Ch 10 - Sense Organs

Why would keen hearing be important to the survival of a potential prey animal? Why would it be important to a predator?

Keen hearing allows a potential prey animal to hear movements caused by its predator and escape. Keen hearing allows a predator to hear the slightest movements made by its potential prey so that it can track and find it.

What are suspensory ligaments?

Ligaments connecting ciliary muscle to the lens, control accommodation (focusing).

What is the proprioceptive sense and why is it so important to the maintenance of balance and an upright posture?

Proprioception is the sense of body position and movement. It allows animals to perform such basic functions as procuring food and escaping danger so they can survive. Maintenance of balance and an upright posture is essential to these activities.

Which layer of the eyeball contains the photoreceptors?

Retina (the inner nervous layer)

What kind of vision do the rods in the retina perceive? What do cones perceive?

Rods are more sensitive to light than cones, but they produce a somewhat coarse image in shades of gray. The cones are more sensitive to color and detail than are the rods, but they do not function well in dim light. So rods are the main receptors for dim light vision, and cones perceive color and detail.

What are the major components of the eye?

cornea, sclera, choroid, iris, pupil, ciliary body, suspensory ligaments, retina, aqueous compartment, vitreous compartment, lens

What is glaucoma?

A group of diseases characterized by increased intraocular pressure (pressure within the eye) that causes pain and can lead to blindness. An instrument called a tonometer is used to test for glaucoma by measuring the intraocular pressure. Most often the problem is due to insufficient drainage of aqueous humor, rather than overproduction.

What are the extraocular structures?

Extraocular structures are not part of the eye itself, but they play important roles in its protection and functioning. They include the conjunctivae, the eyelids, the tear-production and drainage system, and the muscles that delicately move and position the eyeballs.

An animal can intentionally blink its eyelids. Can it intentionally cover its eye with its third eyelid? Why or why not?

No muscles attach to the third eyelid. Its movements are entirely passive.

How does the tapetum aid low-light vision?

After light has passed through the photoreceptors (the rods and cones) in the retina, it reflects off the tapetum and passes back through the photoreceptors again, stimulating them a second time. Therefore, most animals can see better in dim light than we can.

Why do you suppose mild to moderate pain often does not appear to affect the mood or behavior of domestic animals significantly?

Animals do not seem to have the same kind of emotional reaction to pain that humans do. They often seem to hide it well. Hiding signs of pain is a survival instinct for most animals. An animal that shows signs of pain is showing signs of weakness that might encourage other animals, including predators, to attack it.

Where is aqueous humor produced? Where is it drained from the aqueous compartment of the eye?

Aqueous humor is produced in the posterior chamber by cells of the ciliary body. It is drained by the canal of Schlemm, and the fluid is returned to the bloodstream. The canal of Schlemm is a ring like structure located at the edge of the anterior chamber at the angle where the iris and the cornea meet.

How are the senses of taste and smell similar? How are they different?

Both are chemical senses. Whereas many nonhuman animals have less perceptive eyesight but a highly acute sense of smell, humans have a less perceptive sense of smell and a highly acute sense of taste.

Describe the structure of the conjunctiva and eyelids.

Describe the structure of the conjunctiva and eyelids. The conjunctiva is a thin, transparent membrane that covers the front portion of the eyeball and lines the interior surfaces of the eyelids. The portion covering the front of the eyeball is called the bulbar conjunctiva (bulbar refers to the eyeball), and the portion lining the eyelids is called the palpebral conjunctiva (palpebral refers to the eyelids). The space between the bulbar and palpebral portions of the conjunctiva (between the eyelid and the eyeball) is called the conjunctival sac. The eyelids consist of upper and lower folds of skin that are lined by the thin, moist conjunctiva. The lateral and medial corners, where the eyelids come together, are called the lateral and medial canthi (singular, canthus). Along the margin of each eyelid are the tiny openings of the tarsal glands, also known as meibomian glands. These can be seen as a line of little dots along the eyelid margin. They produce a waxy substance that helps prevent tears from overflowing onto the face. Eyelashes (cilia) are most prominent on the upper lid of most animals. Lower eyelashes are usually more sparse and thin, if they are present at all. Domestic animals also have a third eyelid (also called the nictitating membrane) located medially between the eyelids and the eyeball. It consists mainly of a T-shaped plate of cartilage covered by conjunctiva. On its ocular surface (the surface in contact with the eyeball) are lymph nodules and an accessory lacrimal (tear-producing) gland. No muscles attach to the third eyelid. Its movements are entirely passive.

