Mechanisms of accommodation in vertebrates.

Mechanisms of accomodation among Vertebrates vary with phylogeny, structure of the eye, and feeding habits. Some vertebrate eyes are so small as to possess a large depth of field, while others with larger eyes have structures that circumvent the need for an active lenticular focusing mechanism. Cyclostomes and Teleosts are myopic and move the lens backward to accommodate for distance. Selachians, Amphibians, and Snakes are hypermetropic and move the lens forward to accommodate for near. Birds and Reptiles have powerful mechanisms that compress the lens to accommodate for near. Amphibious vertebrates have the greatest accommodative amplitudes of all. The mammalian mode of accommodation is relatively new in the phylogenetic sense. It is poorly developed in most forms except primates, the most advanced of which is man's. The Helmholtz theory of accommodation remains widely accepted. Upon contraction of the ciliary muscle, zonular tension on the lens is released, and capsular elasticity molds the lens into a more spherical shape. Loss of capsular elasticity and hardening of the nucleus account for most of the decrease in accomodation with age.