Source: Richard Glickman-Simon, MD, Assistant Professor, Department of Public Health and Community Medicine, Tufts University School of Medicine, MA

Proper evaluation of the eyes in a general practice setting involves vision testing, orbit inspection, and ophthalmoscopic examination. Before beginning the exam, it is crucial to be familiar with the anatomy and physiology of the eye. The upper eyelid should be slightly over the iris, but it shouldn't cover the pupil when open; the lower lid lies below the iris. The sclera normally appears white or slightly buff in color. The appearance of conjunctiva, a transparent membrane covering the anterior sclera and the inner eyelids, is a sensitive indicator of ocular disorders, such as infections and inflammation. The tear-producing lacrimal gland lies above and lateral to the eyeball. Tears spread down and across the eye to drain medially into two lacrimal puncta before passing into the lacrimal sac and nasolacrimal duct to the nose.

The iris divides the anterior from the posterior chamber. Muscles of the iris control the size of the pupil, and muscles of the ciliary body behind it control the focal length of the lens. The ciliary body also produces aqueous humor, which largely determines intraocular pressure (Figure 1). Cranial nerves II and III control pupillary reaction and lens accommodation; cranial nerve III controls upper lid elevation; cranial nerves III, IV, and VI control eye movement. The six cardinal directions of gaze are controlled by six extraocular muscles (Figure 2) innervated by cranial nerves III, IV, and VI.

Visual testing is an essential part of the ophthalmological exam and is also performed as a part of cranial nerve II assessment during the neurological exam. A focused image is projected onto the retina after its light passes through the cornea, pupil, lens, and vitreous body. The projection is upside down and reversed right to left, which means that light entering from the lower temporal field of vision strikes the upper nasal quadrant of the retina. Photosensitive cells of the retina respond by generating electrical impulses, which are relayed to the optic nerve and passed to the visual cortex through the optic tracts. The right and left visual cortices process images entering from the left and right visual fields, respectively.

Figure 1. Anatomy of the Eye. A diagram showing a sagittal view of the human eye with the structures labeled.

Figure 2. Muscles of the Eye. A cartoon showing a frontal view of the human eye and the extraocular muscles (labeled).