The force of contraction of the human ciliary muscle during accommodation

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Abstract
An apparatus was designed to alter the shape of the human lens by tensile forces applied to the zonular fibers indirectly through the ciliary body. The changes in dioptric power of the lens for monochromatic Na light were measured at the same time. Simultaneous serial photography and direct measurement enabled one to relate a change in shape of the lens to the change in dioptric power. Subsequently, the same lens was isolated and spun around its antero-posterior polar axis while high speed photography recorded its changing profile. By comparing the changes in lens profile due to zonular tension and centrifugal force, respectively, the force developed in the zonule for a given change in the shape of the lens could be calculated. Changes in dioptric power associated with those of shape can thus be related directly to the force of contraction of the ciliary muscle necessary to reduce the initial tension of the zonule in the unaccommodated state. The force of contraction of the ciliary muscle as measured by radial force exerted through the zonule and the change in dioptric power of the lens were not linearly related. The relationship was more exactly expressed by the equation: .**GRAPHIC**. where D = amplitude of accommodation in diopters (m-1), FCB = force of contraction of the ciliary muscle as measured by changes in tension of the zonule (N), Kdf = dioptric force coefficient and was constant for a given age (m-1N-1/2 .times. 102.5). This coefficient was 0.41 at 15 yr and 0.07 at 45 yr of age. In youth for maximum accommodation (10-12 D) the force was .apprx. 1.0 .times. 10-2 N while to produce sufficient accommodation for near vision (3.5 D) the force was less than 0.05 .times. 10-2 N. After the age of 30 yr the force of contraction of the ciliary muscle necessary to produce maximum accommodation rose steadily to about 50 yr of age and thereafter probably fell slightly. At about 50 yr of age the ciliary muscle was some 50% more powerful than in youth. Even if hypertrophy of the muscle did not occur the amplitude of accommodation would be reduced at the most by only 0.8 D of that observed at the onset of presbyopia.

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