ANATOMY AND PHYSIOLOGY OF LENS

The human crystalline lens is transparent structure with a refractive power of 15 to 18  diopters. It is 10 mm in diameter and 4 mm thick. It has a central nucleus surrounded by epinucleus then cortex and a capsule covering it. It is a biconvex structure. The capsule is thick anteriorly and thinnest at posterior pole. New cells are formed at equator and move centrally. Thus oldest cell are present centrally and youngest cells are present peripherally. Lens is held in place by suspensory ligament or zonules which originate at cilliry body. 

During accommodation for near, the ciliary muscle contracts. As it is a circular muscle, the traction on zonules decrease and it assumes more globular shape. Thus its convergence power increases. This ability of the lens to change shape decreases through out life. After 40 years of age it cannot focus light from near and the condition is called presbyopia. Here you need magnifying glasses to help accommodation.

Refractive index = 1.39

Radius of curvature

i. Anterior surface = 10 mm (less convex)

ii. Posterior surface=6 mm

The lens grows through out life

Functions

1. refractive power

2. provide accomodation for near

3. absorption of ultraviolet light

The metabolism of the lens is anaerobic. Glycolysis is responsible for 85% glucose utilization and the pentose phosphate shunt accounts for 15%.

The protein concentration

in the lens is actually the highest amongst body tissues. The main types of proteins are alpha (31%), beta (55%) and gamma (2%) crystallins, and insoluble albuminoids (12%).

The lens is suspended in the eye by zonules which are inserted on the anterior and equatorial lens capsule and attached to the ciliary body.