 Fig. 1  Fig. 2  Fig. 3  Fig. 4  Fig. 5  Fig. 6  Fig. 7 | The special senses consist of olfaction (smell), taste, vision (sight), hearing and equilibrium (balance). These senses are "special" in that their respective receptors are concentrated in specific organs rather than being broadly distributed throughout the body. Olfaction (the sense of smell) is provided by nerve endings which penetrate the ethmoid bone to reach the olfactory mucosa of the nasal cavities (figure 1). The olfactory nerve endings are able to differentiate between a large spectrum of chemical stimuli. When these nerve endings are stimulated, the information is relayed through the olfactory bulb, superior to the ethmoid bone, and through the "olfactory nerve" to the brain. In fact, what we call the "olfactory nerve" is actually regarded as a part of the brain by modern neuroscientists, and we should properly refer to it as the olfactory tract. The real olfactory nerves are those groups of neurons which originate in the olfactory bulb and penetrate the ethoid bone. The olfactory bulb is interesting because it contains stems cells. This allows regeneration of olfactory neurons (and so the preservation of our sense of smell). This capacity of regeneration makes the olfactory bulb very interesting to scientists who are looking for ways to regenerate nervous tissue. The sense of taste, like smell, relies on chemical receptors. These nerve endings are grouped in the taste buds of the tongue and palate (figure 2). There are four functional classes of taste buds, each sensitive to sweet, sour, salt and bitter, respectively. Information from the taste buds is conveyed to the brain via the facial nerve and the glossopharyngeal nerve. The facial nerve serves approximately the anterior two-thirds of the tongue, while the glossopharyngeal nerve serves approximately the posterior one-third of the tongue. The sense of vision is, of course, provided by the eyes. The eye consists of essentially three layers of tissue: retina, choroid and sclera (figure 3). The retina contains the light-sensitive nerve endings. The middle choroid layer (also spelled chorioid) is the vascular layer of the eye. The outer sclera is a tough fibrous layer which maintains the shape of the eye and provides attachment for the extraocular muscles. At the front of the eye, the sclera becomes the transparent cornea which admits light (figure 4). The iris of the eye acts as a diaphragm to regulate the amount of light which passes through the pupil. Behind the pupil, the lens acts to focus light on the retina. Information from the retina is relayed via the optic nerve (figure 5) to the visual cortex of the occipital lobes. Interestingly, each optic nerve divides into two bundles after leaving the eye. The medial bundle from each eye crosses to the other side of the brain through a pathway called the optic chiasma. In this way the visual information from the right sides of both eyes goes to the right side of the brain, and the visual information from the left sides of both eyes goes to the left side of the brain. Hearing and equilibrium are provided by the ears. The ear is divided into three anatomical regions: the outer ear, the middle ear and the inner ear (figure 6). The outer ear consists of the pinna and the external auditory meatus. The tympanic membrane, also called the ear drum, forms the boundary between the outer ear and the middle ear. Within the tympanic cavity of the middle ear are three ossicles: the malleus, incus and stapes. These three small bones convey vibrations from the tympanic membrane to the inner ear. The inner ear consists of the cochlea, the semicircular canals and the vestibular apparatus (figure 7). The cochlea is a spiral of nerve endings supported in a canal of lymph-like fluid. Vibrations from the stapes set the nerve endings of the cochlea in motion. The information from the resulting stimulation is relayed to the brain by the acoustic nerve and interpreted as sound. The vestibular apparatus (the utricle and the saccule) and the semicircular canals provide us with our sense of balance. In particular, the utricle (labelled Recessus ellipticus in figure 7) and saccule (labelled Recessus sphaericus in figure 7) signal the direction of gravity (which way is up or down). The semicircular canals signal acceleration of the head, for example when we nod our head up and down. This information is relayed to the brain via the vestibular portion of the acoustic nerve to help form our sense of equilibrium.
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