How might repeated exposure to loud sounds lead to progressive hearing loss?

Everything up to the generation of an auditory nerve impulse by the hair cells (sensory cells) in the organ of Corti of the cochlea involves movement: the tympanic membrane, the ossicles, the round and oval windows, the endolymph and perilymph in the cochlea, the tectorial membrane, and the microvilli of the hair cells. Loud noises cause exaggerated movements of all those structures. In the short term, irreparable damage rarely results. Over long periods of time, however, loud noises can cause damage to these mechanical structures that the body may not be able to fully repair. Some examples might include damage to the microvilli of the hair cells, thickening or displacement of the tectorial membrane, thickening of the membranes covering the round and oval windows, and damage to the delicate joints between the bony ossicles.

Which type of vision requires more muscular effort: close-up vision or far-away vision? Why?

For close-up vision, the ciliary muscles must contract to take tension off the suspensory ligaments. This allows the lens to assume its natural, more rounded shape. So close-up vision requires muscle contractions in the ciliary body, but distant vision does not.

List the refractive media of the eye.

Four refractive media in the eye help form a clear image on the retina: the cornea, the aqueous humor, the lens, and the vitreous humor. All contribute to the creation of a clear visual image, but the cornea does the majority of the refractive work. Its curved shape and the extreme difference between its optical density and that of the air in front of it result in significant refraction of light rays as they pass through it. The other refractive media, even the adjustable lens, only fine-tune the image that the cornea has formed.

An animal has an area of inflammation located at the dorsal limbus of its right eye. Where is the lesion located?

If the iris is compared with the face of a clock, this lesion would be at approximately 12 o'clock, at the junction of the cornea and the sclera (white) of the eye.

If the medial rectus muscle of an animal's eye was damaged and lost its ability to contract, what would the effect be on the positioning of the affected eye? Why?

If the medial rectus muscle were unable to contract, the pull of the lateral rectus muscle would cause the eyeball to deviate (move abnormally) laterally. Also the animal would not be able to rotate that eyeball toward the midline, because this muscle originates from a small area around the optic canal and fissure and inserts on the medial side of the sclera (near the 9 o'clock position if you are standing in front of the animal).

How would an animal with a plugged nasolacrimal system appear? Why?

If the nasolacrimal system were plugged, tears would be "spilling" out of the eyes because they would have nowhere to drain.

Describe the processes that contribute to the sense of equilibrium.

Maintaining balance is a complicated process that involves information from the equilibrium receptors, as well as from the eyes and the proprioceptors around the body. The receptors are located in portions of the inner ear called the vestibule and the semicircular canals. The vestibule is made up of two saclike spaces, called the utricle and the saccule, that are filled with endolymph and surrounded by perilymph. In each utricle and saccule is a patch of sensory epithelium called the macula. It consists of hair cells and supporting cells covered by a gelatinous matrix that contains tiny crystals of calcium carbonate called otoliths. Gravity causes the otoliths and the gelatinous matrix to put constant pressure on the hairs as long as the head stays still. Movement of the head bends the sensory hairs, which generates nerve impulses that give the brain information about the position of the head. The semicircular canals are located on the other side of the vestibule from the cochlea. Each canal is semicircular and oriented in a different plane at right angles to the other two. Within each bony semicircular canal is an endolymph-filled membranous tube that is surrounded by perilymph. Near the utricle end of each semicircular canal is an enlargement, called the ampulla, that contains the receptor structure called the crista. The crista consists of a cone-shaped area of supporting cells and hair cells with their processes (modified dendrites) sticking up into a gelatinous structure called the cupula, which functions as a float that moves with the endolymph in the membranous canal. When the head moves, the relative movement of the endolymph pulls on the cupula, which bends the hairs. This generates nerve impulses that give the brain information about motion of the head, particularly rotary motion.

