Accessory Structures of the Orbit

Before considering the eye itself, let's survey the accessory structures located in and around the orbit (figs. 16.20 and 16.21). These include the eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles:

• The eyebrows probably serve mainly to enhance facial expressions and nonverbal communication (see p. 204), but they may also protect the eyes from glare

Eyelashes -

Palpebral -fissure

Lateral -commissure

Conjunctiva -Tarsal plate

- Pupil

Superior palpebral sulcus

- Upper eyelid

- Pupil

Superior palpebral sulcus

- Upper eyelid

Superciliary ridge

Eyelashes -

Palpebral -fissure

Lateral -commissure

Conjunctiva -Tarsal plate

Palpebral Ridge

- Lower eyelid

- Inferior palpebral sulcus

Figure 16.20 External Anatomy of the Orbital Region.

- Medial commissure

- Lower eyelid

- Inferior palpebral sulcus

Figure 16.20 External Anatomy of the Orbital Region.

and help to keep perspiration from running into the eye.

The eyelids, or palpebrae (pal-PEE-bree), block foreign objects from the eye, prevent visual stimuli from disturbing our sleep, and blink periodically to moisten the eye with tears and sweep debris from the surface. The eyelids are separated from each other by the palpebral fissure and meet each other at the corners called the medial and lateral commissures (canthi). The eyelid consists largely of the orbicularis oculi muscle covered with skin (fig. 16.21a). It also contains a supportive fibrous tarsal plate, which is thickened along the margin of the eyelid. Within the plate are 20 to 25 tarsal glands that open along the margin. They secrete an oil that coats the eye and reduces tear evaporation. The eyelashes are guard hairs that help to keep debris from the eye. Touching the eyelashes stimulates hair receptors and triggers the blink reflex.

The conjunctiva (CON-junk-TY-vuh) is a transparent mucous membrane that covers the inner surface of the eyelid and anterior surface of the eyeball, except for the cornea. Its primary purpose is to secrete a thin mucous film that prevents the eyeball from drying. It is richly innervated and highly sensitive to pain. It is also very vascular, which is especially evident when the vessels are dilated and the eyes are "bloodshot." Because it is vascular and the cornea is not, the conjunctiva heals more readily than the cornea when injured.

The lacrimal40 apparatus (fig. 16.21b) consists of the lacrimal (tear) gland and a series of ducts that drain the lacrim = tear

Saladin: Anatomy & Physiology: The Unity of Form and Function, Third Edition

16. Sense Organs

Text

Chapter 16 Sense Organs 611

Chapter 16 Sense Organs 611

Eye Anatomy Accessory Structures
Figure 16.21 Accessory Structures of the Orbit. (a) Sagittal section of the eye and orbit. (b) The lacrimal apparatus.

tears into the nasal cavity. The lacrimal gland, about the size and shape of an almond, is nestled in a shallow fossa of the frontal bone in the superolateral corner of the orbit. About 12 short ducts lead from the lacrimal gland to the surface of the conjunctiva. Tears function to cleanse and lubricate the eye surface, deliver oxygen and nutrients to the conjunctiva, and prevent infection by means of a bactericidal enzyme, lysozyme. Periodic blinking spreads the tears across the eye surface. On the margin of each eyelid near the medial commissure is a tiny pore, the lacrimal punctum.41 This is the opening to a short lacrimal canal, which leads to the lacrimal sac in the medial wall of the orbit. From this sac, a nasolacrimal duct carries the tears to the inferior meatus of the nasal cavity—thus an abundance of tears from crying or watery eyes can result in a runny nose. Once the tears enter the nasal cavity, they normally flow back to the throat and we swallow them. When we have a cold, the nasolacrimal ducts become swollen and obstructed, the tears cannot drain, and they may overflow from the brim of the eye.

The extrinsic eye muscles are the six muscles attached to the walls of the orbit and to the external surface of the eyeball. Extrinsic means arising externally; it distinguishes these from the intrinsic muscles inside the eyeball, to be considered later. The extrinsic muscles move the eye (fig. 16.22). They include four rectus ("straight") muscles and two oblique muscles. The superior, inferior, medial, and lateral rectus originate on the posterior wall of the orbit and insert on the anterior region of the eyeball, just beyond the visible "white of the eye." They move the eye up, down, medially, and laterally. The superior oblique travels along the medial wall of the orbit. Its tendon passes through a fibrocartilage ring, the trochlea42 (TROCK-lee-uh), and inserts on the superolateral aspect of the eyeball. The inferior oblique extends from the medial wall of the orbit to the inferolateral aspect of the eye. To visualize the function of the oblique muscles, suppose you turn your eyes to the right. The superior oblique muscle will slightly depress your right eye, while the inferior oblique slightly elevates the left eye. The opposite occurs when you look to the left. This is the primary function of the oblique muscles, but they also slightly rotate the eyes, turning the "twelve o'clock pole" of each eye slightly toward or away from the nose. Most of the extrinsic muscles are supplied by the oculomotor nerve (cranial nerve III), but the superior oblique is innervated by the trochlear nerve (IV) and the lateral rectus by the abducens (VI).

42trochlea = pulley

Saladin: Anatomy & I 16. Sense Organs I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

612 Part Three Integration and Control

Superior

Scleral Spur
(a)

Trochlea

Muscles

Superior oblique Superior rectus Medial rectus Lateral rectus Inferior oblique Inferior rectus

Optic nerve

Superior

Trochlea

Muscles

Superior oblique Superior rectus Medial rectus Lateral rectus Inferior oblique Inferior rectus

Optic nerve

Muscle Rectus Medial

Superior oblique tendon

Muscles

Superior rectus Inferior rectus

Levator palpebrae superioris

Optic nerve

Superior oblique tendon

Muscles

Superior rectus Inferior rectus

Levator palpebrae superioris

Optic nerve

Frontal

Trochlear nerve (IV)

Abducens nerve (VI)

Superior oblique muscle

Lateral rectus muscle

Superior oblique muscle

Lateral rectus muscle

Levator Palpebrae Muscle

Levator palpebrae superioris muscle

Superior rectus muscle

Medial rectus muscle

Levator palpebrae superioris muscle

Superior rectus muscle

Medial rectus muscle

Inferior rectus muscle

Inferior oblique muscle

Oculomotor nerve (III)

Figure 16.22 Extrinsic Muscles of the Eye. (a) Lateral view of the right eye. (b) Superior view of the right eye. (c) Innervation of the extrinsic muscles; arrows indicate the eye movement produced by each muscle.

The eye is surrounded on the sides and back by orbital fat. It cushions the eye, gives it freedom of motion, and protects blood vessels and nerves as they pass through the rear of the orbit.

Was this article helpful?

0 0
Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

Get My Free Ebook


Responses

  • Anniina Puikkonen
    Does the inferior oblique muscle turn into superior oblique?
    8 years ago
  • Gregorio Capon
    What structures are located in facial orbits?
    6 years ago

Post a comment