4.2 Eyes, Ears, and Additional Special Sense Organs

Overview

In this chapter, we will discuss the senses, including common terminology and abbreviations. The first two major senses are seeing and hearing, which will be discussed in detail. The other senses we will discuss are feeling, smelling, and tasting.

Animals are almost always aware of the stimuli in their environment, meaning the senses are in a constant state of work. Given an animal’s need for survival, their different senses have evolved to adapt to their environment. For example, sharks are electrosensitive, making them able to sense other animals through electromagnetic fields (Molnar & Gair, 2021).

Stimuli such as temperature, pain, pressure, and vibration are called somatosensations. Being aware of orientation and body movement and position are all part of somatosensation (Molnar & Gair, 2021).

The Eye

Vision is the ability to detect light stimuli from the environment and interpret them into images. This is the sense of sight. Within the skull are two bony orbits in which the eyeballs are located. The orbits and the surrounding soft tissue protect the eyes.

Path of Vision

Light passes through the cornea and enters the eye. The lens refracts the light towards the retina, which is where the receptors are located that convert light to electrical impulses that can be carried via nerves to the brain (Jennings & Premanandan, 2017).

Fun Facts

Where the eyes are positioned on an animal’s head can greatly affect their vision. Animals with eyes on the front of their face (commonly predators) have a larger field of binocular vision in front of them, allowing for greater depth perception. This is useful for hunting.

Prey animals tend to have eyes on the sides of their heads, which gives them an overall larger field of view, though with less depth perception. This allows them to detect movement in more directions, which can help them see predators.

 

^{CrashCourse. (2015, May 11). Vision: Crash Course anatomy & physiology #18 [Video]. YouTube. https://www.youtube.com/watch?v=o0DYP-u1rNM}

Structures of the Eye

Conjunctiva: Epithelial tissue lining the inside of the eyelids and connecting it to the sclera of the eye

Cornea: The front transparent layer of the eye; helps focus light that enters the eye

Eye muscles: Six major muscles are attached to the eye, allowing it to move

Eyelashes: Also called cilia; hairs that protect the eyes from bright light and foreign objects

Eyelids: Upper and lower eyelids cover the eyes during sleep, protect them from foreign objects or too much light, and spread tears over the eye surface

Globe: The entirety of the “eyeball”

Iris: The pigmented ring of smooth muscle that changes the size of the pupil. It constricts the pupil (miosis) in response to bright light and dilates the pupil (mydriasis) in response to dim light.

Lacrimal apparatus (gland or duct): Also called the tear duct; stores, produces, and removes tears

• The nasolacrimal duct drains tears into the nose.

Nictitating membrane: Also called the third eyelid; conjunctiva that is a transparent sheet that moves sideways across the eye from the medial canthus, cleansing and moistening the cornea without shutting out light

Orbit: Deep depressions in the skull that contain the globes

Pupil: The opening at the centre of the eye that allows light to reach the back of the eye

Retina: The layer of photoreceptive and supporting cells on the inner surface of the back of the eye. It is composed of several layers and contains specialized cells for the initial processing of visual stimuli.

Sclera: Visible as the “white” of the eye (Betts et al., 2013); provides support and protection for the eye

(Sturdy, 2022)

Common Pathological Conditions of the Eye

Blepharitis: Inflammation of the eyelid

Cataract: A clouding of the normally clear lens of the eye

Conjunctivitis: Inflammation of the conjunctiva

Corneal ulcer: Injury of the cornea that causes it to become thinner

Ectropion: A medical condition in which the lower eyelid turns outward

Entropion: A medical condition in which the eyelid (usually the lower lid) folds inward

Glaucoma: Increased intraocular pressure (pressure within the globe)

Keratoconjunctivitis sicca: Also called dry eye; a tear disorder that causes the conjunctiva and cornea to dry out

Nictitating gland prolapse: Also called cherry eye; the prolapse of a gland on the medial portion of the eye

Proptosis: A condition in which the globe bulges out of the orbit

Example

Figure 4.4 is an example of cherry eye. Repairs to the nictitating membrane could include surgically stabilizing it or surgically removing the membrane itself. This is most common in dog breeds such as the Boston terrier, cocker spaniel, and Pekingese (Hamor, 2023).

