Cataract, an opaque condition of the lens of the eye. It is readily distinguished from opacities of the cornea, or clear front part of the eye, by its position just behind the pupil—that round and varying aperture in the iris through which light is admitted into the back of the eye. Cataract may affect the lens alone (lenticular cataract), or the front or back of the capsule of the lens (capsular cataract), or both lens and capsule (capsulo-lenticular cataract). Its colour varies from pearly white to dark amber; but the lighter shades are much the most common. Heat will produce a like change on the lens out of the body, just as it changes white of egg from transparent to opaque. The rounded lens of the fish is seen at table in this opaque condition.

Cataract, especially in some of the forms which are present at birth or appear in infancy, may remain confined to one part of the lens; but in the vast majority of cases it gradually extends till the whole of the lens substance has become opaque. The most useful classification of cataracts is that according to the circumstances under which they develop; and the most important groups are Congenital Cataract, present at birth; Diabetic Cataract, caused by diabetes; Traumatic Cataract, the result of injury; Senile Cataract, coming on spontaneously after middle life (far the most common form); and Secondary Cataract, following some other disease of the eye.

Cataract is painless, and unaccompanied by inflammation. It occasions blindness simply by obstructing the passage of the light; but cataract alone does not produce such complete blindness that the patient cannot tell light, even faint light, from darkness. If an eye affected with cataract be unable to do this, it is certain that there is some serious disease in its deeper structures which would render the removal of the cataract quite useless for the restoration of vision.

With regard to the causes of cataract much is still uncertain. It is known that it results from injuries to the lens and from many serious diseases of other structures in the eye; that it is a common result of diabetes, and possibly of other constitutional diseases; that when it comes on without any such obvious cause the tendency to it is often hereditary, but rarely manifests itself before forty-five; and that after that age it becomes more and more common with advancing years. But what is the nature of the constitutional weakness or the structural defect which leads to its development has yet to be discovered.

No medical or other treatment yet known has any influence in arresting the progress of cataract, nor can it be cured but by a surgical operation. In cases where the cataract begins in the centre of the lens, some improvement of vision can be effected in the earlier stages by enlarging the pupil, and so allowing the rays of light to enter the eye through parts of the lens still transparent. This is sometimes done by the use of dark glasses, but usually by means of Belladonna or Atropia (q.v.) dropped into the eye. The latter method is often resorted to by quacks, who when they find their victims' sight improved by this means, promise a speedy disappearance of the cataract. The patient being able to see somewhat better, after long and increasing dimness of vision, begins to congratulate himself on a cure; the quack, of course, hastens to get his money without waiting for the further result, which is sure to be blank disappointment. So long as there is fair vision with one eye, the operation on the other may be delayed. It is a mistake to delay the operation in children on account of their tender age. The sooner it is done the better, both for the eye and the education of the child.

Till the middle of the 18th century the only recognised operation for cataract was depression or couching, a procedure by which the opaque lens was pushed downwards and backwards into the vitreous humour, leaving the pupil free. This method, though occasionally very successful, was often followed by severe inflammation and total loss of sight, and is now quite abandoned in favour of solution and extraction. The former is generally preferred in patients below thirty, in whom the cataract, like the healthy lens, is usually so soft that it can readily be broken up; and the latter at more advanced ages.

In solution an appropriate needle is passed through the cornea; made to open and lacerate the front of the capsule, the rags of which curl out of the way behind the iris, so that their subsequent opacity does not obstruct the light; then the soft cataractous lens is punctured and torn up so as more effectually to admit the aqueous humour, which naturally fills the space between the lens and the cornea, and which has the remarkable property of absorbing or dissolving the lens or cataract when admitted within the capsule. This operation may require to be repeated several times, at intervals of a few weeks, before the whole cataract is dissolved.

Many methods of performing extraction, differing much in detail, have been practised. But all consist essentially of an incision at or near the margin of the cornea of sufficient length to permit of the removal of the entire lens, or at all events of its hard central nucleus. Most surgeons of the present day remove a portion of the iris (the coloured part of the eye), either at the same time or by a previous operation; and most tear the capsule of the lens and remove only its contents, though some extract the lens with its coverings. The performance of this operation has been much facilitated by the introduction of Cocain (q.v.); and its results have been improved since the antiseptic method has been applied to it. It now proves successful in skilful hands in considerably more than 90 per cent. of eyes where cataract is the only disease present.

After the lens has been removed, it is never re-formed, and the refractive power of the eye is much diminished, while its accommodation is entirely lost. It is therefore necessary that a strong spectacle-glass be worn in order that clear sight may be obtained; and different glasses are required for distant and for near objects (reading, &c.). With such glasses, however, the perception of objects directly looked at is sometimes as clear as ever. See EYE.

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