The special senses include smell, taste, vision, hearing, and equilibrium. Unlike the sense of touch that generally involves free nerve endings, special senses are dependent on specific receptor cells localized in the head region of the animal. Both smell and taste are chemical senses involving chemoreceptors. Olfaction involves the detection of volatile chemicals in a solution. The olfactory epithelium is in the roof of the nasal cavity along the inferior surface of the cribriform plate of the ethmoid bone and extends along the superior nasal concha and upper region of the middle nasal concha. It consists of olfactory receptors, supporting cells, and basal stem cells. Olfactory receptors are bipolar neurons (Fig. 11.1). At their apical end, the dendrites form a knob from which several long cilia project. These cilia lie flat on the nasal epithelium and are covered with a thin mucus layer produced by the supporting cells and olfactory glands. Unlike other cilia, these remain stationary. At their basal end, the axon from the olfactory receptor projects through the cribriform plate and into the olfactory bulb. The supporting cells are columnar epithelial cells surrounding the olfactory receptors. They provide physical support, cushioning, nourishment, and electrical insulation for the olfactory receptor cells. They also contain a yellow‐brown pigment that gives the olfactory epithelium a yellow tint (Box 11.1). Basal stem cells occur between the supporting cells. With ongoing cell division, they provide replacement olfactory receptors as required. The olfactory receptors function for approximately 1 month before they become senescent and are replaced. This population of neurons is very unusual given most neurons are long‐lived and rarely if ever replaced (see Chapter 9). Olfactory (Bowman’s) glands reside in the connective tissue surrounding the olfactory epithelium. These glands produce mucus that coats the surface of the olfactory epithelium to provide a matrix to dissolve odorants or chemicals that can stimulate the olfactory hairs. The facial nerve (cranial nerve VII) innervates supporting cells and olfactory glands. When the olfactory receptors are stimulated, nerve impulses in the facial nerve can also produce stimulation of the lacrimal glands in the eyes and nasal mucous glands. This can cause tearing and a runny nose. In addition to the MOE, the VNO extends cranially and inferior from the MOE. The VNO like the MOE is well positioned to respond to odorants but the VNO is more often linked to the detection of pheromones. Evolutionarily, the vomeronasal system apparently evolved from amphibians and then into primitive mammals, it is especially well developed in rodents (Tirindelli, 2021; Murata et al., 2024). The structure is mostly absent in birds, but less developed structures are present in dogs and cats as well as in multiple farm animals (e.g., cows, sheep, pigs, goats, and horses). The internal space of the VNO is covered by a pseudostratified sensory epithelium such as the MOE. The associated chemosensory neurons project an axon to the accessory olfactory bulb with dendrites positioned into the mucus‐covered epithelial layer. Because this epithelial layer is not directly in the air stream of the nasal passage, chemical signals must enter the nasal or oral cavity, and dissolve or interact with the mucus layer to interact with the sensory receptors. The flehmen reaction, a repetitive oral‐facial gesture characterized by lifting the upper lip and stretching the head, may in part be a mechanism to increase movement of molecules across the receptor area of the VNO. The reaction is seen in horses, bulls, and cats for example. Sniffing is likely a reaction to help force air laden with odorants into the pouch‐like areas of the VNO, Gruenberg Ganglion, or Sepal Organ of Mesera, which is especially prominent in rodents. Fig. 11.1 Olfactory receptors. Olfactory receptor cells are interspersed among supporting cells. The olfactory cilia, embedded in the mucous layer of the olfactory epithelia, detect odorants causing the development of a receptor potential in the olfactory cells. Axons project from the olfactory receptors forming the olfactory nerve cranial nerve I. Odorants dissolved in the mucus of the olfactory epithelium bind to surface receptors on the olfactory cilium membrane, which stimulate G proteins which in turn activate adenylate cyclase. Adenylate cyclase catalyzes the production of cyclic adenosine monophosphate (cAMP), which causes Na+ channels to open. The influx of Na+ depolarizes the olfactory receptor creating an excitatory receptor potential. The unmyelinated axons of the olfactory receptors constitute the first‐order neurons, which pass through the numerous olfactory foramina in the cribriform plate of the ethmoid bone. These axons collectively coalesce into the right and left olfactory nerves (cranial nerve I). They synapse on secondary neurons located within the olfactory bulbs located just below the frontal lobes of the cerebrum. Neurons from the olfactory bulb extend posteriorly in the olfactory