What is nociception and what roles do transduction, transmission, modulation, and perception play in it?

Nociception is the process of experiencing pain. Transduction is the conversion of the painful stimulus to a nerve impulse that occurs at the sensory nerve ending. Transmission of the nerve impulse up the sensory nerve fibers to the spinal cord is the next step. Modulation (changing) of the sensory nerve impulses can occur in the spinal cord, and this can significantly influence the information the brain receives, particularly in cases of chronic and/or severe pain. This modulation process can amplify or suppress sensory impulses through synapses between neurons in the dorsal horns of the spinal cord. Finally, perception of the painful impulses by several areas of the brain occurs. Conscious perception occurs in the cerebral cortex (outer layer of neural tissue of the cerebrum), but other areas of the brain are also involved. These include areas involved with the autonomic nervous system, fear and anxiety, memory, arousal, and behavior and emotion.

Why are heatstroke and hypothermia so dangerous?

Normal cellular functions in warm-blooded animals depend on the core body temperature remaining fairly constant. This is because chemical reactions, including all the metabolic reactions that occur in the body, are temperature dependent. Higher temperatures speed up chemical reactions, and lower temperatures slow them down. Significant variations in the core temperature of the body, such as might occur in heatstroke (significantly elevated body temperature) or hypothermia (significantly decreased body temperature), can have serious consequences and endanger the life of the animal.

What is the aqueous compartment?

One of two fluid-filled compartments that make up the interior of the eye. The aqueous compartment is in front of the lens and ciliary body and contains a clear, watery fluid called aqueous humor. The aqueous compartment is subdivided into two parts by the iris. The space in front of the iris is the anterior chamber, and the space behind the iris and in front of the lens is the posterior chamber.

What is the vitreous compartment?

One of two fluid-filled compartments that make up the interior of the eye. The vitreous compartment is behind the lens and ciliary body and contains a clear fluid with the consistency of soft gelatin, called vitreous humor.

What is the difference between ossicles and otoliths?

Ossicles are small bones in the middle ear that conduct sound waves from the tympanic membrane to the oval window of the cochlea. Otoliths are tiny calcium carbonate stones in the vestibule that respond to gravity to provide information about the position of the head.

What are otoliths and why are they important to the equilibrium sense?

Otoliths are tiny crystals of calcium carbonate in a gelatinous matrix that covers hair cells and supporting cells of the macula. The hair cells are similar to the hair cells of the organ of Corti in the cochlea. Gravity causes the otoliths and the gelatinous matrix to put constant pressure on the hairs as long as the head stays still. Movement of the head bends the sensory hairs, which generate nerve impulses that give the brain information about the position and linear movement of the head. Otoliths are important to the sense of equilibrium because information sensed by the vestibule (which contains the otoliths) regarding linear motion and head position is integrated with information about rotary motion of the head gained by the semicircular canals so that the brain can form a picture of what is happening to the animal's head, and by extension, to its body as a whole. Together, the vestibule and semicircular canals provide the total picture of equilibrium to the brain.

What are cataracts?

Partial or total opacity of the normally transparent lens (Caused by: congenital, trauma, aging, diabetes, drugs like steroid therapy, exposure to radioactivity)

How would the rupture or perforation of an eardrum affect hearing?

Rupture or perforation of an eardrum would decrease the tension of the structure. This would decrease the force of the vibrations that move along the ossicles to the inner ear, impairing hearing.

List and describe the special senses.

