The eye or the eyeball - is a wonderful and extremely complex work of nature. This spherical body is 24 mm in diameter, and weighing 7-8 g. Eye is l

Outer layer of the eye (sclera, cornea, lacrimal glands)

The eye or the eyeball — is a wonderful and extremely complex work of nature. This spherical body is 24 mm in diameter, and weighing 7-8 g. Eye is located in the orbit of the facial skull card (orbit), where he is attached with four straight and two oblique muscles. Their contraction allows movement of the eyes in different directions. Normally, both eyes move synchronously. The walls of the eyeball consists of three layers: the outer — connective, middle — vascular and internal — the retina.

The outer layer of the eye is like its external skeleton, providing a certain form. It consists of two parts: the sclera (from Greek “hard”, “dense”) and the cornea (167 Diagram).

Sclera

The sclera, or protein layer is formed of an opaque strong fibrous tissue, that has intertwined collagen and elastic fibers. Between the fibers are contained connective tissue and pigment cells.

Cornea

Cornea is a transparent shell with diameter of 12 mm, which is placed in front of the sclera. It is the first (second is the crystalline) lens with a very strong light refracting ability. The cornea is formed by the same type of epithelial cells that contain a lot of water, making them transparent. The cornea has no blood vessels, and therefore it is fed through the interstitial fluid.

The auxiliary protective apparatus of the eye — brows, lashes and eyelids and lacrimal gland protect the cornea from damage. Due to the eyebrows sweat misses the eye. The outer part of lashes is covered with delicate skin, and the interior — a thin mucosa — the conjunctiva (“connection” from the Latin), which gradually goes from the lashes to surface of the eye.

Diagram 167. Structure of the eye: 1 — retina; 2 — vitreous; 3 — the visual axis; 4 — the optical axis; 5 — the central fossa; 6 — yellow spot; 7 — the optic nerve; 8 — the sclera; 9 — choroid; 10 — ciliary body; 11 — rear camera; 12 — front camera; 13 — lens; 14 — cornea; 15 — iris; 16 — ciliary muscle; 17 — conjunctiva
Diagram 168. Eye (front view): 1 — the upper eyelid; 2 — nasolacrimal channels holes; 3 — nasolacrimal duct; 4 — lacrimal caruncle; 5 — iris; 6 — lower eyelid; 7 — the pupil; 8 — the sclera; 9 — eyelashes; 10 — lacrimal gland

About 5-7 times a minute there is a short-term tremor of upper and lower eyelids — a protective reflex — blinking. Taken from http://worldofschool.org

The cornea contains many nerve endings (mechanoreceptors), so touching it or rapid approaching of the object to the eyes causes them to close, protecting the cornea from damage. This is a manifestation of the protective nictitans reflex. Cornea regenerates well: after the surgery it may not be sewed, as heals quickly.

Lacrimal glands

Liquid that is produced by lacrimal glands protects cornea and sclera from contamination or drying (diagram 168). Lacrimal glands are located in the outer part of upper-side orbit. A small amount of this fluid gets into the nasal cavity via nasolacrimal channels (diagram 168). When a person cries, the amount of tear fluid is increased to 20-30 ml (normally secretes 1 ml per day), and it flows out from the eyes and nose as the tears.

Categories:
Questions:
  • Explain features of eye protection.

  • Explain the eyeball movement mechanism.

  • Determine the relationship of the cornea structure and functions.