Plate from 18th century encyclopedia showing illustrations of various instruments for amputation, cataract surgery and also various cannulae, and a tourniquet. Couching is the earliest documented form of cataract surgery. As a cataract is a clouding in the lens of the eye, couching is a technique of dislodging the lens, thus removing the opacity. Although couching is no longer part of ophthalmic surgical practice, it was a precursor to modern cataract surgery. A tourniquet is a constricting or compressing device, specifically a bandage, used to control venous and arterial circulation to an extremity for a period of time. A cannula is a tube that can be inserted into the body, often for the delivery or removal of fluid or for the gathering of data. Also shown are bone saws and knives for amputation.