The ciliary body in accommodation.

The ciliary muscle ring contracts about 0.8 mm in radius during maximum accommodation and this change in radius does not significantly alter as the eye ages between 15 and 45 years. Despite this constant movement of the ciliary muscle ring, the force of contraction steadily increases over the same age period from 0.8 to 1.2 gms. The force of contraction of the ciliary muscle per dioptre--the myodioptre--changes both during the act of accommodation and as the eye ages. In the first case as the amplitude of accommodation increases, the myodioptre also increases in direct proportion to the amplitude. In the second case, as the eye ages an additional increase occurs at all amplitudes of accommodation. This increase is initially quite small but between the ages of 40 and 45 it becomes very much greater. As age advances this increased force of contraction is matched by a decrease in movement of the equator of the lens so that the zonule which is attached to both the ciliary body and the lens becomes increasingly stretched during accommodation and so can transmit the increased force of contraction. The zonular fibres between the ages of 15 and 45 do not appear to change in their extensile properties so the elasticity modulus remains constant with a value of 3.5 X 10(5)Nm-2. This Young's modulus is some 10 to 900 times less than the elasticity modulus respectively of lens capsule or tendon collagen but almost the same as aortic elastin.