(2) Vascular tunic. This is the middle layer of the eye and is made up of the
choroid, the ciliary body, and the iris. Altogether, these structures are called the uvea.
(a) Choroid. The choroid is the back part of the vascular tunic. It is a
thin, dark brown membrane which lines most of the internal surface of the sclera. This
membrane has many blood vessels and a large amount of pigment. The layer absorbs
light rays so they are not reflected back out of the eyeball. The blood supply in this
layer nourishes the retina. The choroid is pierced by the optic nerve at the back of the
(b) Ciliary body. The front part of the vascular tunic, the ciliary body, is
the thickest part of the vascular tunic. Ciliary processes and ciliary muscle make up this
part of the layer. The ciliary processes are made up of folds of the internal surface of
the ciliary body; these folds secrete aqueous humor (a clear, watery fluid in the front
and back chambers of the eye). The ciliary muscle is a smooth muscle that changes
the shape of the lens in order to see objects that are close or far away.
(c) Iris. The third part of the vascular tunic is the iris, a structure
composed of circular and radial smooth muscle fibers arranged in a doughnut shape.
The pupil is the black hole; light enters the eyeball through this hole.
(3) Retina (nervous tunic). The retina is the third and inner layer of the eye.
It covers the choroid and consists of an inner nervous tissue layer and an outer
pigmented layer. The pigmented part of the layer extends over the back of the ciliary
body and the iris and is the nonvisual part of the retina. This layer also contains three
zones of neurons which conduct impulses. These zones of neurons are called the
photoreceptor neurons, the bipolar neurons, and the ganglion neurons. Photoreceptor
neurons contain rods and cones, so named because of their shape. Rods, specialists in
dim-light vision, allow us to distinguish between different shades of dark and light and
let us see shapes and movement. Cones permit us to see color and are specialists in
daylight vision. The optic disc is located in this area and is a blind spot with neither rods
nor cones. It is here that the optic nerve leaves the eyeball.
c. Blood Vessels. The structures of the eye include blood vessels, layer
vessels in the sclera, and capillaries in the retina.
d. Cavities and Humor. The anterior cavity of the eye is anterior to the lens.
The anterior chamber is anterior to the iris and posterior to the cornea. The posterior
chamber is posterior to the iris and anterior to the lens. The anterior cavity contains the
aqueous humor secreted by the ciliary body. Too much aqueous humor results in
glaucoma. The posterior cavity of the eye is posterior to the lens and suspensory
ligaments and anterior to the retina. This cavity contains gelatinous substances called
vitreous humor which maintain the globular shape of the eyeball.