The middle layer

The uveal tract (iris, ciliary body and choriod)

Iris

The iris is the part of the eye which gives it its colour. Its main role is in controlling the diameter of the pupil, which corresponds to the amount of light which can enter the eye. There are two types of muscles attached to the iris, the circular sphincter pupillae and the radial dilator pupillae.

• During bright light the sphincter pupillae contract, decreasing the size of the pupil and restricting the amount of light which enters the eye.

• During dim light, contraction of the dilator pupillae causes the pupils to widen. This increases the overall amount of light reaching the retina.

The iris is located inbetween the anterior and posterior chambers. The anterior chamber contains a water based liquid called the aqueous humour. 

Its function is to provide nutrients (e.g amino acids and glucose) to the lens and cornea, as well as to remove waste products. It also helps in maintaining the intraocular pressure within the eye, which contributes to creating the convex shape of the cornea. 

The posterior chamber is not to be confused with the vitreous humour. This is a space which lies directly posterior to the iris but infront of the lens (see structure of the eye ball)

 The vitreous humour is the clear gel made by particular retinal cells, which fills the space between the lens and the retina of the eyeball. 

It has a similar composition to the cornea but contains few cells, no blood vessels and a higher percentage of water. Although it contains little solid matter, it tautly holds the eye. This is because it has a viscosity two to four times that of water, giving it a gelatinous consistency

Unlike the fluid of the anterior chamber, the vitreous humour isn’t replenished, it’s stagnant. Therefore, if a foreign substance, it can only be removed by surgery.

Ciliary body

The ciliary body has two components, the first are the ciliary muscles which affect the zonular fibres attached to the lens. These have important roles in the eyes accommodation by altering the thickness of the lens.  Second are the ciliary processes, which are folds of choroid which produce the aqueous humour.

Choroid

Lying between the retina and the sclera is a membrane called the choroid. In humans it is about 0.5 mm thick and it contains a high number of blood vessels in order to supply oxygen and nutrients to the outer layers of the retina.

It is usefull in reducing the amount of reflection encountered in the eye. This is because it contains the pigment melanin which helps to reduce scattering of light particles by absorbing them.

Choroid image is courtesy of flickr under the creative commons license.

Lens

The lens is a biconvex, transparent feature of the eye which acts to refract light onto the retina. Due to being attached to the ciliary muscles by the zonular fibres, it has the ability to change it's shape in accordance to the distances of different objects. For example, at short distances, the ciliary muscles contract which lengthens the zonular fibres. 

Consequently, the lens also relaxes and becomes thicker which gives the higher refractive power needed to converge the light on to the retina.  When focusing on objects far away however, the lens is instead stretched and flattened by the ciliary muscles, which is necessary for long-range vision. This process of adjusting the lens is called accommodation.