tract, projecting to the lateral olfactory area in the temporal lobe. The olfactory area is part of the limbic system. The olfactory neurons also project to the hypothalamus and other limbic areas, this explains how smell can evoke various memories and emotions. The muscular tongue fills most of the oral cavity. It is composed of interlacing bundles of skeletal muscle fibers and is critical for gripping, repositioning food, mixing food with saliva, and forming a compact mass of food called a bolus. The tongue has intrinsic and extrinsic muscles. The intrinsic muscles are confined to the tongue, not attached to bone, and run in several directions, allowing the tongue to change shape as necessary for prehension, moving food, and making sounds. The extrinsic muscles attach the tongue to the bones of the skull and the soft palate. They allow the tongue to protrude, retract, and move from side to side. The lingual frenulum attaches the tongue to the floor of the mouth. The surface of the tongue is covered with small bumps called papillae (Latin for “bumps”; singular = papilla). Papillae are named for their shape. Filiform papillae are thorn‐shaped, giving the tongue roughness and thus aiding in licking and manipulating food. Their primary function is mechanical. In the ox and cat, they are heavily cornified. Fungiform papillae are mushroom‐shaped, scattered among the more numerous filiform papillae, have taste buds, and are both mechanical and gustatory (Fig. 11.2). Foliate papillae have a series of leaf‐shaped ridges, are located on the lateral borders of the tongue, and have a gustatory function. They are absent in the ox. Vallate, or circumvallate, papillae are the largest and least numerous. They occur in a V‐shaped row near the back of the tongue. They resemble the fungiform papillae but are circled by a cleft populated with taste buds. Marginal taste buds occur along the edge of the rostral portion of the tongue of newborn dogs, but they disappear when puppies switch to solid food. Fig. 11.2 Shape of papillae. (A) Filiform papillae, which are thorn‐shaped. (B) Fungiform papillae, which are mushroom‐shaped. (C) Filiate papillae, which are leaf‐shaped. (D) Vallate, or circumvallate papillae, which are similarly shaped to fungiform, but have clefts along the side that contain taste buds. Taste is an important sense in animals because it helps differentiate nutritious feedstuffs from toxic materials. Animals also have innate preferences for certain flavors, such as those that are sweet, while tending to reject those that are unpleasant, such as bitter flavors. Each papilla has one to hundreds of taste buds. Taste buds are also located in the palate, pharynx, epiglottis, and upper third of the esophagus. Each gourd‐shaped taste bud has three types of taste cells, called light, intermediate, and dark cells, as well as basal cells (Fig. 11.3). The different taste cells are believed to be either cells at different stages of differentiation, with the lightest being the most mature, or alternatively cells of different cell lineages. The basal cells are likely stem cells for the taste cells, which are short‐lived. There is a small opening at the surface of the epithelium called the taste pore. Each taste cell has microvilli extending into the taste pores. The microvilli are the only part of the cell exposed to the oral cavity. Each taste cell is innervated by sensory gustatory neurons. The space between the taste cells and sensory neurons functions as a synapse. Taste cells are electrically excitable and have voltage‐gated Na+, K+, and Ca++ channels on their surfaces. Until recently, it was believed that animals had four primary tastes such as salty, sour, sweet, and bitter. In recent years, an additional classification was added, called umami, which means “delicious” in Japanese. Monosodium glutamate stimulates this taste response. Foods can provide a different taste by stimulating varying combinations of these five tastes. In addition, volatile components in feedstuffs also stimulate the sense of smell, which also strongly impacts the sense of taste. The binding of molecules soliciting taste stimuli, called tastants, to surface receptors on the taste buds elicits depolarization of the taste cell either directly or by inducing activation of second messengers within the cell. The changes in receptor potential produced by depolarization change the influx of Ca++ by opening voltage‐gated Ca++ channels. This causes the release of an unidentified neurotransmitter which excites the sensory neuron. Fig. 11.3 Taste bud. Each taste bud consists of three types of taste cells dark, intermediate, and light cells. In addition, there are small, round basal cells located at the base of the taste bud. The basal cells are thought to be stem cells that produce taste cells. Sweet taste, elicited by sugars, signifies the presence of carbohydrates and consequently a link to energy metabolism. Imagine the reactions of a black bear raiding a beehive full of honey. Two types of receptor groups are known to be involved in soliciting the sweet