Big Picture

The primary organ responsible for vision is the eye. The eyeball is located within a bony orbital encasement, which protects it. The lacrimal apparatus keeps the eye moist and free of dust and other irritating particles through the production and drainage of tears. Eyelids protect the eye from external stimuli such as dust, wind, and excessive light.

Bony Orbit

The bony orbit is the region of the skull that surrounds the eye and is composed of the following structures (Figure 18-1A):

• Superior wall. Formed by the frontal bone and the lesser wing of the sphenoid bone.
  
  
  
  • Supraorbital foramen. Transmits the supraorbital nerve [cranial nerve (CN) V-1] and vessels to the scalp.
  
  
• Lateral wall. Formed by the zygomatic bone and the greater wing of the sphenoid bone.
  
• Inferior wall. Formed by the maxillary, zygomatic, and palatine bones.
  
  
  
  • Infraorbital foramen. Transmits the infraorbital nerve (CN V-2) and vessels to the maxillary region of the face.
  
  
• Medial wall. Formed by the ethmoid, frontal, lacrimal, and sphenoid bones.
  
  
  
  • Anterior and posterior ethmoidal foramina. Transmits the anterior and posterior ethmoidal nerves and vessels, to the nasal cavity and the sphenoid and ethmoid sinuses.
    
  • Nasolacrimal canal. Formed by the maxillary, lacrimal, and inferior nasal concha bones. Drains tears from the eye to the inferior meatus in the nasal cavity.
  
  
• Optic canal. Transmits the optic nerve (CN II) and the ophthalmic artery.
  
• Superior orbital fissure. An opening between the greater and lesser wings of the sphenoid bone that transmits the oculomotor, trochlear, ophthalmic, and abducens nerves (CNN III, IV, V-1, and VI, respectively), and the ophthalmic veins.
  
• Inferior orbital fissure. Communicates with the infratemporal and pterygopalatine fossae. The inferior orbital fissure transmits CN V-2 and the infraorbital artery and vein.

Lacrimal Apparatus

The lacrimal gland lies in the superolateral corner of the orbit (Figure 18-1B).

• Tears. The lacrimal gland secretes tears that spread evenly over the eyeball through blinking and cleanse the eye of dust and foreign particles.
  
• Drainage. Tears drain from the eyeball, via the nasolacrimal duct, into the inferior nasal meatus of the nasal cavity.
  
• Innervation. The lacrimal gland is innervated by visceral motor parasympathetic neurons from CN VII (Figure 18-4C).

Because the mucosa of the nasal cavity is continuous with the mucosa of the nasolacrimal duct system, a cold or “stuffy” nose often causes the lacrimal mucosa to become inflamed and swollen. Swelling constricts the ducts and prevents tears from draining from the eye surface, causing “watery” eyes.

Eyelids

Eyelids protect the eye from foreign particles and from bright light (Figure 18-1C). The external surface of the eyelids is covered by skin, whereas the conjunctiva covers the internal surface.

Conjunctiva

The conjunctiva is a mucous membrane that lines the internal surface of the eyelids. The conjunctiva is reflected at the superior and inferior fornices onto the anterior surface of the eyeball and forms the conjunctival sac when the eyes are closed.

Eyelid Muscles

Tears produced by the lacrimal gland are secreted continually and are spread over the conjunctiva and cornea by movement of the eyelids (blinking). The following muscles are involved in movement of the eyelids:

• Orbicular oculi muscle. Considered a muscle of facial expression. Its circular fibers attach to the anterior surface of the bony orbit as well as in the eyelid. When the facial nerve (CN VII) activates the orbicular oculi muscle, the eyes close.

The corneal, or blink, reflex is elicited by stimulation of the cornea. The nasociliary nerve (CN V-1) mediates the sensory portion of this reflex, with CN VII initiating the motor response by innervating the orbicularis oculi muscle.

• Levator palpebrae superioris muscle. The levator palpebrae superioris muscle elevates the upper eyelid and is innervated by CN III.
  
• Superior tarsal (Müller’s) muscle. Composed of smooth muscle and attaches to the inferior surface of the levator palpebrae superioris muscle and the upper eyelid. These two muscles work synergistically to keep the eyelid elevated. The superior tarsal muscle is innervated by postganglionic sympathetic neurons that originate in the superior cervical ganglion and are carried to the orbit by the internal carotid and ophthalmic arteries.

Horner’s syndrome is caused by damage to the sympathetic innervation to the head (Figure 18-1D). Signs of Horner’s syndrome found on the ipsilateral side of the injury include the following:

• Ptosis (drooping upper eyelid). Results from the loss of sympathetic innervation to the superior tarsal muscle.
  
• Anhidrosis (decreased sweating). Results from the loss of sympathetic innervation to sweat glands.
  
• Miosis (constricted pupil). Results from the loss of sympathetic innervation to the dilator pupillary muscle.

Big Picture

The eyeball consists of the sclera, choroid, and retina.

Sclera

The sclera is the white, fibrous covering of the eye into which muscles insert (Figure 18-2A).

• Cornea. The sclera is continuous anteriorly as the cornea, which forms a bulging, transparent region specialized for refracting light as it enters the eye.

Choroid

The choroid is the vascular, middle layer of the eye where blood vessels course (Figure 18-2A). The choroid layer is also composed of the following structures:

Ciliary Apparatus

The anterior region of the choroid is thickened to form the ciliary muscle, which is composed of smooth muscle and is innervated by parasympathetic neurons from CN III (Figure 18-2B).

• Ciliary muscle. This circular muscle surrounds the periphery of the lens. Contraction causes the diameter of the muscle to decrease, whereas relaxation causes the diameter of the muscle to increase. Understanding this concept is essential to understanding how the lens focuses on images near and far.
  
• Lens. A transparent biconvex structure enclosed in an elastic covering. The lens is held in position by radially arranged suspensory ligaments, which are attached medially to the lens capsule and laterally to the ciliary muscles.

Accommodation

Process by which the lens charges to maintain a focused image from distant or near objects.

• Distant vision. Light rays from distant objects are nearly parallel and do not need as much refraction to bring them into focus (Figure 18-2C). Therefore, to focus on distant objects, the ciliary muscle relaxes, which stretches the suspensory ligaments and flattens the lens. When the lens is flat (least rounded), it is at optimal focal length for distant viewing.
  
• Near vision. Light rays from close objects diverge and require more refraction to focus (Figure 18-2D). To focus on near objects, parasympathetic neurons in CN III cause ciliary muscle contraction and thus relaxation of the tension on the suspensory ligaments, allowing the lens to become more rounded. When the lens is round, it is at optimal focal length for near viewing.

Iris