Of Cutting the IRIS.
THERE are two Cases where this Operation may be of some Service; one, when the Cataract is from its Adhesion immoveable; and the other, when the Pupil of the Eye is totally closed up by a Disorder of the Muscular Fibres of the Iris, which gradually contracting the Orifice, at last leaves the Membrane quite imperforate. This last Distemper has hitherto been deemed incurable. The Adhesion of the Cataract I have spoken of in the preceding Chapter, and considered it as a Species of Blindness not to be relieved: But Mr. Cheselden has invented a Method of making an artificial Pupil, by slitting the Iris, Which may relieve in both the Instances here stated.
In doing this Operation, the Patient must be placed as for couching, and the Eye kept open and fixed by the Speculum Oculi, which is absolutely necessary here, for the very Reason I would discard it in the other; since the Flaccidity of the Membrane from the Issue of the Aqueous Humour, would take away its proper Resistance to the Knife, and make it, instead of being cut through, tear from the Ligamentum Ciliare: then introducing the Knife in the same Part of the Conjunctiva you wound in couching, insinuate it with its Blade held horizontally, and the Back of it towards you, between the Ligamentum Ciliare and Circumference of the Iris, into the anterior Chamber of the Eye, and after it is advanced to the farther Side of it, make your Incision quite thro' the Membrane; and if the Operation succeeds, it will, upon wounding, fly open, and appear a large Orifice, though not so wide as it becomes afterwards.
The Place to be opened in the Iris, will be according to the Nature of the Disease. If the Membrane itself be only affected with a Contraction, the middle Part of it, which is the natural Situation of the Pupil, must be cut; but if there be a Cataract, the Incision must be made above or below the Cataract, though I think it more eligible to do it above.
The contracted Iris, from a paralytick Disorder, is so often complicated with an Affection of the Retina, that the Success is very precarious in this Case. This Operation, by what I have seen, has answered best in Adhesions of the Crystalline Humour, though to speak truly, but very seldom even there. As I would not mislead any one who shall practise an Operation not yet much known in the World, I do confess that either the Danger of the Iris separating from the Ligamentum Ciliare, or of the Wound not enlarging sufficiently, do upon the whole make the Event very doubtful. I once performed it with tolerable Success, and a few Months after the very Orifice I had made, contracted, and brought on Blindness again. Since it has been discovered by the Extraction of the Crystalline, that a large Wound may be made through the Cornea without any bad Consequence, I should imagine this Operation would be much improved by introducing the Knife perpendicularly thro' the Cornea and Iris, and cutting both at the same time, so that the Incision of the Iris should be exactly in the same Parts, and of the same Dimension as by the other Method.
In these two Chapters I have not once used the Word Uvea, but have made mention of the Ligamentum Ciliare, two or three times; both which Parts are but little understood for want of proper Explanation; but which must be rightly conceived of, in order to understand what I have said upon these Diseases.
The generality of Anatomists call that Membrane which I have spoken of under the Name of Iris, the Uvea, and its anterior Lamina, the Iris; others again, call the Membrane, Uvea, and the Colour of it, Iris; but both one and the other Distinction confound Learners exceedingly, and take their rise from a want of proper Attention to the History of Anatomy. The Ancients, who have given most of the Names we now employ in the Description of the Eye, were versed chiefly, if not altogether, in the Dissection of Brutes, amongst which, those of the graminivorous kind, have a party-coloured Choroides, one half of it being dark, and the other of a light shining Green; this last, from its Resemblance to an unripe Grape, was called the Uvea; but the succeeding Writers amongst the Moderns, applying themselves to human Dissections only, and not duly considering the Difference of the human Choroides, which is nearly of an uniform Colour, and of that above described, have retained the Appellation, though we have not the Thing. Hence has arisen the great variety of Misapplication of this Word, which ought no more to be adopted in the Anatomy of the human Eye, than the Tunica Nictitans, which is proper to Certain Beasts and Birds.
The Ligamentum Ciliare is that circular Line on the Globe of the Eye, where the Sclerotis, Choroides, Retina, Cornea, Processus ciliares, and Iris, terminate; forming a whitish Ring somewhat denser than any other Part of the Coats; but since the Institution of this Term, the Description of the Part it implies has been very much neglected, and the Term itself confounded with the Processus Ciliares; wherefore it was necessary to define it, that the Process of the Operation of the Iris might be better comprehended.
A. The Couching-Needle, the broad Part of which towards the Point is flat on one Side; but on the other, is a little convex, to give it more Substance and Strength.
The Handle of this Instrument is white Ivory, inlaid with a Streak of Black in that Part of it lying even with the convex Surface of the Blade: The Meaning of which is, that by holding the Handle with the Streak upwards, we may be guided to depress the Membrane of a milky Cataract with the flat Surface, tho' the Substance of the Cataract swimming in the Eye obscures the Needle, and prevents its being directed in a proper Position by the Sight.
B. A Speculum Oculi, which is made to open or shut by an Iron Button sliding along a Slit in the Handle. This Instrument is composed of one Piece of Steel, in such a manner that it would fly open by its Elasticity, if the two Branches of the Handle were not confined by the Button. The Circle of it should be covered with Velvet, to make it lie softer on the Eye-lids.
C. The Knife for cutting the Iris, the Blade of which has two edges, resembling a Lancet, which are more advantageous than one only, In cutting the Cornea for the extraction of the Cataract.
D. The Figure of the Eye.
The small Arch on the Forepart of the Figure, is the Cornea; the two straight Lines, tending to each other are the Iris, and the Opening between them is the Pupil; the Space between the Cornea and the Iris, is the anterior Chamber of the Eye; the Spherodial Body is the Crystalline Humour; The Space between the Iris and Crystalline Humour is the Posterior Chamber; and the two short Lines which arise from the meeting of the Cornea, Iris, &c. and run upon the Crystalline Humour, are the Processus Ciliares. The Design of this Representation is to shew the Smallness of the posterior Chamber, and how some Light may pass obliquely between the Iris and Crystalline Humour, thro' the Interstices of the Ciliary Processes, and occasion that Degree of Sight which People with Cataracts have.