The only affection of the lens requiring special notice is Cataract. This consists in an opacity of the lens or its capsule, such as to interfere with vision. It sometimes occurs as a congenital affection, the lens being imperfectly formed. It is most frequently a senile change, but it also occurs as a result of injury and is an occasional consequence of diabetes.

At birth the lens is soft throughout, but as years go on the centre becomes dense, so as to form a hard nucleus while the periphery remains soft. The hard nucleus is not liable to change, but the soft cortex may undergo alterations. These consist in a formation of fat amongst the fibres which constitute the lens, and the loosening and separation of these fibres. The process goes on till sometimes the affected part of the lens consists of a detritus in which are fat, choles-tearine, and remains of fibres. The dense nucleus may sink down in this softened substance. In senile cataract there is always the dense nucleus, which may be of considerable bulk.

In Capsular cataract the epithelial cells, which line the capsule, undergo changes, which may be spontaneous or the result of inflammation. They multiply and cause opacity by their abundance and by retrograde changes to which they are liable.