VASCULAR TUNIC ANATOMYThe vascular tunic of the eyeball is also referred to as the uvea. It is comprised of the iris, the choroid, and the ciliary body. The choroid, which is highly vascular, nearly immeasurably thin, and lines nearly the entire surface of the sclera, is designed to prevent the light rays from bouncing right back out of the eyeball. It achieves this through the numerous pigment producing melanocytes, which explains the brown coloring of the choroid. The choroid opens up at the rear of the eyeball, permitting the passage of the optic nerve.
VASCULAR TUNIC STRUCTUREThe anterior segment of the choroid thickens to create an internal muscular ring toward the front of the eyeball. This muscular ring is known as the ciliary body. The ciliary muscles are naturally found inside the ciliary body, and are created by smooth muscle fiber bands.
There is a collection of various extensions arising from the ciliary body, which is known as ciliary processes. These attach to zonular fibers and are ultimately attached to the lens capsule through these fibers. Suspensor ligaments are actually made up of these zonular fibers.
Tight layers of protein fibers arranged in a continuing shell (like an onion) creates the transparent lens. This lens is then encapsulated by a thin but clear lens capsule, which makes a handy attachment point for the suspenosry ligament.
VASCULAR TUNIC DIAGRAM
Image: Vascular Tunic
The lens shape is the determining factor at which angle the incoming rays of light are refracted. The lens becomes flatter when the ciliary muscle relax, which causes a greater tension on the suspensory ligament. When the ciliary muscles contract, the ligament then relaxes and the lens becomes a bit more spherical.
When the suspensory ligament is more relaxed, there is increased tension on the lens capsule. This means that the lens is more convex. When the eye focuses in on something close, the lens becomes more spherical, allowing for focus. Alternatively, when they eyeball is focusing on objects that are far away the lens flattens for better focus.
VASCULAR TUNIC FUNCTIONSThe choroid is also continuous with the anterior segment of the vascular tunic, which is known as the iris of the eye. From an external view of the eye, this is the colored portion. Smooth muscles which lie in a continuous, radial, but circular pattern make up the iris. The pupil’s diameter is controlled by autonomic contraction of these smooth muscle fibers. The pupil is in effect, an opening in the center of the iris, not a growing and shrinking dot on top of the iris. The iris contains a pupillary constrictor muscle, which is stimulated to either contract or relax through light stimulation. A constricted muscle decreases the light permitted into the eye. The iris also contains a pupillary dilator muscle, which contracts when the eye requires additional light, and the pupil becomes larger.