response. One includes the G‐protein‐coupled receptors TAS1R2/TAS1R3. When activated, this leads to a cascade involving a G protein gustducin, phospholipase Cβ2 (PLCβ), and a transient receptor potential channel M5 (TRPM5). The other receptor involved is a glucose transporter. With the passage of glucose by the transporter, metabolism leads to increased adenosine triphosphate (ATP), which inhibits the opening of K+ATP channels. This promotes depolarization and therefore promotes gustatory sensory signaling. Interestingly, there is also strong evidence for the presence of “sweet” detectors or receptors in multiple organs and tissues including the oral cavity generally, intestine, pancreas, brain, respiratory epithelium, and bone. Finally, it seems likely that leptin, a hormone produced by adipocytes may bind to its receptor in sweet‐sensitive cells. Leptin binding leads to the activation of phosphoinositide 3‐kinase (PI3K), then the production of phosphatidylinositol (3,4,5)‐triphosphate (PIP3), and then the phosphorylation of Abd then the phosphorylation of a serine/threonine protein kinase (AKT). AKT indicates a family of serine/threonine kinases with at least three isoforms capable of acting on a very large number of substrates. Ultimately, phosphorylation reactions lead to increased activation of K+ATP, resulting in hyperpolarization and therefore a suppressed sweet response. This seems relevant given leptin’s involvement in appetite control and hunger responses (Wang et al., 2024; Yoshida and Ninomiya, 2024; Fig. 11.4). Fig. 11.4 Sweet taste. There are two mechanisms for sweet taste. (1) A sweet tasting binds to a specific receptor inducing the formation of a G protein that activates adenylyl cyclase (AC), thus causing the formation of cAMP. The cAMP activates protein kinase A (PKA) which then closes membrane‐bound K+‐channels, thus causing depolarization of the taste cell. This allows voltage‐gated Ca++ channels to open, and Ca++ enters the cell, causing the release of the neurotransmitter that excites the gustatory sensory neurons. (2) G proteins activate phospholipase C (PLC), which catalyzes the formation of 1,4,5‐triphosphate (IP3). IP3 causes the release of calcium from endogenous stores in the endoplasmic reticulum. Ca++ then causes the release of the neurotransmitter. A bitter taste, often associated with toxic compounds, is elicited by divalent cations, some amino acids, and alkaloids. Some bitter‐tasting compounds (such as quinine) are membrane permeable, while others (such as denatorium) are not, suggesting multiple mechanisms for transduction. The mechanisms for bitter taste appear like those for sweet tastes (Fig. 11.5). Denatorium causes the increase in intracellular calcium through an IP3‐dependent mechanism. Other bitter compounds activate gustducin that stimulates taste cell phosphodiesterase reducing intracellular cAMP and cyclic guanosine monophosphate (cGMP) levels. Knockout mice, in which the gustducin gene has been deleted, are insensitive to selected bitter tastes. Some bitter compounds are also thought to bind directly to K+‐selective ion channels blocking them. This prevents the movement of K+ out of the cell, thus causing depolarization release of neurotransmitters and signaling for the sensation of bitterness. A salty taste is mostly caused by the cation Na+. Taste cells responding to salt have a Na+‐selective ion channel that can be blocked with amiloride (Fig. 11.5). This amiloride‐sensitive channel is different from the voltage‐gated sodium channels responsible for the upstroke in regular action potentials. In these cells, the sodium channel is not voltage‐sensitive and remains open all the time. When an animal consumes feedstuffs high in sodium, this increases the Na+ concentration outside the salt‐sensitive taste cell. The extra Na+ moves down its concentration gradient into the cell to elicit depolarization. The sour taste in food or feed depends on the acidity of the feedstuff. The higher the H+ ion concentration, the greater the sour sensation. The H+ ions can affect the sour taste receptors in two ways. First, H+ can permeate amiloride‐sensitive Na+ channels, which leads to depolarization of the cell because the cell cannot distinguish between Na+ and H+. Second, H+ can bind to and block K+‐selective channels, preventing the movement of K+ out of the cell (Fig 11.5). Because amino acids are a major part of dietary intake, it should come as no surprise that animals have a taste for amino acids. Studies with glutamate and aspartate show at least two pathways for this taste. Glutamate can activate an ion channel, allowing Na+ and Ca++ to enter the cell (Fig. 11.6). This inward current causes voltage‐gated Ca++ channels to open, causing depolarization that triggers neurotransmitter release. The second mechanism entails glutamate binding to a G protein‐coupled receptor, probably resulting in a decrease in intracellular cAMP levels. Information from taste buds in the anterior two‐thirds of the tongue and on the palate travels in the chorda tympani, a branch of the facial nerve (cranial nerve