Chapter 7 The Sensory Systems 265 the eye within the orbital socket (Figure 7.2). The specific functions of these muscles are listed in the table in Figure 7.3. Internal Structures The eyeball is a hollow chamber, mostly spherical but somewhat oblong in shape. Three layers of tissue form the walls of the eyeball. The tough, fibrous sclera makes up the outer layer of the eye (Figure 7.4). The sclera includes the “white of the eye” as well as the transparent cornea over the anterior center of the eye. The cornea is called the “window of the eye” because light passes through it. The cornea has no blood supply and is therefore the one body tissue that can be transplanted from one person to another with no concern for rejection. The middle layer of the eye, called the choroid (KOR-oyd), contains a rich supply of blood vessels that provide nourishment to the eye. These blood vessels contribute to a crimson-purple pigmentation that darkens the interior of the eye, preventing light reflections. Anteriorly, the choroid also includes the iris, which gives the eye its color. The iris can widen or narrow to control the size of the pupil, the opening through which light passes into the interior of the eye. © Body Scientific International Figure 7.1 Lubricating structures of the eye. A—Anterior view. B—Lateral view. Some of the structures identified in these drawings serve a function other than lubrication. What are these structures and what is their primary purpose? Excretory duct of lacrimal gland Lacrimal gland Skull Conjuctiva Eyelid Eyelashes Tarsal gland Eyelid B Lateral view A Anterior view Lacrimal gland Lateral commissure Medial commissure Excretory ducts of lacrimal gland Ciliary gland Lacrimal canaliculus Nasolacrimal duct © Body Scientific International Figure 7.2 Lateral view of the extrinsic muscles of the eye. Medial rectus muscle Superior oblique muscle Superior rectus muscle Lateral rectus muscle Inferior oblique muscle Inferior rectus muscle The Extrinsic Eye Muscles Muscle Action superior rectus upward eye motion inferior rectus downward eye motion lateral rectus lateral eye motion medial rectus medial eye motion superior oblique downward and lateral eye motion inferior oblique upward and lateral eye motion Figure 7.3 Goodheart-Willcox Publisher Copyright Goodheart-Willcox Co., Inc.