| The Special Senses of The Horse |
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By Stephen Champion, BSc (Hons) BVM&S MRCVS, The wild equid, from which the modern domestic horse has evolved, must constantly check its environment if it is to survive. It must see, hear and smell predators and distinguish them from members of its own group. It must be able to taste and discard harmful substances in its forage. It must be in touch with its surroundings and internal self through the many senses in its skin, muscles and internal organs. This article will concentrate on the horse’s so called special senses; collections of specialised sensory cells in its eyes, ears, nose and mouth. The Eyes Normal vision is extremely important to the horse's daily existence. A sound, full, clear, intelligent eye is something which must always add a high value to our equine friends. The essence of the visual system is to collect light and focus it onto the retina in the back of the eye, where the image is then transmitted to the brain via the nervous system. To the point where the light, and hence the image, is focused on the retina, the eye functions very much like a camera lens. In people and domestic animals, the lens (the soft structure that sits behind the pupil) can have its shape changed by small muscles called ciliary muscles and this brings an image into focus on the retina. If the light image is not in focus, then the image perceived by the brain is not in focus. The discussion of a ramped retina in horses began in the 1930s, since it is known that, relative to other species, the horse"s lens does not have a great ability to change shape and therefore might not be the sole mechanism of focusing light on the retina. The retina, and hence the back of the horse"s eye, is not perfectly round, but in fact is sloped or "ramped." In short, this means that the horse might be able to "find" a location on the retina where the light is in focus by raising or lowering its head. It is generally believed that the horse must elevate its head to "focus" on objects close to him, and conversely lower his head to focus on objects far away. The Ears The ear is divided into three portions: the outer, the middle, and the inner ear. The outer ear is the visible portion that can be a strong indicator of the general mood of your horse, a tip-off as to where its attention is directed, and even a sentinel to a well-timed kick! The ear drum, technically called the tympanic membrane, lies at the end of the ear canal; it is the membrane that picks up sound waves and starts the process of hearing. On the inner side of the tympanic membrane starts the middle ear, the tympanic cavity, which contains the three smallest bones in the body: the malleus, incus, and stapes (Latin for hammer, anvil, and stirrup, respectively). The malleus has one end attached to the tympanic membrane and starts a chain with the other bones attached end to end. The end of the stapes sits on a deeper membrane that separates the middle ear from the inner ear. Within the inner ear is a complicated labyrinth of channels that are fluid-filled. The channels are lined with thousands of sensory cells that signal both the auditory nerve (involved with hearing) and the vestibular nerve (involved with balance). It is via this complex and delicate system that sound waves are transmitted to the brain and become what we hear. The Nose Horses depend on their sense of smell the way we depend on language. It allows them to identify their owner at 100 paces and alerts him to the presence of yucky medications in his sweet feed even though you"ve doctored them with herbs and molasses! As prey animals, it behoves them to be able to detect even the slightest scent of danger on the wind. They"re also quick to detect the "smell of fear" in other animals and in humans (probably an emanation of chemical signals we cannot detect). The horse has prodigious nostrils and inside are long and cavernous nasal passages that facilitate the intake of large quantities of air during exercise, together with the chemical messages in the air. The horse"s olfactory receptors, millions of elongated nerve cells that are specialised to analyse smells, are located in the mucous membranes in the upper portion of the nasal cavity. When airborne odour molecules come into contact with the mucous membranes, they interact with the microscopic tufts of hair protruding from the receptor cells. The receptors are connected by the olfactory nerves directly to the twin bulbs, right at the front of the brain. This is only half of the story, for horses really have two olfactory systems. There"s a second pair of olfactory organs lurking under the floor of the horse"s nasal cavity - the vomeronasal organs or Jacobson"s organs. Their main purpose is the detection and analysis of pheromones, the sexual chemical signals emanating from other horses and, on occasion, from humans! Most of us, at one time or another, have witnessed a horse tilt up his head and curl his upper lip in a "horse laugh." Although the expression is amusing, it actually has a practical purpose. The posture is called flehmen, meaning “testing”, and it appears to help horses trap pheromone scents in the vomeronasal organs so they can be analysed more thoroughly. The Gustatory Organs The receptors of the sense of taste are the taste buds, found as small nests of specialised cells on the tongue and soft palate. In order for food to be discerned, it has to be in solution. This is one of the reasons why horses salivate. Information about taste is transmitted through several different nerves to the medulla oblongata of the brain, where the taste is analysed by the horse. The principal taste sensations are sweetness, sourness, saltiness and, most probably, Bute! |
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