 

Common Eye Procedures and Instruments

Blepharoplasty: Surgical repair of an eyelid

Enucleation: Surgical removal of an eyeball

Fluorescein stain: A non-invasive diagnostic procedure used to detect and evaluate corneal integrity (detects corneal ulcers). The bright yellow stain is placed on the cornea, and if there is damage, the dye will stick to it and glow when looked at under a blue light.

Ophthalmoscope: An instrument used to view the eye

Schirmer tear test: A test used to determine whether the eye produces enough tears to keep it moist

Tonometer: An instrument that measures intraocular pressure

Tonometry: The measurement of intraocular pressure

The Ear

Hearing, or audition, is the conversion of sound waves into a neural signal that is made possible by the structures of the ear. Audition is important to animals for many different interactions. It enables an animal to detect and receive information about danger, such as an approaching predator, and to participate in communal exchanges like those concerning territories or mating (Hinic-Frlog, n.d.). The ear is also important in maintaining equilibrium or balance.

Auditory Pathway

In mammals, sound waves are collected by the external, cartilaginous part of the ear called the pinna (pl. pinnae), then travel through the auditory canal and cause vibration of the eardrum (tympanum) (Hinic-Frlog, n.d.). Sound is then conducted via three small bones, which then causes movement of fluid, which is detected via receptors and converted to an electrical signal for the nervous system to transmit.

^{CrashCourse. (2015, May 4). Hearing & balance: Crash Course anatomy & physiology #17 [Video]. YouTube. https://www.youtube.com/watch?v=Ie2j7GpC4JU}

Structures of the Ear

The structure of the ear can be divided into three parts: outer, middle, and inner.

The structure of the ear can be divided into three parts: outer, middle and inner.

Outer ear

The outer ear consists of

• The pinna (auricle), which is the ear flap
• The vertical and horizontal ear canals

Middle ear

The middle ear consists of

• The tympanum (tympanic membrane), which is also known as the eardrum
• The ossicles, which are three small bones that collect and amplify sounds and transfer energy from the moving tympanum to the inner ear

Inner ear

The inner ear is often described as a bony labyrinth because it is composed of a series of canals embedded within the skull that are responsible for hearing and balance.

 

Common Pathological Conditions of the Ear

Aural hematoma: A collection of blood within the pinna

Example

An aural hematoma, shown in Figure 4.9, can be caused by trauma or an infection. The clinical signs include a thickened and swollen pinna. A common sign a client will see is the animal shaking its head in an effort to relieve the discomfort. Treatment may include drainage and placing sutures in the ear, as well as a prescription for antibiotics.

 

Deafness: Complete or partial hearing loss

Otitis externa: An acute or chronic inflammation of the external ear canal

Common Ear Procedures and Instruments

Cytology: The study of cells

Example

We can swab the external ear canal and then perform cytology to check for ear mites, bacteria, and yeast, all of which can lead to otitis externa.

Otoscope: An instrument used to visually examine within the ear canal

Otoplasty: Surgical repair of the ear

Additional Special Senses

The first step in sensation is reception, which is the activation of sensory receptors by stimuli from external sources such as being touched or feeling changes in temperature (Hinic-Frlog, n.d.). The senses provide information about the body and its environment. The other senses discussed here are touch and pressure, pain, temperature, proprioception, smell and taste.

Touch and Pressure

Sensory nerves in the skin allow the animal to perceive pressure, temperature, and pain. The nerve endings are found throughout the skin, and some areas are more sensitive than others. Whiskers (vibrissae), for example, are long, tactile hairs that are sensitive to touch and give the animal more information about its environment, such as the size of hole they might want to get into.

Pain

Sensory receptors that respond to pain are known as nociceptors. These receptors are found throughout the entire body. They tell the animal that tissues are dangerously hot, cold, compressed, or stretched or that there is not enough blood flowing in them. The animal may then be able to respond and protect itself from further damage.

