The contents of eyeball include aqueous humor, lens, vitreous body and iris corneal angle.
The outermost layer of eyeball is fibrous layer, also called corneal sclera layer, including sclera and cornea.
Cornea can be divided into four layers, which are epithelial cell layer, matrix layer, posterior elastic layer and endothelial cell layer in turn from outside to inside.
1, epithelial cell layer: composed of squamous epithelium without keratinization.
2. Matrix layer: about 90% of the corneal thickness, mostly collagen fibers, arranged in a plate structure.
3. Posterior elastic layer: the basement membrane of endothelial cells, which will not regenerate after injury. When the cornea is injured and the posterior elastic layer jumps out, the cornea must be repaired by surgery to prevent perforation.
4. Endodermis: a single layer of flat cells, which can control the sodium and potassium pump to adjust the water content of cornea and maintain the transmittance of cornea. If corneal endothelium is damaged, water will enter corneal stroma, causing corneal edema and reducing corneal transmittance.
The cornea remains transparent and clear for several reasons:
1. Sodium and potassium pumps in corneal endothelium make corneal stroma in a relatively dehydrated state.
2. Corneal epithelium and inner layer cells are closely arranged and hydrophobic, which can prevent water from entering corneal stroma and causing edema.
3. Collagen fibers are regularly arranged in corneal stroma.
There are no pigments and blood vessels in the cornea.
The sclera is the outer membrane of the eyeball, commonly known as the white part of the eye, which is mainly composed of fibrous connective tissue, accounting for about 5/4 of the whole eyeball, and is subdivided into three layers: the sclera surface layer, the sclera matrix layer and the sclera inner layer.
The iris is a colored part visible to the naked eye (as shown in the figure below). Brown iris contains more pigment, while blue iris contains more pigment. Iris opening is the pupil, and iris smooth muscle is responsible for adjusting the size of the pupil, thus producing pupil light reflection.