VII). Signals from the taste buds in the posterior third of the tongue are carried via the glossopharyngeal nerve (IX). Taste buds on the epiglottis and esophagus are innervated by the vagus (cranial nerve X). Fibers carrying this information enter the solitary tract in the medulla and enter the gustatory area of the nucleus of the solitary tract. Second‐order fibers then ascend to the thalamus, synapsing in the parvocellular region of the ventral posterior medial nucleus. This area then projects to the area along the border between the anterior insula and frontal operculum in the ipsilateral cerebral cortex. Note that this pathway does not cross to the contralateral side (Box 11.2). Fig. 11.5 Salty, sour, and bitter taste. (1) Salty taste is elicited by cations, primarily Na+, which enter through a Na+‐selective ion channel that can be blocked by amiloride. The entry of Na+ causes depolarization of the taste cell and release of the neurotransmitter, which causes excitation in the gustatory neurons. (2) Sour taste is caused by the entry of H+ ions that permeate amiloride‐sensitive Na+ channels; this leads to depolarization of the cell because the cell cannot distinguish between Na+ and H+. A sour taste can also be elicited by H+ binding to and blocking K+‐selective channels, preventing the movement of K+ out of the cell. (3) Bitter taste is caused by either increasing intracellular calcium through an IP3‐dependent mechanism or by activating the G‐protein gustducin that stimulates taste cell phosphodiesterase (PDE), thus reducing intracellular cAMP and cGMP concentrations. Fig. 11.6 Umami taste. There are two proposed umami pathways: (1) the first involves a glutamate binding to a receptor which activates a G protein (i.e., a metabotropic receptor). The activated G protein causes a decrease in cAMP levels, resulting in a decrease in the activation of phosphokinase A (PKA), and thus a decrease in the metabolic pathway downstream. (2) The second pathway involves an ionotropic receptor in which glutamate interacts with a receptor on the taste bud surface causing the influx of Na+ and Ca++. The resulting depolarization results in neurotransmitter release. Table 11.1 Human scent survival on different materials at three temperatures. Modified from Zahid et al. (2024). Vision is a major sensation in many animals. This is reflected in the large number of receptors within the eye and the large percentage of the cortex that is devoted to vision. Vision is particularly important in birds. Although the eyes make up only about 1% of the weight of the head in humans, they make up approximately 15% in a starling. The accessory structures of the eye include the eyelids, eyelashes, eyebrows, lacrimal (tearing) apparatus (Fig. 11.7), and extrinsic muscles of the eye. The upper and lower eyelids, or palpebrae, cover the eye during sleep, protect the eye from excessive light and foreign objects, and assist with lubricating the eye (Fig. 11.8). The layers of the eyelid consist of the superficial epidermis, dermis, subcutaneous tissue, fibers of the orbicularis oculi muscle, a tarsal plate, tarsal glands, and conjunctiva. The tarsal plate is a fold of connective tissue giving support to the eyelid. Embedded in the tarsal plate is a row of sebaceous glands called the tarsal, or Meibomian, glands. They secrete fluid that helps prevent the eyelids from adhering to one another. The conjunctiva is a thin, protective mucous membrane consisting of stratified columnar epithelium with goblet cells and areolar connective tissue. It lines the inside of the palpebrae and the anterior surface of the eyeball, excluding the cornea. Fig. 11.7 External structures of the eye in the goat. (A) Frontal aspect of the eye and lacrimal apparatus. (B) Eyelids and eye of the goat. Reprinted from Constantinescu (2001). Used by permission of the publisher. Fig. 11.8 Front view of the eye of the dog. Reprinted from Constanstinescu (2002). Used by permission of the publisher. The gap between the two eyelids is the palpebral fissure. At either corner of the eyelid is the lateral and medial commissure, respectively. A small, reddish elevated area found in the medial commissure is the lacrimal caruncle. It contains both sebaceous (oil) and sudoriferous (sweat) glands. Domestic species have a nictitating membrane or third eyelid (Fig. 11.9). This fold of mucous membrane arises from the ventromedial border of the eye. A serous gland, the gland of the third eyelid is at its base. The margins of the eyelids display cilia or eyelashes. The eyebrows are positioned just above each eyelid. Eyelashes and the eyebrow both help keep foreign objects, perspiration, and direct sunlight out of the eye. Located at the base of the hair follicles of the eyelashes, sebaceous ciliary glands release a lubricating fluid. Infection of these glands can produce a sty. The lacrimal apparatus (Fig. 11.9) is a group of structures that produce and drain tears (lacrimal fluid). Lacrimal glands, located in the dorsolateral portion of the orbit, secrete lacrimal fluid through excretory lacrimal ducts