Question

Human eye lens is
A. Spherical and can be moved forward
B. Biconvex and cannot be moved forward
C. Spherical and cannot be moved forward
D. Biconvex and can be moved forward

Answer 1

Hint: The eye lens shape is similar to that of lenses used in microscopes. The lens is able to change its aperture depending upon the position of the object with the help of ciliary muscles. No movement is necessary.

Complete answer:
The human eye lens is a fibrous crystalline structure formed of alpha and beta crystalline proteins. It is colorless, transparent, and enclosed in a lens membrane. It is biconvex and cannot be moved forward because it is held back by the suspensory ligaments.

Additional Information:

Eye lens:
- The suspensory ligaments are also known as Zonula of Zinn. In humans the eye lens is biconvex but it is elliptical or subspherical in case of frogs.
- The eyeball is divided into two chambers by the lens, the external chamber is filled with aqueous humor which is watery and secreted by the ciliary body. The internal chamber is also known as the vitreous chamber filled with vitreous jelly which is also known as Wharton’s jelly, besides water it contains mucoprotein and hyaluronic acid.

Protective parts of the eye are:
- Eyebrows: Hair like structures directed outward, they carry away sweat and raindrops trickling from above forehead away from the eye.
- Eyelids or Palpebrae: Two eyelids are present in humans but on the upper eyelid can be moved.
- Eyelashes: These are stiff hair-like structures present at the free edge of eyelids. Help to prevent entry or falling of dust particles, and small organisms into the eyes.
- Eye glands: Several secretions help to keep the eye surface moist and secretions like lysozyme act as an antibacterial agent.

So, the correct answer is,” Human eye lens is biconvex and cannot be moved forward”

Note:
The eyeball has attachments to six extrinsic muscles which include anterior rectus, posterior rectus, inferior rectus, superior rectus, inferior oblique muscles, and inferior oblique muscles. In frogs, out of the two eyelids, the lower eyelid is movable, unlike humans. The nictitating membrane which is a vestigial organ in humans is still functional in frogs and birds. It helps to protect the eye. The non-functional nictitating membrane in the human eye is called plica semilunaris.