Very various operations have been proposed, and exceedingly numerous and complicated instruments invented for this purpose. We can notice here only one or two of the most approved procedures:—
1. Incision is the simplest.
This is practicable and effectual only in cases where the iris is so far healthy as still to retain its contractile power, and so far free from adhesions as to be able to make use of it. The best example of such a case is that of a cataract, in which after extraction a prolapse of the iris has occurred to such an extent as to obliterate the pupil, and where, at the same time, the only adhesions are to the wound, none to the cornea.
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.
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.
The simplest and safest operation is the following:—
The patient recumbent, and the lids held apart by a speculum, the eyeball should be steadied by the forceps of an assistant. A broad cutting needle should then be introduced at the lower or outer edge of the corneal margin. This must be very gently withdrawn so as to retain as much aqueous humour as possible. Into the wound thus made the surgeon must introduce the blunt hook (known as Tyrrell's) at first with its point forwards, then, on arriving opposite the edge of the pupil, which it is intended to enlarge or replace, with its point turned backwards, so as to hook over the edge of the iris and thus drag on it. Once the hook has fairly got hold, it must again be rotated forwards, and withdrawn in the same direction as it was put in. The iris thus pulled out of the wound is to be cut off with a pair of fine scissors, so as to remove a sufficient amount to make a new pupil of the required size.
But in those cases in which the whole or greater part of the pupillary margin is adherent, the blunt hook will not do, because there exists no edge round which to hook it. One of two plans is generally chosen to remedy this:—
(1.) A free incision made with a double-edged needle; through this a pair of canula forceps is introduced, with which a portion of iris is seized and dragged to the external wound; it can then either be cut off or tied (see Iridesis); or,
(2.) A previous attempt may be made to free a portion to form an edge to catch hold of, either by incision or by Corelysis (q.v.)