Special Senses: Eye and Ear



Special Senses


Eye and Ear













Functions of the Special Senses


When we want to relate our understanding of how someone is feeling, we say, “I hear you” or even “I see what you’re saying!” Our experience in the world is filtered through our senses and our interpretations of them.


The senses include vision, hearing, taste, smell, and touch. They allow us to experience our environment through specific nervous tissue that transmits, processes, and then acts on our perceptions. This chapter covers the eyes and ears. Disorders of taste and touch are covered in Chapter 12, and olfaction (smell) is discussed in Chapter 11.




Specialties/Specialists






The Eye


Anatomy and Physiology


The eye can be divided into the ocular adnexa—the structures that surround and support the function of the eyeball—and the structures of the globe of the eye itself.




Ocular Adnexa


Each of our paired eyes is encased in a protective, bony socket called the orbit. Our binocular vision sends two slightly different images to the brain that produce depth of vision. The right eye is called the oculus dextra, the left eye is called the oculus sinistra, and the term for “each eye” is oculus uterque. Within the orbit, the eyeball is protected by a cushion of fatty tissue. The eyebrows mark the supraorbital area and provide a modest amount of protection from perspiration and sun glare. Further protection is provided by the upper and lower eyelids and the eyelashes that line their edges (Fig. 14-1).


image
Fig. 14-1 Ocular adnexa.


The corners of the eyes are referred to as the canthi (KAN thy) (sing. canthus); the inner canthus is termed medial (toward the middle of the body), and the outer canthus is lateral. The area where the upper and lower eyelids meet is referred to as the palpebral fissure (PAL puh brul FISH ur). This term comes from the function of blinking, called palpebration (pal puh BRAY shun). The eyelids are lined with a protective, thin mucous membrane called the conjunctiva (kun jungk TYE vuh) (pl. conjunctivae) that spreads to coat the anterior surface of the eyeball as well.


Also surrounding the eye are two types of glands. Sebaceous glands in the eyelid secrete oil to lubricate the eyelashes, and lacrimal glands above the eyes produce tears. The sebaceous glands for the eyelashes are called meibomian (mye BOH mee un) glands. These glands can be a source of complaint when they become blocked or infected. The other type of gland, the lacrimal (LACK rih mul) gland, or tear gland, provides a constant source of cleansing and lubrication for the eye. The process of producing tears is termed lacrimation (lack rih MAY shun). The lacrimal glands are located in the upper outer corners of the orbit. The constant blinking of the eyelids spreads the tears across the eyeball. They then drain into two small holes (the lacrimal puncta) in the medial canthus, into the lacrimal sacs, and then into the nasolacrimal ducts, which carry the tears to the nasal cavity.





The extraocular (eck strah OCK yoo lur) muscles attach the eyeball to the orbit and, on impulse from the cranial nerves, move the eyes. These six voluntary (skeletal) muscles are made up of four rectus (straight) and two oblique (diagonal) muscles.







The Eyeball


The anatomy of the eyeball itself is traditionally described as being three layers, or tunics (TOO nicks) (Fig. 14-2). The outer layer, or fibrous tunic, consists of the sclera (SKLAIR uh) and cornea (KOR nee uh). The middle layer, or vascular tunic, consists of the uvea (YOO vee uh), which is made up of the choroid (KOR oyd), ciliary (SILL ih air ee) body, and iris (EYE ris). The inner layer, or nervous tunic, consists of the retina (RET in uh). These three layers are essential to the process of seeing. All parts work together with impressive harmony. The eye muscles coordinate their movements with one another; the cornea and pupil control the amount of light that enters the eye; the lens focuses the image on the retina; and the optic nerve transmits the image to the brain.




Two important mechanisms also contribute to the ability to see. As light hits the eye, it passes first through the cornea, which bends the rays of light (refraction) so that they are projected properly onto receptor cells in the eye. Then, muscles in the ciliary body adjust the shape of the lens to aid in this refraction. The lens flattens to adjust to something seen at a distance, or thickens for close vision—a process called accommodation. Errors of refraction are the most common reason for lens prescriptions.




The Uvea

The uvea is the middle, highly vascular layer of the eye. It includes the iris, the ciliary body, and the choroid. The iris (pl. irides), pupil, lens (pl. lenses), and ciliary body are located directly behind the cornea. The iris is a smooth muscle that contracts and relaxes to moderate the amount of light that enters the eye. In most individuals, this is the colored part of the eye (brown, gray, hazel, blue) because of its pigmentation. Individuals with albinism, however, have reddish-pink irides (sing. iris) because a lack of pigment makes visible the blood cells traveling through the vessels supplying the iris.



The pupil is the dark area in the center of the iris where the light continues its progress through to the lens. The lens is an avascular structure made of protein and covered by an elastic capsule. It is held in place by the thin strands of muscle that make up the ciliary body. The fluid produced by the capillaries of the ciliary body is called the aqueous (AY kwee us) humor. It nourishes the cornea, gives shape to the anterior eye, and maintains an optimum intraocular pressure. It normally drains through tiny veins called the canals of Schlemm. The aqueous humor circulates in both the anterior chamber, between the cornea and the iris, and the posterior chamber, behind the iris and in front of the lens. Between the lens and the retina is a jellylike substance, the vitreous (VIT ree us) humor (also called the vitreous body), which holds the choroid membrane against the retina to ensure an adequate blood supply. The choroid is the vascular membrane between the sclera and the retina.




The Retina

The inner layer of the eye, called the retina, contains the sensory receptors for the images carried by the light rays. These sensory receptors are either rods, which appear throughout the retina and are responsible for vision in dim light, or cones, which are concentrated in the central area of the retina and are responsible for color vision.



During daylight, the area of the retina on which the light rays focus is called the macula lutea (MACK yoo lah LOO tee uh). The fovea (FOH vee uh) centralis is an area within the macula that contains only cones and provides the sharpest image. The area that allows a natural blind spot in our vision is the optic disk, where the optic nerve leaves the retina to travel to the brain. There are no light receptors there.




image Exercise 3: The Eyeballs


Match the parts of the eye with the correct combining forms. More than one answer may be correct.



Decode the adjective forms.




image Exercise 4: The Eyeballs


Label the drawings below with correct anatomic terms and combining forms where appropriate.


image


imageYou can review the anatomy of the eye by going to Evolve at evolve.elsevier.com/Shiland and clicking on Body Spectrum Electronic Anatomy Coloring BookSensesEye. Choose Hear It, Spell It to practice spelling the anatomy and physiology terms for the eye. To practice pronouncing these terms, click on Hear It, Say it.






Pathology
















image Exercise 5: Disorders of the Ocular Adnexa


Match the disorders with their definitions.


Oct 6, 2017 | Posted by in GENERAL SURGERY | Comments Off on Special Senses: Eye and Ear
Premium Wordpress Themes by UFO Themes
%d bloggers like this: