The eyeball, or globe, is a remarkably engineered structure. Understanding its basic anatomy, from its outermost layers to its innermost tissues, gives you the framework to understand virtually every ocular condition and procedure you will encounter. Globe anatomy is a high-yield topic on the CPO exam.
The Three Coats of the Eye
The globe is organized into three concentric layers, called coats or tunics. Each coat has a distinct structure and function.
1. The Fibrous Coat (Outermost)
The fibrous coat forms the tough outer shell of the globe, providing structural protection and maintaining the eye's shape. It has two distinct regions:
- Sclera: The opaque, white portion covering the posterior five-sixths of the globe. Dense collagen fibers give it its characteristic whiteness and rigidity. The sclera is where the extraocular muscles attach.
- Cornea: The transparent anterior one-sixth of the fibrous coat. Its transparency and precise curvature make it the eye's most powerful refractive surface, contributing approximately 43 of the eye's total 60 diopters of refractive power.
The junction between the cornea and sclera is called the limbus, an important anatomical landmark used in surgical measurements.
2. The Vascular Coat (Middle)
The middle layer is the vascular coat, also called the uvea or uveal tract. It is richly supplied with blood vessels and pigmented cells. The uvea has three components, from anterior to posterior:
- Iris: The colored, ring-shaped structure visible through the cornea. Its muscles control the size of the pupil, regulating the amount of light entering the eye.
- Ciliary body: Located behind the iris, the ciliary body serves two critical functions: it produces aqueous humor (the clear fluid filling the front of the eye) and it controls the shape of the lens through the zonular fibers for accommodation.
- Choroid: A dense network of blood vessels lining the posterior globe between the sclera and the retina. The choroid nourishes the outer retinal layers.
3. The Neural Coat (Innermost)
The innermost layer is the neural coat, which is essentially the retina. The retina is the light-sensitive tissue that converts light into electrical signals and transmits them to the brain via the optic nerve. The retina contains two types of photoreceptors: rods (for low-light and peripheral vision) and cones (for color and fine detail vision).
Anterior and Posterior Segments
The interior of the globe is divided into two main compartments by the lens:
| Segment | Location | Key Structures | Contents |
|---|---|---|---|
| Anterior | In front of the lens | Cornea, iris, ciliary body, lens | Aqueous humor |
| Posterior | Behind the lens | Vitreous, retina, choroid, optic nerve | Vitreous humor |
The anterior segment is further divided into the anterior chamber (between the cornea and iris) and the posterior chamber (between the iris and lens). Aqueous humor flows from the posterior chamber through the pupil into the anterior chamber, then drains through the trabecular meshwork at the drainage angle.
Size and Shape of the Globe
The average adult globe is approximately 24 mm in axial length (front to back). Eyes longer than average tend to be myopic (nearsighted); shorter eyes tend to be hyperopic (farsighted). The globe weighs about 7.5 grams and has a volume of approximately 6.5 mL.
Key Takeaways
- The globe has three coats: the fibrous coat (sclera and cornea), the vascular coat/uvea (iris, ciliary body, choroid), and the neural coat (retina).
- The uvea is responsible for aqueous humor production (ciliary body), pupil control (iris), and retinal nourishment (choroid).
- The globe is divided into anterior and posterior segments by the lens; each is filled with its respective humor (aqueous anteriorly, vitreous posteriorly).
- The limbus is the junction between the cornea and sclera and is an important surgical landmark.
- The average axial length is 24 mm; deviations explain common refractive errors.