This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
When the lens is opaque it constitutes the disease known as cataract: this name is also applied to opacities of the capsule of the lens. When the lens alone is opaque it is called a lenticular cataract; when the capsule alone is affected, it is a capsular cataract. Both are sometimes involved, constituting a lentiadocapsular cataract. The lens is made up of layers like an onion. Some of these layers may become opaque, leaving a surrounding rim of clear tissue. The nucleus within the affected layer is also clear. This form is called a zonular or lamellar cataract. A capsular cataract may affect the anterior portion of the capsule, forming an anterior polar cataract, or the posterior layer of the capsule, forming a posterior polar cataract.
If the cornea has been perforated by a central ulceration, the aqueous humor escapes, the lens falls forward, and its anterior capsule becomes adherent at the site of perforation. As the aqueous humor reaccumulates, it pushes the lens back, leaving a small portion of inflammatory tissue clinging to its anterior capsule, thus forming an anterior polar cataract. A posterior polar cataract is the result either of disease, such as choroiditis, in which the posterior capsule becomes involved, or of a persistence of the remains of the hyaloid artery, a fetal structure.
Fig. 93. - Diagrammatic horizontal section of right eye. X 3 1/2. (Piersol).
Secondary cataracts are the opacities of the capsule or inflammatory bands and tissues which are left, or which occur, after the removal of the lens. The lens in childhood is soft; it grows harder as age increases. If the aqueous humor obtains access to the lens through a wound of the anterior capsule, the lens becomes opaque, constituting a traumatic cataract. In operating for cataract in childhood, the lens, being soft, is first rendered opaque by the aqueous humor admitted through a puncture made in the capsule; if it is admitted repeatedly to the lens by the surgeon's needle (needling or discission operation) the lens matter is completely dissolved. The fluid lens matter can also be removed by a suction instrument. In old people the nucleus becomes hard and opaque, forming a senile cataract.
The aqueous humor does not dissolve the opaque lens after the age of thirty-five years. Senile cataract rarely occurs before the forty-fifth year, so there is a period of ten years in which a cataract may be a nuclear cataract without being senile.
To remove a nuclear or a senile cataract, a slit is made through the cornea near its scleral junction, a piece of the iris may (or may not) be removed, the anterior capsule is cut with a cystotome and the opaque lens pressed out through the opening so made, then through the pupil (either artificial or dilated with atropine), and finally through the sclerocorneal incision. The posterior capsule is not injured, and it prevents the vitreous humor from escaping. If inflammation follows the operation, the iris and ciliary region throw out lymph and the remains of the capsule become opaque, forming a secondary or capsular cataract. This is removed by tearing or cutting it across with needles or extremely fine scissors.