Accommodation (vertebrate eye)

Accommodation (vertebrate eye)

Diagram illustrating the process of accommodation in the eye.

Accommodation in the vertebrate eye refers to the process by which the eye changes optical power to maintain a clear image or focus on an object as its distance varies. This process is essential for clear vision and involves the lens of the eye changing shape.

Mechanism of Accommodation

Accommodation is primarily controlled by the ciliary muscle, which is a ring of smooth muscle in the eye's middle layer. When the ciliary muscle contracts, it releases tension on the zonular fibers, allowing the lens to become more spherical and increase its optical power. This is necessary for focusing on near objects. Conversely, when the ciliary muscle relaxes, the zonular fibers become taut, flattening the lens and allowing the eye to focus on distant objects.

Laser focus on a horse lens showing accommodation.

Changes in the Lens

The lens is a transparent, flexible structure that can change shape due to its elastic properties. During accommodation, the lens becomes thicker and more curved, increasing its refractive power. This change in shape is facilitated by the lens's unique structure, which includes layers of lens fibers that can slide over each other.

Path of light rays through the eye during accommodation.

Age-Related Changes

As individuals age, the lens becomes less flexible, a condition known as presbyopia. This reduced flexibility impairs the ability to focus on close objects, necessitating corrective lenses for reading or other close work.

Models of Accommodation

Several models have been proposed to explain the mechanics of accommodation. The Helmholtz model suggests that the lens changes shape due to the action of the ciliary muscle and zonular fibers. The Schachar model proposes that the lens changes shape due to equatorial tension on the lens capsule.

Diagram of the Schachar model of accommodation.

Visual Demonstrations

• 
  Human lens focusing from side and back views.
• 
  Video showing the eye lens focusing from 10 meters to 26 millimeters.

Anatomy of the Lens

The lens is composed of several layers, including the lens capsule, lens epithelium, and lens fibers. The lens capsule is a thin, elastic membrane that surrounds the lens and plays a crucial role in accommodation by allowing the lens to change shape.

Scheimpflug image showing the layers of the human lens.

Related Pages

• Presbyopia
• Ciliary muscle
• Lens (anatomy)
• Zonular fibers
• Helmholtz model of accommodation
• Schachar model of accommodation