Couching is performed by means of a sharp-pointed, slender instrument, called the couching needle, introduced into the globe of the eye, about one-eighth of an inch behind the cornea, carried in front of the cataract, and then moved down wards and backwards, so as to displace the opaque lens from the axis of vision. The cataract will be absorbed in time, when it is thus removed from its natural connections. If the lens should be in a milky or caseous state, the pupil will not become clear at the time of the opera tion ; but the opaque fluid, or any frag ments, will be absorbed afterwards. if there be adhesion to the iris, beware of too much violence, and rather repeat the operation. Needles have been made of various shapes, those of Mr. Hey and Scarpa are the best.
Extraction is performed by cutting the lower half of the cornea, near its junc tion with the sclerotica, with a slender knife carried across the eye by one •mo tion of the hand. A scratch is then made in the crystalline capsule, through which the cataract escapes. The dexterity re
quired in performing extraction, and par ticularly in making the cut through the cornea, is only to be -acquired by long practice ; and this operation has conse quently become confined almost entirely. to the oculists.
Much has been written on the respec tive advantages of the two operations : the merit of ease of performance, sim plicity and mildness in the subsequent symptoms, belong to couching ; also that of the power of repeating the operation. Extraction is more difficult, attended with great injury to , c the cause entirely.
After both operations light must be carefully excluded, and every precaution taken against inflammation. The use of convex glasses is required as an assist ance to the sight, rendered necessary by the loss of the lens.
The capsule of the lens sometimes be comes opaque after the operation, and thus a secondary membranous cataract is formed, requiring the operation.