EXERCISE 17 The Special Senses Materials Objectives For Vision • Describe the structure and function of the accessory visual structures. ● Dissectible eye model • Identify the internal structures of the eye when provided with a model, ● Chart of eye anatomy diagram, or preserved animal eye, and list the functions of each. ● Preserved cow or sheep eye • Define blind spot, refraction, hyperopia, myopia, and astigmatism, ● Dissecting tray and instruments and discuss image formation on the retina. ● Disposable gloves ● Metric ruler • Discuss the importance of the accommodation and convergence ● Common straight pins reflexes. ● Snellen eye chart (floor marked with chalk • Identify the structures of the external, middle, and internal ear to indicate 20-ft distance from posted by correctly labeling a diagram. Snellen chart) ● Ishihara’s color-blindness plates • Describe the anatomy of the spiral organ and explain its role in For Hearing and Equilibrium hearing. ● Three-dimensional dissectible ear model • Describe how one is able to localize sounds and to differentiate and/or chart of ear anatomy sensorineural from conduction deafness. ● Otoscope (if available) • Describe the anatomy of the equilibrium organs of the internal ● Alcohol swabs ear, and explain their relative roles in maintaining balance. ● Prepared microscope slide of cochlea ● Absorbent cotton • State the purpose of the Romberg test, and describe the role of ● Pocket watch or clock that ticks vision in maintaining equilibrium. ● 12-inch ruler • Describe the location, structure, and function of the olfactory and ● Tuning forks (range of frequencies) taste receptors. ● Rubber mallet • List several factors that influence taste. ● Demonstration area: Cochlea slide set up under a compound microscope for student observation For Smell and Taste ● Small mirror n contrast to the small and widely distributed general receptors (touch, tem- perature, pressure, and pain), the special sense receptors are large, complex ● Paper towels sensory organs (eyes and ears) or localized clusters of receptors (taste buds and ● Granulated sugar I olfactory epithelium). This chapter focuses on the functional anatomy of each of ● Cotton-tipped swabs the special sense organs individually, but keep in mind that sensory inputs are ● Disposable autoclave bag overlapping. ● Paper cups; paper plates ● Beaker containing 10% bleach solution ● Prepared dropper bottles of oil of cloves, Anatomy of the Eye oil of peppermint, and oil of wintergreen or corresponding flavorings found in the Accessory Structures condiment section of a supermarket The adult human eye is a sphere some 2.5 cm (1 inch) in diameter. Only about ● Equal-sized cubes of foods, such as cheese, apple, dried prunes, banana, and one-sixth of the eye’s anterior surface is observable (Figure 17.1); the remain- hard-cooked egg white (in an opaque der is protected by a cushion of fat and the walls of the bony orbit. The accessory container, such as a foil-lined egg carton) structures of the eye include the eyebrows, eyelids, conjunctivae, lacrimal ap- ● Chipped ice paratus, and extrinsic eye muscles (Table 17.1, Figure 17.1 and Figure 17.2). ● Absorbent cotton 195 M17_MARI6332_06_SE_C17.indd 195 10/09/15 2:04 pm 196 Exercise 17 Lacrimal sac Orbicularis Medial commissure oculi muscle Eyebrow Lacrimal caruncle Tarsal plate Upper eyelid Palpebral Lacrimal gland conjunctiva Excretory ducts of lacrimal glands Tarsal glands Lateral commissure Cornea Lower eyelid Palpebral Lacrimal punctum fissure Lacrimal canaliculus Nasolacrimal duct Eyelashes Inferior meatus Bulbar of nasal cavity conjunctiva Nostril Conjunctival sac (a) (b) Figure 17. 1 External anatomy of the eye and accessory structures. (a) Lateral view; some structures shown in sagittal section. (b) Anterior view with lacrimal apparatus. 17 Table 17.1 Accessory Structures of the Eye (Figures 17.1 and 17.2) Structure Description Function Eyebrows Short hairs located on the supraorbital margins Shade and prevent sweat from entering the eyes. Eyelids (palpebrae) Skin-covered upper and lower lids, with eyelashes Protect the eyes and spread lacrimal fluid (tears) projecting from their free margin with blinking. Tarsal glands Modified sebaceous glands embedded in the tarsal Secrete an oily secretion that lubricates the plate of the eyelid surface of the eye. Ciliary glands Typical sebaceous and modified sweat glands that Secrete an oily secretion that lubricates the lie between the eyelash follicles surface of the eye and the eyelashes. An infection of a ciliary gland is called a sty. Conjunctivae A clear mucous membrane that lines the eyelids Secrete mucus to lubricate the eye. Inflammation (palpebral conjunctivae) and the anterior white of of the conjunctiva results in conjunctivitis, the