When the disease has passed off it may leave a very contracted or closed pupil. After a sufficient time has elapsed—several months— an operation may be performed for the purpose of making an artificial pupil end restoring some degree of sight by affording a passage for rays of light. The operation is called iridectomy (see p. 481).
Cataract is an affection of the crystalline lens (p. 448), by which it gradually ceases to be transparent and becomes more and more opaque. There is no mistaking a case of ordi nary cataract. The window of the eye is quite clear, the iris has its usual appearance, but through the pupil is seen the whitish lens. It was called cataract by the ancients from the Greek verb "to flow down", because of the idea that the dimness of sight was due to some watery material flowing down behind the pupil and obscuring the sight. It is caused by sonic alteration in or interference with the nourish ment of the lens. It is commonest in old people from the general failure due to age, a large number of cases being in persons over forty years of age, and the largest number in persons over sixty. In diabetes (p. 407), a disease pro foundly affecting the nourishment of the body, it is very common. It is also caused by inter ference with the lens through accident. Any injury of the eye, which has affected the lens, is likely speedily to lead to cataract. Children are sometimes born with cataract. In some of these cases the opacity extends over the whole lens, in others it is confined to a small part of the lens, and appears as a white spot beyond the dark pupil.
Cataract usually, in cases of old persons, comes on slowly, so that the person feels a gradually increasing dimness of sight, which spectacles do not benefit. There is no pain, and the loss of sight never occurs suddenly. But as dimness of sight may arise from many causes, only an examination of the eye is sufficient to decide whether cataract or other disease is the cause. When the cataract is complete, that is when the entire lens is quite opaque, so that for all practical purposes the person is blind, the light from a lamp or candle can still be perceived in all directions, and the person can point to its direction if it is moved about.
Treatment.—In favourable cases, that is in cases where there is no other disease that would interfere with sight though there were no cat aract, the opaque lens can be removed by an operation, and good sight restored. Of course, after the lens has been removed, strong convex glasses must he used to enable the person to read. The favourable cases are known by the person being able to follow the light from a lamp moved in all directions, and to point to its direction. This operation consists in cut ting round the margin of the cornea until a sufficient opening is made for the lens to pass through. An instrument is passed through the opening to tear open the capsule which holds the lens. When this has been properly done, gentle pressure of the lids urges the lens out. Immediately after the operation the patient should be able to see clearly with the eye. A person desiring this operation to be performed should always place himself or herself in the hands of an eye surgeon of repute, since there are a considerable number of risks (to the eye only) that attend the operation. For example, if undue pressure be exerted on the eyeball, not the lens only, but some of the vitreous humour behind it, may be pressed out. If much of this escapes, the sight of the eye will be lost. The operation, therefore, requires skill and delicacy. As soon as the lens has been removed the eye is closed with plaster and a bandage, and the pa tient is put to bed and kept lying quietly on the back in a dark room for several days. If all goes well the bandage is not undone for four or six days, and it is at least a week before the person is permitted to get up, and several weeks before he is permitted to begin gradually to accustom the eye to light.
An old operation, called couching, consisted in passing in a needle and with it pushing the lens out of its place to leave the pupil clear. The lens thus was left in the eye ; but it was so apt to set up inflammation by its presence in au unusual position that this operation has been abandoned.