that empty onto the surface of the conjunctiva of the upper eyelid. The fluid moves over the anterior surface of the eye and enters two small openings on the upper and lower palpebrae near the medial corner called the lacrimal puncta. From there, the fluid enters the lacrimal canals, two ducts leading into the lacrimal sac. The nasolacrimal duct carries fluid from the lacrimal sac into the nasal cavity just below the inferior nasal concha. The lacrimal fluid contains water, salts, mucus, antibodies, and lysozyme. This fluid protects, lubricates, and moistens the eyeball. The moisture seen on the noses of domestic animals is mostly lacrimal fluid. Movement of the eyeball is controlled by six striated muscles called extraocular muscles, in contrast to intraocular muscles located within the eyeball (Fig. 11.10). The lateral and medial rectus muscles move the eye laterally and medially, respectively. The superior rectus and inferior rectus muscles elevate and depress the eye, respectively. The inferior oblique muscle elevates and turns the eye laterally, while the superior oblique depresses and turns the eye laterally. Fig. 11.9 Eye and accessory structures of the dog. Reprinted from Constanstinescu (2002). Used by permission of the publisher. Fig. 11.10 Muscles of the eye of the dog. The left drawing shows the lateral aspect of the eye while the right drawing shows the posterior aspect of the eye. Reprinted from Constanstinescu (2002). Used by permission of the publisher. Vision receptors are positioned within the eyeball at the rear. The eyeball has three layers: (1) fibrous tunic, (2) vascular tunic, and (3) retina (Fig. 11.11). The fibrous tunic, or external layer of the eyeball, is avascular and consists of the anterior cornea and posterior opaque sclera. The transparent cornea covers the iris, the colored portion of the front of the eye. The curved cornea helps bend light toward the retina. The cornea has three layers. The outer layer consists of nonkeratinized stratified squamous epithelium, the middle layer includes collagen fibers and fibroblasts, and the inner layer is a simple squamous epithelium. The sclera covers the entire outside surface of the eye except for the cornea. It contains dense connective tissue and provides rigidity and strength to help maintain the shape of the eyeball. The scleral venous sinus (canal of Schlemm) is located at the junction of the sclera and cornea. Aqueous humor drains into this sinus. Figures 4.20, 4.21, and 4.22 (Chapter 4) give examples of the histological structure of the cornea from our discussion of epithelial cell types. Fig. 11.11 Median section of the eye of a dog. M, muscle; N, nerve. Reprinted from Constanstinescu (2002). Used by permission of the publisher. The vascular tunic, or uvea, is the middle layer of the eyeball. It includes three parts: choroid, ciliary body, and iris. The choroid is the highly vascularized, dark brown, posterior portion of the vascular tunic, it lines most of the inside of the sclera. Its brown pigment is produced by melanocytes which help absorb light and limit light scatter. The choroid is incomplete where the optic nerve exits the rear of the eyeball. Many species of domestic animals, including cats, dogs, horses, and ruminants, have an additional layer in the choroids called the tapetum lucidum. This causes the animal’s eyes to appear to glow when shined with a light. The tapetum lucidum reflects light back toward the retina so that the animal can see in low light. In the anterior, the choroid becomes the ciliary body that extends from the ora serrata, the serrated front margin of the retina. It is just posterior to the junction of the sclera and cornea. It includes the ciliary processes and ciliary muscles. The ciliary processes are folds of tissue containing capillaries that secrete aqueous humor. Extending from the ciliary processes to the lens are zonular fibers (suspensory ligaments). The ciliary muscles are bundles of smooth muscles that alter the shape of the lens to allow for near or far vision. The iris is the colored portion at the front of the eyeball that is shaped like a disc with a hole, the pupil, in the center. The color of the iris depends on the number of pigmented cells. Lots of pigmented cells produce a brown color; a low number results in a blue color. Other colors reflect the variation in number of cells. The shape of the pupil varies. It can be round, elliptical, or slit like. Cats have an elliptical pupil that opens and closes faster than round pupils. The iris lies between the cornea in front and the lens to the rear and is attached to the ciliary processes. Consisting of circular and radial smooth muscle fibers, the ciliary process regulates the amount of light entering the eye. Parasympathetic signals stimulate the circular muscles to contract during bright light and close vision, causing the pupil to constrict. During dim light or distant vision, sympathetic signals stimulate the radial muscles to contract, causing the pupil to dilate. Most of us have experienced the uncomfortable response of walking out of an