Special senses include: taste, smell, hearing, vision, and equilibrium. The sense of taste, also called the gustatory sense, is a chemical sense. Its receptors are located in the mouth in structures called taste buds. When they detect chemical substances dissolved in the saliva, the taste receptors generate nerve impulses that travel to the brain and are interpreted as tastes. The sense of smell is also called the olfactory sense. It is a chemical sense very similar to taste. The sense of smell is more important in most nonhuman animals than it is in humans. Hearing, also called the auditory sense, is a mechanical sense that converts vibrations of air molecules into nerve impulses that are interpreted by the brain as sound. Equilibrium is a mechanical sense that helps the animal maintain its balance by keeping track of the position and movements of the head. Vision is a process in which light rays entering the eye are transformed into signals that pass to the brain, enabling the animal to see its surroundings.

What is the difference between superficial and central temperature receptors?

Superficial temperature receptors are located in the skin and detect upward or downward changes in skin temperature. Heat receptors increase their generation of nerve impulses when the temperature increases, and cold receptors increase theirs when the temperature falls. These increased impulses get the attention of the conscious mind and let it know that things are out of balance. At constant temperatures, the receptors generate steady, low-level streams of nerve impulses that are sensed at the subconscious level and do not intrude on the conscious mind. Central temperature receptors keep track of the core (interior) temperature of the body by monitoring the temperature of the blood. Central temperature receptors are located in the hypothalamus. An animal's rectal temperature indicates its core temperature.

Which skull bone houses the middle and inner ear structures?

Temporal bone

The cornea and sclera are both made up substantially of collagen fibers. Why is the cornea transparent, but the sclera opaque?

The cornea contains no blood vessels, and the amount of water it contains is carefully controlled.

What is esthesia?

The ability to perceive sensations. The study of the sensory system is called esthesiology. Anesthesia is the loss of esthesia, or the complete loss of sensation (general or local). Analgesia is a related state in which the perception of pain is decreased but not completely absent.

What is the choroid?

The choroid is located between the sclera and the retina. It consists mainly of pigment and blood vessels that supply blood to the retina. Most of the pigment is dark melanin, but in most domestic animals, except swine, the choroid forms a highly reflective area in the rear of the eye called the tapetum. The tapetum is like a brightly colored mirror, and it is responsible for the bright light that seems to shine from an animal's eyes in the dark, when a light is directed into them. Its purpose seems to be to act as a light amplifier to aid low-light vision.

What is the ciliary body?

The ciliary body is a ring-shaped structure located immediately behind the iris. It contains the tiny muscles that adjust the shape of the lens to allow near and far vision.

What is the cornea?

The cornea admits light into the eye. It is transparent, composed of collagen fibers, contains no blood vessels but has many pain receptors. Its transparency is maintained by careful control of the amount of water it contains.

Describe the structure and function of the components of the external ear.

The external ear consists of structures that collect sound waves and transmit them to the middle ear. Its main parts are the pinna, the external auditory canal, and the tympanic membrane (eardrum). The pinna is the part of the ear that we can see from the outside. It is a funnel-like structure composed mainly of elastic cartilage and skin that collects sound wave vibrations and directs them into the external auditory canal. The external auditory canal is a soft, membrane-lined tube that begins at the base of the pinna and carries sound waves to the tympanic membrane (eardrum). The tympanic membrane (eardrum) is a paper-thin connective tissue membrane that is tightly stretched across the opening between the external auditory canal and the middle ear cavity. When sound wave vibrations strike it, the tympanic membrane vibrates at the same frequency through a process called sympathetic vibration.

List and describe the extraocular eye muscles.