eye (bulbar conjunctiva) (commonly called “pinkeye”). Medial and lateral commissures Junctions where the eyelids meet medially and Form the corners of the eyes. The medial laterally commissure contains the lacrimal caruncle. Lacrimal caruncle Fleshy reddish elevation that contains sebaceous Secretes a whitish oily secretion for lubrication of and sweat glands the eye (can dry and form “eye sand”). Lacrimal apparatus Includes the lacrimal gland and a series of ducts Protects the eye by keeping it moist. Blinking that drain the lacrimal fluid into the nasal cavity spreads the lacrimal fluid. Lacrimal gland Located in the superior and lateral aspect of the Secretes lacrimal fluid, which contains mucus, orbit of the eye antibodies, and lysozyme. Lacrimal puncta Two tiny openings on the medial margin of each Allow lacrimal fluid to drain into the superior and eyelid inferiorly located lacrimal canaliculi. Lacrimal canaliculi Two tiny canals that are located in the eyelids Allow lacrimal fluid to drain into the lacrimal sac. Lacrimal sac A single pouch located in the medial orbital wall Allows lacrimal fluid to drain into the nasolacrimal duct. Nasolacrimal duct A single tube that empties into the nasal cavity Allows lacrimal fluid to flow into the nasal cavity. Extrinsic eye muscles Six muscles for each eye; four recti and two Control the movement of each eyeball and hold oblique muscles (see Figure 17.2) the eyes in the orbits. M17_MARI6332_06_SE_C17.indd 196 10/09/15 2:04 pm The Special Senses 197 Superior oblique Muscle Action muscle Trochlea Lateral rectus Moves eye laterally Medial rectus Moves eye medially Superior Superior rectus Elevates eye and turns it medially oblique tendon Inferior rectus Depresses eye and turns it medially Inferior oblique Elevates eye and turns it laterally Superior rectus Superior oblique Depresses eye and turns it laterally muscle (b) Lateral rectus muscle Figure 17. 2 Extrinsic muscles of the eye. (a) Lateral view of the right eye. (b) Summary of actions of the extrinsic eye muscles. Inferior Inferior cones and is the area of greatest visual acuity. Focusing for rectus oblique detailed color vision occurs in the fovea centralis. muscle muscle Internal Chambers and Fluids (a) The lens divides the eye into two segments. The anterior seg- ment (anterior to the lens) contains a clear, watery fluid called Activity 1 the aqueous humor. The posterior segment behind the lens is filled with the gel-like vitreous humor. The aqueous humor Identifying Accessory Eye Structures is continually formed by the capillaries of the ciliary process. It helps to maintain the intraocular pressure of the eye and Observe the eyes of another student, and identify as provides nutrients for the avascular lens and cornea. Aqueous many accessory structures as possible. Ask the student humor is drained into the scleral venous sinus, a drainage duct to look to the left. Which extrinsic eye muscles produce located at the junction of the sclera and cornea. The vitreous this action? humor reinforces the posterior part of the eyeball, and helps to 17 keep the retina pressed firmly against the wall of the eyeball. Right eye Left eye Light Ask your partner to look superiorly. Which two extrinsic muscles of each eye can bring about this motion? Axons of ganglion Right eye cells Left eye Ganglion cells Internal Anatomy of the Eye The eye itself is a hollow sphere. Anatomically, the wall is Nervous constructed of three layers: the fibrous layer, the vascular tissue layer, and the inner layer. Bipolar layer Distribution of Photoreceptors cells The inner neural layer is composed of three major popula- tions of neurons. There are, from outer to inner aspect, the photoreceptors (rods and cones), the bipolar cells, and the ganglion cells (Figure 17.3). The rods are the specialized photoreceptors for dim light. The cones are color photore- Photo- ceptors that permit high levels of visual acuity, but they only receptors function under conditions of high light intensity. The photo- Rods receptor cells are distributed over the entire neural retina, ex- Cone cept where the optic nerve (the bundled axons of the ganglion cells) leaves the eyeball. This site is called the optic disc, or blind spot (see Figure 17.4). Lateral to each blind spot is the macula lutea (yellow spot), an area of high cone density. In Figure 17. 3 Microscopic anatomy of the retina. (See its center is the fovea centralis, a tiny pit that contains only also Plate 15 in the Histology Atlas.) M17_MARI6332_06_SE_C17.indd 197 10/09/15 2:04 pm 198 Exercise 17 Activity 2 Identifying Internal Structures of the Eye Obtain a dissectible eye model or observe a chart of eye anatomy to identify the structures described below. As you work, refer to Figure 17.4 and Table