Eyeball is made up of 3 coats or tunic.
1. Sclerotic layer(Fibrous tunica)
Outer most and opaque,fibrous and non-vascular layer easily seen as white of the eye.It is a coat of desne connective tissue made up of collagen fibres and firbroblastic .Sclera covers entire eyeball except cornea, gives shape to eyeball.Sclera in frog is cartilaginous.
In the centre,sclerotic layer it merges with the transparent round window called cornea.
The cornea and exposed part of sclera are covered externally by a thin, transparent membrane the conjunctiva.
2. Choroid layer( Vascular tunica)
Choroid also known as uvea middle.It is vascular layer which supplies nutrients to the eye.It is distinguished into part choroid,cilliary body and iris.
It is highly vascular posterior portion of vascular tunica.The choroid occurs in the main part of eyeball adhered to the sclerotic. (The pigment is reddish in rabbit and black,brown or bluish in man).
b. Ciliary body
Ciliary body is vascular and pigmented like choroid,made up of ciliary processes and ciliary muscles(only circular type).The ciliary body is hidden by iris. The ciliary body helps in accommodation by altering the focus of eye from object or the shape of lens near or far vision.
Beyond the ciliary body, the vascular tunic sharply turns inwards,forming a circular ,shelf-like diaphragm called iris.The colour of the iris is responsible for colour of eye eg.brown,black,blue or green.In albinos,iris is deficient of pigment.