Temperature

Nerve endings in the skin that respond to hot and cold stimuli are known as thermoreceptors. The majority of these receptors are in the dermis and will perceive temperatures as hot or cold, allowing the animal to react accordingly.

Proprioception

Proprioception is the sensation that allows animals to know where parts of their body are in space, even if they cannot see them. This is easy to understand when you see an animal walk on all four legs, yet they can’t see their back legs as they walk.

Taste 

Both taste (gustation) and odour stimuli are molecules taken in from the environment (Hinic-Frlog, n.d). All odours that we perceive are molecules in the air we breathe. Different species have different numbers and types of tastebuds, so the same food might taste differently to each species. For example, it’s been determined that cats can’t taste sweet flavours, whereas dogs can.

Smell

Smell, known as olfaction, is a very primitive sense and allows an animal to sense the presence of food, other animals, or chemicals that can impact their survival. Animals also release different smells as messages to communicate between members of their own species and with other species. Many species have a more powerful sense of smell than that of humans. Bears, dogs, and elephants are known for their superior sense of smell.

A pheromone is a chemical released by an animal that affects the behaviour or physiology of other members of the same species. Pheromonal signals can have profound effects on animals that inhale them, but pheromones apparently are not consciously perceived in the same way as other odours. There are several different types of pheromones that are released in urine or as glandular secretions. Certain pheromones are attractants to potential mates, others are repellants to potential competitors of the same sex, and still others play roles in mother-infant attachment. Some pheromones can also influence the timing of puberty, modify reproductive cycles, and even prevent embryonic implantation.

The structure that is involved with the detection of pheromones is the vomeronasal organ. When an animal curls their upper lip, it helps bring the pheromones towards this organ and allows the animal to get a better sense of them (Hinic-Frlog, n.d). This is called the flehmen response and is shown in Figure 4.11.

 

References

Betts, J. G., Young, K. A., Wise, J. A., Johnson, E., Poe, B., Kruse, D. H., Korol, O., Johnson, J. E., Womble, M., & DeSaix, P. (2013). Anatomy and physiology. OpenStax. https://openstax.org/details/books/anatomy-and-physiology, licensed under CC BY 4.0

Hamor, R. E. (2023). The conjunctiva in animals. In Merck veterinary manual: Eye diseases and disorders. Merck. https://www.merckvetmanual.com/eye-diseases-and-disorders/ophthalmology/the-conjunctiva-in-animals

Hinic-Frlog, S. (n.d.). Introductory animal physiology. University of Toronto Mississauga. https://ecampusontario.pressbooks.pub/introanimalphysiology, licensed under CC BY 4.0

Jennings, R., & Premanandan, C. (2017). Veterinary histology. Ohio State University. https://ohiostate.pressbooks.pub/vethisto, licensed under CC BY-NC 4.0

Molnar C., & Gair, J. (2021). Concepts of biology – 1st Canadian edition. BCcampus. https://opentextbc.ca/biology/, licensed under CC BY 4.0

Woodward, M. (2020). Otitis externa in animals. In Merck veterinary manual: Ear disorders. Merck. https://www.merckvetmanual.com/eye-diseases-and-disorders/ophthalmology/the-conjunctiva-in-animals

Image Credits
(images are listed in order of appearance)

Eye with labels by National Eye Institute, Public domain

Anatomy and physiology of animals Well developed binocular vision by Sunshineconnelly, CC BY 3.0

Dog eye head by Sabrinasfotos, Pixabay license

Prolapsed gland of the third eyelid by Joel Mills, CC BY-SA 3.0

Split corneal dystrophy by Joel Mills, CC BY-SA 3.0

Haine Oftamoskop by Janeer, CC BY-SA 3.0

The Ear by Kelly Robertson, NorQuest College. Used with permission.

Anatomy and physiology of animals The ear by Sunshineconnelly, CC BY 3.0

Otitis Othaematom Hund by Kalumet, CC BY-SA 4.0

Psoroptes-cuniculi-ear-canker-mite by Daktaridudu, CC BY-SA 4.0

Flehmendes Pferd 32 c by Waugsberg, CC BY-SA 3.0