The extraocular eye muscles attach to the sclera of the eye. They are the small skeletal muscles that hold the eyeballs in place and delicately and accurately move them. They are capable of a wide range of movements that allow the eyes to track and examine objects with great precision. Humans share six extraocular muscles with domestic animals: four straight muscles and two oblique muscles. The four straight muscles are called the rectus muscles. (The word rectus means "straight.") They include the dorsal, ventral, medial, and lateral rectus muscles. Their names indicate where they attach to the eyeball. The two oblique muscles are the dorsal and ventral oblique muscles. Many animals also have a seventh extraocular muscle—the retractor bulbi muscle. It retracts the eyeball slightly deeper into the orbit when it contracts. This may assist the activity of the other muscles by enhancing their mechanical advantage. Most movements of the eye involve a combination of several of these muscles contracting together.

What are the major layers of the eyeball?

The eyeball consists of three major layers: the outer fibrous layer, the middle vascular layer, and the inner nervous layer. The fibrous layer consists of the cornea and the sclera. The vascular layer (also called the uvea) includes the choroid, the iris, and the ciliary body. The inner layer is the retina. The term fundus is sometimes used to describe the caudal interior surface of the eye. The retina is the main component of the fundus, along with the optic disc.

Describe the structure and function of the components of the inner ear.

The hearing portion of the inner ear is contained in a snail shell-shaped spiral cavity in the temporal bone called the cochlea. Within the cochlea is a soft, multilayered, fluid-filled portion that contains the receptor organ of hearing—the organ of Corti. The organ of Corti runs the length of the cochlea in a long tube called the cochlear duct, which is filled with a fluid called endolymph. A U-shaped tube containing another fluid, perilymph, lies on either side of the cochlear duct. The organ of Corti runs along the cochlear duct, on a shelf called the basilar membrane, like a long ribbon. Its main parts are hair cells, supporting cells, and the tectorial membrane. The hair cells are the receptor cells of hearing. Sound waves cause the tympanic membrane and ossicles to vibrate. As the stapes vibrates back and forth, it pushes and pulls on the membrane covering the oval window of the cochlea. This sets the fluid in the cochlea in motion, which causes the hair cells and tectorial membrane to rub against each other. This bends sensory hairs, generating nerve impulses that are transmitted to the brain and interpreted as sound.

What is the function of the iris?

The iris controls the amount of light that enters the posterior part of the eyeball. The pupil enlarges in low light conditions and gets smaller in bright light.

What is the iris?

The iris is a pigmented muscular diaphragm that controls the amount of light that enters the posterior part of the eyeball. The opening at its center is called the pupil. The pupil enlarges in low-light conditions and gets smaller in bright light.

Describe the origin of tears and explain how tears flow onto and drain from the eye.

The lacrimal apparatus includes the structures that produce and secrete tears and the structures that drain them away from the surface of the eye. Tears are produced by the lacrimal glands and the accessory lacrimal glands of the third eyelids. The lacrimal glands are the primary source of tears. They are located dorsal and lateral to each eye inside the bony orbits that protect the eyeballs. Several small ducts from each gland deposit tears in the dorsal conjunctival sacs; from there, tears wash down over the surface of the eyes, aided by blinking movements of the eyelids. Tears are constantly being produced, so they must constantly be drained from the surface of the eye to prevent their spilling down the animal's face. Two small openings, one each in the upper and lower eyelid margins, drain the tears away from the surface of each eye. The openings are called the lacrimal puncta, and they are located near the medial canthus of each eye. From the lacrimal puncta on each side, the tears flow down two small ducts to the lacrimal sacs and then down single ducts, called the nasolacrimal ducts, to the nasal cavity.

What is the lens of the eye?

The lens of the eye is a soft, transparent structure made up of layers of microscopic fibers that are arranged like the layers of an onion. It is elastic and biconvex (meaning it bulges out on both sides). Its normal shape is fairly rounded, but it can be pulled into a flatter shape if tension is applied equally around its equator. The front (rostral) surface of the lens is in contact with aqueous humor, and its back (caudal) surface is in contact with vitreous humor. The main role of the lens is to help focus a clear image on the retina regardless of whether the object being viewed is close up or far away. It does this with the help of the muscles of the ciliary body through a process called accommodation.

