Globes

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The ocular globes or simply, the eyes are paired spherical sensory organs, located anteriorly on the face within the orbits, which house the visual apparatus.

Gross anatomy

Location

The globe is suspended by the bulbar sheath in the anterior third of the bony orbit.

Size

Each globe is an approximately spherical structure and 25 mm in diameter .

Structure

The surface of the globe is made up of three layers, the outermost fibrous layer, the intermediate vascular layer, and the innermost neural layer. This trilaminar structure brings about a further subdivision of the eye into two major segments; the anterior segment, which contains aqueous humor, and the posterior segment, which contains the gel-like vitreous humor. Both of these substances allow diffusion of nutrients to adjacent structures and act to maintain the shape of the globe.

The fundus of the globe refers to the retina, optic disc, macula and fovea collectively, and hence funduscopy/fundoscopy (alternative spellings: both acceptable).

Fibrous layer

The fibrous layer of the globe is made up of the opaque sclera and the transparent cornea. The sclera makes up the posterior 5/6 of the fibrous layer and provides structure and protection to the globe, as well as serving as a point of insertion to the extraocular muscles. The cornea makes up the protruding anterior 1/6 of the fibrous layer. The cornea owes its transparent clarity to both the structural arrangement of the collagen and its constant, relatively dehydrated state . The point where the sclera and cornea are continuous is the sclerocorneal junction or the limbus.

Vascular layer

The vascular layer of the globe is made up of the uvea, which is comprised from anterior to posterior of the iris, ciliary body and choroid:

  • the iris is a pigmented muscular structure located deep to the cornea, with radially arranged dilator pupillae muscles, and concentrically arranged sphincter pupillae muscles; these muscles dilate and constrict the pupils in response to sympathetic (from long ciliary nerve), and parasympathetic nerve impulses (from CN III), respectively
  • the ciliary body is a muscular structure, with a globular medial aspect from which the zonule fibers originate, to tether to the lens; posterolaterally, the ciliary body is flattened and extends to the ora serrata , where it is continuous with the choroid, about halfway to the equator of the globe
  • the choroid is the darkly pigmented vascular portion of the uvea, continuing posteriorly along the wall of the globe, to give blood supply to the neural layer of the globe
Neural layer

The neural layer of the globe is made up of the retina, which extends over the insertion of the optic nerve, around the walls of the globe, deep to the choroid; to the ora serrata, where it fades. The retina is a multilayered structure:

  • the deepest layer is the melanin-rich retinal pigment epithelium (RPE), (from which it can be detached); the RPE continues to line the inner aspects of the uveal tract
  • deep to this are layers of neuronal structures, including the pigment cells, rods and cones, bipolar cells, horizontal cells, and ganglion cells, whose long axons extend from the retina, through the optic nerve, and ultimately to synapse at the lateral geniculate body of the thalamus

The retina has a number of important areas:

  • the optic disc corresponds to the entry point of the optic nerve, and the physiological blindspot
  • the macula lutea is the yellow colored area located temporally to the disc
  • and the fovea centralis is the small avascular region located in the central depression of the macula that is involved in sharp color vision
Segments and chambers
  • the anterior segment of the eye is bounded by the cornea anteriorly and the lens posteriorly; the anterior segment is further divided into two chambers that communicate via the pupil
    • the anterior chamber, which lies between the cornea and the iris; the point at which they meet containing the drainage apparatus of the anterior segment, the canal of Schlemm
    • and the posterior chamber, which lies between the iris and the lens; both of these areas are filled with aqueous humor
  • the posterior segment is bounded anteriorly and posteriorly by the hyaloid membranes, and is filled with gel-like vitreous humor, which is involved in maintaining the position of the retina and the shape of the globe
    • a fluid-filled, fibrous structure known as the hyaloid canal projects from the posterior aspect of the lens to the center of the optic disc

Relations

Arterial supply

Arterial supply to the globe is provided by multiple arteries, all of which are branches of the ophthalmic artery (from the internal carotid artery):

  • central retinal artery is the major vessel of the retina
  • long posterior ciliary arteries supply the iris, ciliary body and anterior aspect of the choroid
  • short posterior ciliary arteries supply the posterior aspect of the choroid

Venous drainage

Venous drainage of the globe is via the vorticose veins (a.k.a. vortex veins) of the choroid and the central retinal vein, which drain into the superior ophthalmic vein and inferior ophthalmic vein, and ultimately the cavernous sinus.

Medial veins of the face drain via facial-cavernous anastomoses that communicate via the ophthalmic veins.

Innervation

The globe receives its sensory innervation from the long ciliary nerves and short ciliary nerves (branches of the nasociliary nerve from the ophthalmic division of the trigeminal nerve). The visual special sensory function of the eye is supplied by the optic nerve.

Eye movements

The six extraocular muscles control eye movement from their common origin at the annulus of Zinn.

Variant anatomy

Variations in the size of the globe, as well aberrant connections between the extraocular muscles have been described ; variants such as these may bring about congenital refractive errors such as myopia and hyperopia, or may be incidental findings on imaging:

Radiographic features

Multiple features can be demonstrated in the globe, through a variety of modalities:

Ultrasound
  • the cornea and iris appear as thin echogenic lines; with the anechoic aqueous humor intervening posteriorly on the cornea, and on either side of the iris
  • the lens is defined by biconvex echogenic boundaries, with an anechoic interior area
  • the vitreous humor is anechoic, with increasing frequency of linear and spot echoes, as a result of the aging process
  • although the retina, choroid and sclera do not produce echoes, together these structures appear as a posteriorly concave surface, outside of which, an echogenic area extending from the cornea to the location of the optic disc
  • the optic nerve appears hypoechoic in contrast to the surrounding echogenic fat
CT
  • lens and ciliary bodies appear dense in comparison to the fluids of the anterior and posterior segment
  • aqueous and vitreous humor appear isodense despite marked differences in their viscosity
MRI
  • the cornea and sclera appear hypointense on both T1W and T2W images, however, the cornea may be accentuated by a hyperintense tear film in T1W
  • the lens appears hypointense on both T1W and T2W images .
  • the uveal tract appears hyperintense on T1W, and hypointense on T2W
    • the retina is not easily distinguished from the choroid in the absence of pathological detachments or contrast enhancement
    • because the choroid and ciliary body enhance strongly with IV gadolinium
  • both the aqueous and vitreous humor are hypointense on T1W and hyperintense on T2W

Development

The globes develop during the third week of embryonic life and are completed by week ten. Both the vascular and fibrous layers of the eye are derived from the mesoderm, with the exception of the iris. The iris, along with the entire neural layer of the globe, originates from the neural ectoderm, whereas the lens develops from surface ectoderm of the anterior cranium .

Hyaloid canal and artery

During embryological development, the hyaloid canal is the pathway of the hyaloid artery which supplied the lens and retina, and involutes prior to birth . If there is a persistent hyaloid artery, however, this can lead to visual impairment and hemorrhages in the posterior compartment.

Related pathology

Sclera/cornea
Iris/pupil
Ciliary body/lens
Uvea
Choroid
Vitreous humor
Retina
Optic nerve
Vascular