Topographic Features of the Globe
The eyeball, or globe, is not a true sphere. The radius of curvature of the prolate (polar radius greater than equatorial radius, or “pointy”) cornea is 8 mm, smaller than that of the sclera, which is 12 mm. This makes the globe an oblate “squashed” spheroid (equatorial radius greater than polar radius). The anteroposterior diameter of the adult eye is approximately 23–25 mm. The average transverse diameter of the adult eye is 24 mm (Fig 2-1).
The eye contains 3 compartments: the anterior chamber, the posterior chamber, and the vitreous cavity. The anterior chamber, the space between the iris and the cornea, is filled with aqueous fluid. Anterior chamber depth varies among individuals and in regional populations; the average depth is 3.11 mm. The average volume of the anterior chamber is 220 μL. The posterior chamber is the anatomical portion of the eye posterior to the iris and anterior to the lens and vitreous face. It is also filled with aqueous fluid and has an average volume of 60 μL. The largest compartment is the vitreous cavity, which makes up more than two-thirds of the volume of the eye (5–6 mL) and contains the vitreous gel (also called vitreous, vitreous body, or vitreous humor). The total volume of the average adult eye is approximately 6.5–7.0 mL (Table 2-1).
The eyeball is composed of 3 concentric layers: an outer protective layer, a middle vascular layer, and an inner neural layer. The outermost layer consists of the clear cornea anteriorly and the opaque white sclera posteriorly. This corneoscleral layer is composed of collagen and protects the internal ocular tissues.
Figure 2-1 Sagittal section of the eye with absent vitreous and major structures identified. Dimensions are approximate and are average for the normal adult eye.
(Illustration by Christine Gralapp.)
Table 2-1 Dimensions and Contents of the Adult Eye
The cornea occupies the center of the anterior pole of the globe. Because the sclera and conjunctiva overlap the cornea anteriorly, slightly more above and below than medially and laterally, the cornea appears elliptical when viewed from the front. The limbus, which borders the cornea and the sclera, is blue-gray and translucent.
The middle layer of the globe, the uvea, consists of the choroid, ciliary body, and iris. Highly vascular, it serves nutritive and supportive functions, supplying oxygen to the outer retina and producing aqueous humor.
The innermost layer is the retina. This photosensitive layer contains the photoreceptors and neural elements that initiate the processing of visual information.
Other important surface features of the globe, such as the vortex veins, the posterior ciliary artery and nerves, and extraocular muscle insertions are discussed in Chapter 1; the optic nerve and its surrounding meningeal sheaths are discussed in Chapter 3.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.