Question

Write the function of the following part of the human eye.
Crystalline lens.

Answer 1

The human eyes work on the same basic principles as a camera. A single lens focuses light from all parts of the visual field onto a sheet of light-sensitive cells. The region of the environment from which each eye collects light rays, is called the visual field.

Complete answer:
The adult human eyeball is roughly/nearly a spherical structure. The wall of the eyeball is composed of three concentric layers. The external layer is called sclera, middle layer is called choroid and inner layer is called retina.
The middle coat (layer) of the eyeball, the choroid coat, is deeply pigmented with melanin and well supplied with blood vessels. Choroid looks bluish in colour. Choroid is not visible from outside because it is covered with sclera. The role of the choroid layer is to prevent the reflections of light rays within the eyes. This is the same function accomplished by the dull black paint within the camera.

The choroid layer occurs thin over the posterior two-thirds of the eye-ball, but it becomes thick in the anterior part to form the ciliary body. The ciliary body itself gets formed forward to form a pigmented and opaque structure called the iris. Iris is the visible coloured portion of the eye and is responsible for black, brown, green or blue colour of the eyes.
The lens of the eye is located just behind the iris. The eye lens is a converging type of lens which bends the light rays. The eye lens is a transparent crystalline structure which is held in place by the 'suspensory' ligaments attached to the ciliary body.
Ordinarily, the suspensory ligaments are kept under tension and the lens is correspondingly flattened. But, contraction of the muscles of the ciliary body attached to these suspensory ligaments relaxes them and permits the lens to take on a more or less spherical shape.

Function of lens:
1.It helps to focus the object.
2.It helps to converge the light.

Note: The changes in lens shape enable the eye to shift its focus (accommodate) from far objects to near objects and vice versa. Hence, accommodation (focussing) is the reflex mechanism by which light rays from objects at various locations in the near visual field are brought to focus on the retina.