ophthalmic examination into sunlight when these normally occurring parasympathetic signals were temporarily blocked to allow examination of the retina. The innermost layer of the eye, the retina, lines the posterior portion of the eyeball. The retina contains two layers: an outer pigmented layer and an inner neural layer. The pigmented layer is a one cell‐thick layer of melanin‐containing epithelial cells, like the choroid. These cells also act as phagocytes and store vitamin A. The neural portion of the retina is multilayered and develops directly from the brain during embryonic development. Its three major layers are (1) the photoreceptor layer, (2) the bipolar cell layer, and (3) the ganglion cell layer (Fig. 11.12). The outer and inner synaptic layers separate these layers from each other. Before reaching the photoreceptor layer, light must first pass through the ganglion and bipolar cell layers. The light activates the photoreceptors positioned close to the pigmented layer. This generates signals that travel through the outer synaptic layer to the bipolar cells, and through the inner synaptic layer to the ganglion cells. Interspersed among these cells are two other types of neurons: horizontal cells and amacrine cells. These latter neurons form lateral connections that modify signals along the photoreceptive pathway. Axons from the ganglion cells collectively form the optic nerve, which exits the eye at the optic disc. Because the optic disc lacks photoreceptors, it is also called the blind spot. The blind spot normally is not apparent because the brain “fills in” information from this area. However, you can demonstrate its presence to yourself by covering your right eye and gazing at the plus sign below. As you move the position of the book closer or farther from your eye, you will see the large dot disappear: There are two types of photoreceptors: rods and cones. In most animals, rods outnumber cones 20 : 1. Birds are an exception where there are more cones than rods. Rods have a low light threshold, and are more effective in dim light, allowing for the perception of varying shades of gray. Cones require brighter light but provide color and high acuity vision. The macula lutea is an oval region found in the exact center of the posterior of the retina. It contains mostly cones. At its center is a small pit, the central fovea, which contains only cones and where the bipolar and ganglion cells are displaced to the sides. This allows light to pass unimpeded to this population of cones. The density of cones in the retina decreases moving from the macula toward the periphery. Because the central fovea has a high concentration of cones, this is the region of the eye with the greatest visual acuity (sharpness of vision). So, an animal will focus an object on the fovea to generate greater detail. The avian retina is avascular and contains a unique structure called the pecten. This is a black‐pigmented structure extending from the ventral retina to near the area where the optic nerve exits. It contains blood vessels and pigmented stromal cells and is thought to serve a nutritive function for the retina. Fig. 11.12 Microscopic anatomy of the retina. Copyright © 2004 Pearson Education. Inc. publishing as Benjamin Cummings. The lens is a biconvex, transparent, and avascular structure that can change its shape to focus light on the retina. Located behind the iris, the lens is held in place by the suspensory ligament attaching it to the choroid process. The lens, enclosed in a thin, elastic capsule, consists of two regions: lens epithelium and lens fibers. The lens epithelium consists of cuboidal cells located on the anterior surface of the lens. These cells differentiate into the lens fibers that form the bulk of the lens. Arranged like layers of an onion, lens fibers are non‐nuclear and contain few organelles. Lens fibers are made of folded proteins called crystallins. The lens divides the eye into the anterior and posterior segments. The iris subdivides the anterior segment into the anterior chamber, located between the cornea and iris, and the posterior chamber, located between the iris and lens. The anterior segment is filled with aqueous humor. It is a clear, watery fluid similar in composition to plasma to nourish the lens and cornea. Aqueous humor is continually derived as a filtrate from the capillaries of the ciliary processes entering the posterior chamber. It flows forward through the pupil of the iris into the anterior chamber. From there, it drains into the venous blood via the scleral venous sinus (canal of Schlemm). Normally produced and removed at the same rate, aqueous humor, along with the vitreous humor discussed below, maintains the intraocular pressure. However, if the drainage of aqueous humor is blocked, intraocular pressure increases, causing compression of the retina and optic nerve. This can lead to glaucoma and blindness. The posterior segment of the eye is the larger of the two segments. It contains vitreous humor, a clear gel‐like substance. The vitreous humor pushes the retina against the pigmented