Describe how the sense of taste works.

The majority of the taste buds are located on the sides of certain small, elevated structures on the tongue called papillae, although a few can be found in the lining of the mouth and throat (pharynx). Taste buds are tiny, rounded structures made up of gustatory (sensory) cells and supporting cells. Tiny openings on the surface of each taste bud, the taste pores, allow dissolved substances to enter the taste buds and contact the sensory receptors. The sensory receptors are tiny, hairlike processes (modified dendrites) from the gustatory cells that project up into the taste pores. When appropriate chemical substances dissolved in the saliva come in contact with the sensory processes, nerve impulses are generated that travel to the brain and are interpreted as particular tastes.

Describe the structure and function of the components of the middle ear.

The middle ear cavity is a hollowed-out area of the temporal bone that is lined by soft tissue membranes. It is filled with air and contains three small bones called ossicles and the opening of the Eustachian tube, which connects it with the pharynx (throat). Laterally it is separated from the external ear by the tympanic membrane, and medially it is separated from the inner ear by the membranes that cover the oval and round windows of the cochlea. Three small bones called ossicles link the tympanic membrane with the cochlea of the inner ear, where the receptors for hearing are located. The outermost bone, the malleus (hammer), is attached to the tympanic membrane. The malleus forms a complete synovial joint with the middle bone, the incus (anvil), which in turn forms a joint with the medial-most bone, the stapes (stirrup). The other end of the stapes is attached to the membrane that covers the oval window of the cochlea. The ossicles act as a system of levers that transmit the sound wave vibrations from the tympanic membrane to the cochlea. The Eustachian tube, also called the auditory tube, connects the middle ear cavity with the pharynx. Its purpose is to equalize the air pressure on the two sides of the tympanic membrane.

What is wind-up pain?

The neurons of the spinal cord are bombarded with painful stimuli during surgical procedures despite the fact that the conscious mind is temporarily disconnected from the process by the general anesthetic. Because the spinal cord is capable of changing (modulating) the information it forwards on to the brain, this sensory assault during surgery can cause the pain signals going to the brain to be amplified once the animal wakes up from the general anesthesia. This can make the animal's postoperative pain level even more severe than the tissue damage caused by the surgery would seem to warrant. This exaggerated pain response is referred to as wind-up, and it can cause significant stress on a postsurgical patient.

Why is the optic disc the blind spot of the eye?

The optic disc contains only nerve fibers and a few blood vessels but no photoreceptor cells, so no visual images are formed there.

How would an animal probably feel if they had a middle ear infection that caused the opening of the Eustachian tube to swell closed?

The purpose of the eustachian tube is to equalize the air pressure on the two sides of the tympanic membrane. With an infection that closes it off, this could not happen and would cause pain as the membrane bulges in and out, because the tympanic membrane has many pain receptors.

What is the retina?

The retina lines the back of the eye and contains the actual sensory receptors for vision, the rods and cones.

What is the sclera?

The sclera is the "white" of the eye. Like the cornea, it consists mainly of collagen fibers and makes up the majority of the outer fibrous layer of the eye. The junction of the cornea and the sclera is called the limbus.

Describe how the sense of smell works.

The sense of smell is organized in two patches of olfactory epithelium located up high in both nasal passages. Sensory (olfactory) cells are mixed with supporting cells in these epithelial patches. Hairlike processes (modified dendrites) from the surfaces of the olfactory cells project up into the mucous layer that covers the nasal epithelium. When odor molecules dissolve in the mucus and contact the sensory processes, nerve impulses are generated that travel to the brain and are interpreted as particular smells.

List the four general types of stimulus that can trigger a response from sensory receptors.

The sensory receptors of common domestic animals are sensitive to only four general types of stimulus: 1. Mechanical stimuli (e.g., touch, hearing, balance). 2. Thermal stimuli (e.g., hot and cold). 3. Electromagnetic stimuli (e.g., vision). 4. Chemical stimuli (e.g., taste and smell).

