Sence organs: Eye I

Histology 2016-2017
Department of Anatomy &Histology: Dr.Rajaa Ali
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Sence organs: Eye I
INTRODUCTION
Information about the external environment is conveyed to the central nervous system through sense organs (general and special). General sense organs concerned with pain, temperature, touch and pressure are situated in the skin .
Special sense organs, e.g. olfactory mucosa (smell) and taste buds (taste) have already been described under respiratory and digestive systems respectively.

GENERAL FEATURES
o The eyeball is the peripheral organ of vision.
o It is spherical in shape and is lodged in a bony cavity, the orbit.
o Certain accessory structures are intimately associated with the eyeball, namely, the extraocular muscles to move the eyeball, the eyelids to protect it in front and the lacrimal gland to lubricate the cornea.

STRUCTURE
Coats of the Eyeball (Box 18.1)
o The wall of the eyeball has three concentric coats which enclose three compartments, namely, anterior, posterior and vitreous chambers (Fig.).
o These chambers contain fl uid media of eye. The three coats are:


1. Outer fi brous coat/Sclerocorneal layer
2. Middle vascular coat/Uveal tract
3. Inner photosensitive coat/Retina
Outer Fibrous Coat/Sclerocorneal Layer
o This coat consists of the opaque posterior four-fi fth, the sclera and the anterior transparent one-fi fth, the cornea.
o The sclera and cornea become continuous with each other at the limbus or the sclerocorneal junction.

Sclera
o It is composed mostly of dense collagen fi bres and few elastic fi bres.
o Sclera is separated externally from Tenon’s capsule (fascial sheath of eyeball) by episcleral layer of loose connective tissue (Tenon’s space) and internally from the choroid by lamina fusca (suprachoroidal lamina) of dark pigmented fi broelastic tissue.
o Posteriorly it is pierced by axons of ganglionic cells of retina at lamina cribrosa to form optic nerve.