Uveal Tract

The middle coat of the eye is called the uvea (from the
Latin for “grape”) because the eye looks like a reddish-blue
grape when the outer coat has been dissected away. The
posterior part of the uvea, the choroid, is essentially a
layer of blood vessels and connective tissue sandwiched
between the sclera and the retina. The forward portion of
the uvea, the ciliary body and iris, is more complex, containing
as it does the ciliary muscle and the sphincter and
dilator of the pupil.
The blood supply responsible for nourishing the retina
consists of the retinal and uveal circulations, both of which
derive from branches of the ophthalmic artery. The two
systems of blood vessels differ in that the retinal vessels,
which supply nutrition to the innermost layers of the retina,
derive from a branch of the ophthalmic artery, called
the central artery of the retina, that enters the eye
with the optic nerve, while the uveal circulation, which
supplies the middle and outer layers of the retina as well as
the uvea, is derived from branches of the ophthalmic artery
that penetrate the globe independently of the optic nerve.
The ciliary body is the forward continuation of the
choroid. It is a muscular ring, triangular in horizontal section,
beginning at the region called the ora serrata and
ending, in front, as the root of the iris. The surface is
thrown into folds, called ciliary processes, the whole being
covered by the ciliary epithelium, which is a double layer
of cells; the layer next to the vitreous body, called the inner
layer, is transparent, while the outer layer, which is continuous
with the pigment epithelium of the retina, is
heavily pigmented. These two layers are to be regarded
embryologically as the forward continuation of the retina,
which terminates at the ora serrata. Their function is to
secrete the aqueous humour.
The ciliary muscle is an unstriped, involuntary, muscle
concerned with alterations in the adjustments of focus—
accommodation—of the optical system; the fibres run
both across the muscle ring and circularly, and the effect
of their contraction is to cause the whole body to move
forward and to become fatter, so that the suspensory ligament
that holds the lens in place is loosened.