How might examination of the bulbar and palpebral portions of the conjunctiva be useful as part of the overall physical examination of an animal?

The transparency of the conjunctiva allows the underlying tissues to show through, so it can be used as a window to see the blood vessels that are hidden elsewhere in the body by opaque structures, such as the skin. By looking through the conjunctiva at the lining of the eyelid, we can often detect abnormalities such as anemia (pale color caused by decreased blood flow), jaundice (yellowish color), and cyanosis (dark purplish color).

How many senses are there?

There are ten senses: five general senses and five special senses. General senses include: visceral sensations, touch, temperature, pain, and proprioception. Special senses include: taste, smell, hearing, vision, and equilibrium.

What is the basic cause of motion sickness?

There is a disagreement among the sensory receptors for the vestibular system, eyes, and proprioception, which can result in the unpleasant sensations of motion sickness, such as headache, nausea, and vomiting. The eyes look around the interior of the vehicle and see that nothing is apparently moving, but the equilibrium receptors and proprioceptors detect motion.

What are the three main layers that make up the overall liquid film that moistens and protects the surface of the eye?

These are an inner mucous layer, a middle tear layer, and an outer oily layer. The inner mucous layer comes from cells in the conjunctiva. It contains antibacterial substances that help protect the eye from infection. The middle tear layer comes from the lacrimal glands and the accessory lacrimal glands of the third eyelids. It serves to keep the cornea moist. The outer oily layer comes from the tarsal (meibomian) glands. It helps reduce evaporation of the underlying tear layer and prevents tears from flowing over the lid margin.

Why do nonhuman animals often greet others by sniffing them?

They live in more of a smell-oriented world. They communicate and understand information by their sense of olfaction. For example, dogs obtain a huge amount of information from sniffing the air or an object where other animals have been.

Why do touch and pressure sensations fade so rapidly from the conscious mind unless they change or are severe?

Touch and pressure sensations are not threats to the well-being of the animal and tend to fade from the conscious mind, allowing the CNS to focus on other more important sensations.

What sometimes makes upper respiratory tract infections dangerous for domestic animals?

URI infections effectively eliminate the animals' sense of smell. They often stop eating and drinking completely, because they cannot smell anything. If this continues for very long, they can be in real danger from dehydration.

Which layer of the eyeball are the iris and ciliary body part of?

Uvea

Why are visceral sensations important to the survival of an animal?

Visceral sensations keep the CNS informed about the overall prevailing conditions inside and outside the body. The result is the initiation of behaviors designed to ensure the well-being of the animal.

List and describe the visceral senses.

Visceral senses include the sensations of hunger and thirst, which indicate deficiencies of nutrients and water. Other visceral sensations originate in internal organs, particularly hollow organs. They have only certain, specific kinds of receptors, particularly stretch receptors. With the exception of the urinary bladder, anything that stretches the wall of the organ, such as a bubble of gas in the intestine or a stone (calculus) in the ureter, can be intensely painful. The pleura and peritoneum (membranes that line and cover the contents of the thorax and abdomen, respectively) are well supplied with sensory receptors. As long as conditions are normal and the pleural and peritoneal surfaces slide over each other smoothly, no sensation is felt. However, if the surfaces become roughened by inflammation and/or infection, the resulting pleuritis or peritonitis is very painful.

How is the physical concept of inertia important to the functioning of the semicircular canals?

When the head moves in the plane of one of the semicircular canals, inertia causes the endolymph to lag behind the movement of the canal itself. The relative movement of the endolymph pulls on the cupula, which bends the hairs. This generates nerve impulses that give the brain information about the rotary motion of the head.

How would arthritis in the tiny joints of the ossicles affect hearing? Could this possibly affect the hearing of older animals?

Yes, arthritis could affect hearing in older animals. Arthritis could decrease the conduction of sound waves from the tympanic membrane to the oval window. The force of the vibrations would be decreased and the animal's hearing might be impaired.