Accommodation (eye)
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Accommodation is the process by which the eye increases optical power to maintain a clear image (focus) on the retina.Cassin, B. and Solomon, S. Dictionary of Eye Terminology. Gainsville, Florida: Triad Publishing Company, 1990. The principal focusing ability of the (terrestrial) eye is due to the difference in refractive index between air and the curved cornea, but the variable curvature of the lens allows for an additional adjustment. This varies from a maximum of over 15 diopters in an infant to only about 1.5 diopters in a person 70 years old, as the lens becomes less flexible with age.
Contents
Theories of mechanism
- Helmholtz - Hermann von Helmholtz proposed his theory of accommodation in the middle of the 19th century. When viewing a far object, the circularly arranged ciliary muscle relaxes causing the lens zonules and suspensory ligaments to pull on the lens, flattening it. The source of the tension is the pressure that the vitreous and aqueous humours exert outwards onto the sclera. When viewing a near object, the ciliary muscles contract (resisting the outward pressure on the sclera) causing the lens zonules to slacken which allows the lens to spring back into a thicker, more convex, form.
- Schachar
Notes
References
See also
Disorders of accommodation
- Accommodative esotropia
- Accommodative spasm/Accommodative excess
- Accommodative infacility
- Accommodative insufficiency
- Accommodative lag
- Latent hyperopia
- Presbyopia
- Pseudomyopia
Other
- Accommodation reflex
- Amplitude of accommodation
- Cycloplegia
- Cyclospasm
- Edinger-Westphal nucleus
- Negative relative accommodation
- Positive relative accommodation
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