layer of the choroid, allowing the retina to receive a clear image. Unlike aqueous humor, vitreous humor forms during embryonic development and lasts a lifetime. Running through the center of the vitreous humor from the lens to the optic disc is the hyaloid canal, a narrow channel that is occupied by the hyaloid artery during fetal development. Occasionally, debris called vitreous floaters are visible within the vitreous humor. Derived from minute clumps of collagen and hyaluronic acid (a structural carbohydrate) these clumps can cast shadows on the retina that appear as amorphous strings or dots in your field of vision. These floaters typically increase with age, nearsightedness, eye injury, or disease. The eye can be likened to a camera. The image of an object is focused on the retina by the lens with the amount of light entering the eye dependent on the pupil. This is analogous to adjusting the F stop on your camera. The retina, lens, and pupil correspond to the film, lens, and aperture of the camera, respectively. Three processes are important in the formation of a clear image: refraction, accommodation, and pupil diameter. When light rays pass from one medium to another of a different density, the speed of light changes. As a result, the light rays are bent, or refracted (Fig. 11.13). For example, when light rays move from air into water, light rays are bent at the interface of the two mediums. This is easily illustrated by placing a pencil in a glass of water and noting its appearance at the interface between the air and water. With respect to the eye, light rays are refracted, or bent, at the anterior and posterior surfaces of both the cornea and lens. Approximately 75% of the refraction occurs at the interphase with the cornea. Note that images are inverted and backward, as they are focused on the retina. The brain reinterprets this image so that objects are not perceived as inverted. In the eye, the angle at which the light rays are bent depends on the shape of the lens. The more convex the lens, the greater the degree to which light rays are bent. As an object is moved closer to the lens, the light rays must be bent at a greater angle to allow the image to focus on the retina. The process of increasing the refractive power of the lens is called accommodation. Therefore, as an object moves closer to the eye, the lens must become more rounded, that is, made more convex, to maintain focus on the image on the retina. Accommodation is accomplished by the actions of the ciliary muscle. When the ciliary muscle is relaxed, the zonular fibers surrounding the lens pull on the lens, thus making it fatter or less convex. When the ciliary muscle contracts, it pulls the ciliary body and choroid forward, thus decreasing the tension of the zonular fibers on the lens. As a result, because of elastic fibers in the lens, it becomes more convex, that is, more rounded, which increases focusing power, causing greater bending of the light rays. In addition to the accommodating mechanisms described above, some species of birds possess a static mechanism, allowing them to keep objects in focus regardless of their distance. This is accomplished by asymmetries in the eye, allowing it to be emmetropic (i.e., light rays focus directly on the retina) in its upper portions, while becoming increasingly myopic (i.e., nearsighted) toward its lower portions. This allows a bird to keep objects on the horizon in focus in the upper portion of the eye while simultaneously keeping nearer objects in focus in the lower portion of the eye. Horses appear to have a limited accommodating ability due to weak ciliary muscles. To compensate, horses have a ramp retina in which the distance between the lens and the retina varies from dorsal to ventral positions. This allows horses to use a form of static accommodation in which they move their head to focus an object at different locations on the retina, depending on the distance of the object from the eye. The far point of vision is that distance beyond which no accommodation (no change in lens shape) is needed for focusing. The near point of vision is the closest point at which the animal can focus clearly. It is the point of maximum accommodation of the lens. This point gradually gets farther away with older animals. Fig. 11.13 Refraction of light rays. (A) Refraction of light rays at the interphase between air and water. (B) The refraction of light to focus a far object on the retina. (C) Note the more spherical shape of the lens necessary to focus a near object on the retina. A normal eye is said to be emmetropic. As animals age, the lens loses its elasticity, and therefore its ability to accommodate, a condition called presbyopia. When an animal can see close objects, but distant objects are blurred, it is called myopia or nearsightedness. It occurs because the eyeball is too long relative to the focusing power. Consequently, distant objects are focused in front of the retina (Fig. 11.14). In hyperopia or farsightedness, the animal can see distant objects but is unable to focus near objects because