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 This must be very gently withdrawn so as to retain as much aqueous humour as possible.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 Once the hook has fairly got hold, it must again be rotated forwards, and withdrawn in the same direction as it was put in.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._) IRIDESIS.--_Critchett's Operation of Ligature._[89]--Patient being put under chloroform, the ball is fixed by the wire speculum, and also by a fold of conjunctiva being seized by forceps. An opening is then made with a broad needle through the margin of the cornea, _close_ to the sclerotic, just large enough to admit the canula forceps, with which a small portion of iris close to its ciliary attachment is seized and drawn out; a piece of fine floss silk, previously tied in a small loop round the canula forceps, is slipped down and carefully tightened round the prolapsed portion. This speedily shrinks, and the loop may generally be removed about the second day. The chief advantage claimed for this method is the ease with which the size of the new pupil can be regulated. It is also suitable in cases of conical cornea, where it is wished to change the form of the pupil into a narrow slit. _N.B._--The ends of the ligature must be left sufficiently long to avoid any risk of their being drawn out of sight into the substance of the cornea, or even into the ball, by retraction of the fibres of the iris. CORELYSIS.--_Freeing of the Pupil._--An operative procedure for separating posterior adhesions of the iris to the lens. In it the surgeon hopes to act, not on the iris, as in the operations for artificial pupil, but only on the bands of false membrane which distort the pupil.