evident.

The Depression of a Cataract of any Colour would be the Cure, if that alone was the Distemper of the Eye; but it generally happens, that the Yellow Cataracts adhere to the Iris so firmly as to become immoveable; besides, when they follow in Consequence of a Blow, which is often the Case, either the Cells of the Vitreous Humour are so much disturbed and broken, or the Retina affected that a degree of Blindness will remain, though the Cataract be depressed, and that one Cause removed.

To judge whether the Cataract adheres to the Iris, if you cannot at once distinguish it by your Sight, shut the Patient's Eye, and rub the Lids a little; then suddenly opening it, you will perceive the Pupil contract, if the Crystalline Humour does not prevent the Action by its Adhesion: And when this is the case in any kind of Cataract, the Operation can hardly be advised, though where the Adhesion has been slight, I have now and then performed it with Success.

Another Consideration of the greatest moment, before undertaking the Cure, is to be assured of the right state of the Tunica Retina, which is very readily learnt, where there is no Adhesion of the Cataract, from the Light falling between the Iris and Crystalline Humour, which if the Eye is not sensible of, it is a certain Indication of another Malady, and absolutely forbids the Operation. Generally, this Cataract takes its rise from Head-achs, Convulsions, and nervous Disorders. How the Eye perceives in this case, vide the Copper-plate.

The Operation for the soft Species of Cataract, which may perhaps properly be stiled Milky, has been by some Writers falsly said never to succeed. Of this there are two sorts; some where we do not perceive any Membrane, but which are almost uniformly soft, and admitting the Needle through them as through Water, are consequently immoveable; and others where the Humour is liquefied, and contained in its own Membrane; now pretty much thickened by the Disease, which last frequently does well; for upon breaking the Membrane, the Fluid bursts out and precipitates, and the Membrane itself, if it is not depressed in process of time shrinks into a small Compass, or wastes quite away. Whether the whole Cataract after subsiding continues to lie at the bottom of the Eye, or is quite wasted by being separated from its Vessels, I have never had an Opportunity of knowing positively by dissecting one that had been couched; but by what we see of those, which have not been totally depressed below the Pupil, and continue in that state for ever after, we may suppose that they only waste a little: I know one Instance of a Woman whose Cataract after couching became quite loose in the Eye, and in an erect Posture sunk to the bottom, but by stooping the Head forward, she could bring it quite over the Pupil. On the other hand, I once couched a Person, when upon the first Attempt to depress the Cataract it suddenly sprung up, and made its Way thro' the Pupil into the Anterior Chamber of the Eye, where I left it without endeavouring to dislodge it again. In about six Weeks it began to diminish, and at the End of ten Weeks was entirely wasted, and the Patient saw extremely well.

When none of the Objections I have stated forbid the Operation, it may be thus done: Having placed your Patient in a convenient Light, and in a Chair suitable to the Height of that you yourself sit in, let a Pillow or two be placed behind his Back, in such a manner that the Body bending forward, the head may approach near to you; then inclining the Head a little backward upon the Breast of your Assistant, and covering the other Eye as to prevent its rolling, let the Assistant lift up the superior Eye-lid, and yourself depress a little the inferior one: This done, strike the Needle thro' the Tunica Conjunctiva, something less than one tenth of an Inch from the Cornea, even with the Middle of the Pupil, into the posterior Chamber, and gently endeavour to depress the Cataract with the flat Surface of it. If after it is dislodged, it rises again, though not with much Elasticity, it must again and again be pushed down. If it is membranous; after the Discharge of the Fluid, the Pellicule must be more broke and depressed: If it is uniformly fluid, or exceedingly elastick; we must not continue to endanger a terrible Inflammation by a vain Attempt to succeed. If a Cataract of the right Eye is to be couched and the Surgeon cannot use his left Hand so dexterously as his right, he may place himself behind the Patient, and use his right Hand.

I have not recommended the Speculum Oculi, because upon the Discharge of the Aqueous Humour through the Puncture, the Eye being somewhat emptied, more readily admits of the Depression of the Crystalline Humour, than when pressed upon by the Instrument.

As to the Method of treating the succeeding Inflammation, (when it happens, for sometimes there is none) I can advise nothing particular, but to refrain from those Collyria, that are charged with Powders; for the thinner Parts flying off, leave a gritty Substance in the Eye, which must be pernicious: Bleeding, and other gentle Evacuations are found absolutely necessary. The Use of cool Applications externally is most easy to the Eye but after all, there will sometimes ensue a troublesome Ophthalmy, which, with the Uncertainty there always is of Success after the Operation, have deterred most Surgeons from undertaking it, and 'till lately from studying the Nature of the Disease; but I fancy the Operation will come into greater Repute, when more generally practised by Men of good Character; for it is less the Difficulty, than the Abuse of it by Pretenders, Which has brought it into Discredit.

Since the Publication of the sixth Edition of this Treatise, a Method of removing the Cataract by opening the Cornea, and extracting the Crystalline itself, has been discovered. The Experience of a little more Time will evince whether it be preferable or not to the old Operation. For the Manner of performing it, and the Success attending it, I must refer the Reader for the present to the Philosophical Transactions, and to the third Edition of my Critical Enquiry, where I have said all I yet know on this Subject.

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