Sympathetic fibers to the eyeball may not enter the ganglion as a separate sympathetic root. Rather, the postganglionic sympathetic fibers may leave the plexus associated with the internal carotid artery in the cavernous sinus, join the ophthalmic nerve [V1], and course into the ciliary ganglion in the sensory root from the nasociliary nerve. In addition, the sympathetic fibers carried in the nasociliary nerve may not enter the ganglion at all and may course directly into the eyeball in the long ciliary nerves (Fig. 8.107). Whatever their path, postganglionic sympathetic fibers reach the eyeball and innervate the dilator pupillae muscle.
The globe-shaped eyeball occupies the anterior part of the orbit. Its rounded shape is disrupted anteriorly, where it bulges outward. This outward projection represents about one-sixth of the total area of the eyeball and is the transparent cornea (Fig. 8.108).
Posterior to the cornea and in order from front to back are the anterior chamber, the iris and pupil, the posterior chamber, the lens, the postremal (vitreous) chamber, and the retina.
The anterior chamber is the area directly posterior to the cornea and anterior to the colored part of the eye (iris). The central opening in the iris is the pupil. Posterior to the iris and anterior to the lens is the smaller posterior chamber.
The anterior and posterior chambers are continuous with each other through the pupillary opening. They are filled with a fluid (aqueous humor), which is secreted into the posterior chamber, flows into the anterior chamber through the pupil, and is absorbed into the scleral venous sinus (the canal of Schlemm), which is a circular venous channel at the junction between the cornea and the iris (Fig. 8.108).
The aqueous humor supplies nutrients to the avascular cornea and lens and maintains the intra-ocular pressure. If the normal cycle of its production and absorption is disturbed so that the amount of fluid increases, intra-ocular pressure will increase. This condition (glaucoma) can lead to a variety of visual problems.
The lens separates the anterior one-fifth of the eyeball from the posterior four-fifths (Fig. 8.108). It is a transparent, biconvex elastic disc attached circumferentially to muscles associated with the outer wall of the eyeball. This lateral attachment provides the lens with the ability to change its refractive ability to maintain visual acuity. The clinical term for opacity of the lens is a cataract.
The posterior four-fifths of the eyeball, from the lens to the retina, is occupied by the postremal (vitreous) chamber (Fig. 8.108). This segment is filled with a transparent, gelatinous substance—the vitreous body (vitreous humor). This substance, unlike aqueous humor, cannot be replaced.
Walls of the eyeball
Surrounding the internal components of the eyeball are the walls of the eyeball. They consist of three layers: an outer fibrous layer, a middle vascular layer, and an inner retinal layer (Fig. 8.108).
The outer fibrous layer consists of the sclera posteriorly and the cornea anteriorly.
The middle vascular layer consists of the choroid posteriorly and is continuous with the ciliary body and iris anteriorly.
The inner layer consists of the optic part of the retina posteriorly and the nonvisual retina that covers the internal surface of the ciliary body and iris anteriorly.
The arterial supply to the eyeball is from several sources:
The short posterior ciliary arteries are branches from the ophthalmic artery that pierce the sclera around the optic nerve and enter the choroid layer (Fig. 8.108).
The long posterior ciliary arteries, usually two, enter the sclera on the medial and lateral sides of the optic nerve and proceed anteriorly in the choroid layer to anastomose with the anterior ciliary arteries.