the eyeball is too short. Therefore, there is not enough accommodating power to focus the light rays of a near object, and the animal instead true focus is at a point behind the retina. An irregular curvature in either the lens or cornea results in astigmatism. The amount of light that can enter the eye is controlled by the diameter of the pupil. Circular muscle fibers regulate the pupil diameter. During the accommodation papillary reflex, parasympathetic signals from the oculomotor nerves cause the pupil to constrict, to prevent the most divergent light rays from entering the eye. These light rays would fall on the periphery of the retina where they would not be focused properly. Fig. 11.14 Refraction problems. (A) A normal (emmotropic) eye that can focus light rays on the retina. (B) A nearsighted (myopic) eye is too long; thus, the light rays from distant objects are focused in front of the retina. (C) In a farsighted (hypermyopic) eye, the eyeball is too short so that near objects are focused behind the retina, and the lens does not have enough accommodating power to focus the light rays on the retina. The field of vision is the spatial area that can be seen by a single eye, providing monocular vision. The location of the eyes within the head has an impact on the field of vision. The field of vision of the two eyes generally overlaps, providing an area of binocular vision. The eye location varies between species and within breeds of species. The wider set the eyes, the greater the panoramic field of vision. Although herbivores tend to have their eyes set wide, which provides them with a panoramic field of vision (Fig. 11.15), they cannot see directly in front of the nose or behind their hindquarters (Box 11.3). As mentioned earlier, photoreception involves the light energy focused on the retina being converted to an electrical signal carried by the optic nerve. Photoreceptors consist of rods and cones, named for the shape of the outer segment (Fig. 11.16). The outer segment of rods is cylindrical whereas that of the cones is tapered, or cone shaped. The tips of the rods and cones lie next to the pigmented layer. In birds, oil droplets are also found in cones, which may help filter out UV radiation. Rods and cones consist of an outer segment involved in photoreception and an inner segment containing the cell nucleus, Golgi complex, and mitochondria. The proximal end of each photoreceptor consists of bulb‐shaped synaptic endings containing synaptic vesicles. Within the outer segments are stacks of membranous discs in which the visual pigments, or photopigments, are embedded. In rods, the discs are discontinuous and stacked like pancakes, one on top of the other. In cones, the disc membranes are continuous with the plasma membrane, and the interior of the discs is continuous with the extracellular space. In rods, one to three new discs are added to the base of the outer segment every hour, thus pushing old discs toward the distal end where they are sloughed off and phagocytized by the pigmented epithelial cells. Cones also renew their discs, but it apparently occurs in a circadian rhythm and is not as well understood. There is one type of rod and three (four in birds) types of cones, distinguished by different visual pigments. Although rods are more sensitive to light and are stimulated by all visible wavelengths, they perceive only gray tones, whereas cones allow for the differentiation of color. The light‐absorbing photopigment in rods is rhodopsin. It consists of a glycoprotein called opsin and a vitamin A derivative called retinal. Vitamin A is found in carotenoid‐rich vegetables, including carrots, spinach, broccoli, and yellow squash, as well as vitamin A‐containing tissues such as the liver. Although the retinal is the light‐absorbing part of all photopigments, the opsins found in each of the three types of cones differ, permitting them to absorb primarily blue, green, or yellow‐orange wavelengths of light. The photopigments respond to light in the following sequence (Fig. 11.17): Fig. 11.15 Field of vision. (A) There is a blind area immediately between the eyes of a horse and behind the horse’s body. (B) The cat has a larger binocular area than the horse, but also has a greater blind area behind its body.
11
Special Senses
Olfaction: The Sense of Smell
Anatomy of Olfactory Receptors
Physiology of Olfaction
Olfactory Pathway
Gustation: The Sense of Taste
Tongue
Taste
Sweet Taste
Bitter Taste
Salty Taste
Sour Taste
Umami
Taste Pathway
Scent Material
Temperature
30 °C
40 °C
50 °C
Leather
93 days
65 days
19 days
Plastic
88 days
53 days
32 days
Denim
66 days
47 days
39 days
Bank notes
59 days
41 days
17 days
Silk Cloth
28 days
18 days
11 days
Vision
Accessory Structures of the Eye
Lacrimal Apparatus
Extrinsic Eye Muscles
Anatomy of the Eyeball
Fibrous Tunic
Vascular Tunic
Retina (the Sensory Tunic)
Lens
Chambers of the Eye
Physiology of Vision
Refraction
Accommodation
Refraction Problems
Pupil Diameter
Field of Vision
Photoreception
Chemistry of Visual Pigments
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
