The Three Coats of the Eye
The globe (eyeball) is composed of three concentric layers, each with distinct functions:
Outer Layer: The Fibrous Coat
The fibrous coat provides structural support and protection. It consists of:
- Cornea: The transparent anterior one-sixth of the eye. It is the primary refracting surface, responsible for approximately two-thirds of the eye's focusing power.
- Sclera: The opaque, white posterior five-sixths. It provides a tough, protective shell and serves as the attachment site for the extraocular muscles.
- The junction between the cornea and sclera is called the limbus.
Middle Layer: The Vascular Coat (Uvea)
The uvea is the vascular, pigmented middle layer containing three structures from posterior to anterior:
- Choroid: A highly vascular layer that supplies blood to the outer retina. It lies between the sclera and retina.
- Ciliary body: Produces aqueous humor and contains the ciliary muscle, which controls accommodation (lens focusing). The zonular fibers extend from the ciliary body to hold the lens in position.
- Iris: The colored, muscular diaphragm that controls pupil size. The sphincter pupillae constricts the pupil; the dilator pupillae dilates it.
Inner Layer: The Neural Coat (Retina)
The retina is the light-sensitive neural tissue lining the inner surface of the posterior globe. It contains photoreceptor cells (rods and cones) that convert light into electrical signals transmitted to the brain via the optic nerve.
Segments and Chambers
The eye is divided into two major segments:
- Anterior segment: Everything from the cornea to the posterior surface of the lens. Contains the anterior and posterior chambers filled with aqueous humor.
- Posterior segment: Everything behind the lens, including the vitreous cavity, retina, choroid, and optic nerve.
Within the anterior segment:
- Anterior chamber: Between the cornea and the iris/pupil
- Posterior chamber: Between the iris/pupil and the anterior lens surface (not to be confused with the posterior segment)
Do not confuse the "posterior chamber" with the "posterior segment." The posterior chamber is a small space behind the iris in the anterior segment. The posterior segment is the large area behind the lens containing the vitreous and retina. This is a common exam question.
Ocular Fluids
- Aqueous humor: A clear, watery fluid produced by the ciliary body that fills the anterior and posterior chambers. It nourishes the avascular lens and cornea, maintains IOP, and drains through the trabecular meshwork into Schlemm's canal.
- Vitreous humor: A clear, gel-like substance filling the vitreous cavity (the large space behind the lens). It helps maintain the shape of the globe and holds the retina in place against the choroid.
Confusing aqueous and vitreous humor. Aqueous is a continuously produced and drained watery fluid in the anterior segment. Vitreous is a gel-like substance in the posterior segment that is essentially static (not actively produced or drained). Their dysfunction leads to different pathologies: aqueous drainage problems cause glaucoma; vitreous degeneration causes floaters and potential retinal detachment.
Key Takeaways
- The eye has three layers: fibrous (cornea/sclera), vascular/uveal (choroid/ciliary body/iris), and neural (retina)
- The anterior segment is divided into anterior and posterior chambers, both filled with aqueous humor
- The posterior segment contains the vitreous cavity, retina, choroid, and optic nerve
- Aqueous humor is continuously produced by the ciliary body and drains through the trabecular meshwork
- Impaired aqueous drainage leads to elevated IOP, the primary risk factor for glaucoma