For clinical anatomy, it is useful to think of the lens like a peanut M&M: composed of the "candy shell" capsule, the "chocolate" cortex, and the "peanut" dense nucleus. The lens cells are elongate, spanning from the front to the back of the lens, meeting at the "sutures." After being produced at the equator, lens cells are sequentially compressed in to the lens center by new growth, making the nucleus denser with age (nuclear sclerosis).
The lens focuses images on the retina. To do that, it must be transparent and in the proper position: suspended in place (behind the iris and centered in the pupil) by the lens zonules.
A cataract is any opacity within the lens. Such opacities occur due to disruption (genetic protein miscoding and misalignment, or physical or metabolic disruption) of the normally orderly lamellar arrangement of the lens cells, and, in varying degrees, cause light scatter and blurred vision or blindness when severe. Despite this potential visual impairment, even with a complete cataract, the pupil should have normal resting size and react normally to light.
