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Introduction I find it hard to recognize some relatives of ours, like the rotifer, the sycamore, iguanas, and sea stars. —­They Might Be ­Giants In their song about evolution, “My ­ Brother the Ape,” the American rock band They Might Be ­ Giants observed that­ humans are relatives of trees, lizards, and starfish. But can one ­ really make a comparison between a tree and a person? Indeed, we have a lot in common with plants. Like plants,­ people are made of cells. The cells are made up of proteins. One aspect shared by all living ­ things is that we evolved in a gravitational field owing to the im­ mense mass of the Earth. A feature all living organisms share is the ability to orient with re­ spect to gravity, to tell up from down. Even the most basic single-­ cell forms of life such as bacteria and algae “detect ” the pull of gravity, prob­ ably by differences in density at dif­ fer­ ent parts of the cell. If the stems of plants are placed flat, they ­ will grow faster on the upper side and turn upward as heavier substances gather in greater concentration on the lower side of the stem. Specialized organs used to sense gravity are already seen in primitive animals such as jellyfish , which appeared more than 600 million years ago on the Dizziness 2 evolutionary time scale. In ­ these animals, a pouch filled with seawater, the otocyst, contains tiny stones, or “liths,” whose density is much greater than the surrounding fluid. Gravity­ causes ­ these tiny stones to rest their weight on specialized sensing cells in the walls of the pouch, allowing the animals to regulate their position in space. This primitive otocyst is the forerunner of the inner ear in more advanced animals. In his famous work, On the Origin of Species, Charles Darwin used a tree to illustrate the concept of evolution. On this so-­ called tree of life, the primitive otocyst is located at the main trunk while the inner ears of the most advanced animals are located at the most peripheral branches. In early fish, the pouch previously open to the outside becomes closed and is filled with fluid secreted by cells in the wall of the cyst. The otolith organ remains relatively unchanged , but the other parts of the inner ear—­ the sensors for rotation of the head, the semicircular canals, and the sensors for hearing, the cochlea—­ develop ­ later but are already pres­ ent in modern fish that evolved about 100 million years ago. The vestibular part of the inner ear, the otolith organs and the semicircular canals, are relatively unchanged from fish to primates , including ­ humans. In its simplest form, dizziness is a prob­ lem with orientation , a feeling of disorientation. Since the inner ear is a key organ for providing the brain with orientating information, damage to the inner ear can cause dizziness. Although it seems intuitive now, it is remarkable how long it took the medical community to understand this ­ simple concept. The symptom of dizziness had been recognized for several thousand years, but it ­ wasn’t ­ until the pioneering work of Prosper Ménière in the mid nineteenth ­ century that it was appreciated that dizziness could originate from damage to the inner ear. Prior to that time, a patient with dizziness was said to have “ce­ re­ bral Introduc tion 3 congestion,” a condition resulting from excessive blood filling the brain. Bloodletting and leeches to relieve the congestion­ were the treatments of choice. Ménière, who directed a Paris institute for ­ people who could neither hear nor speak, made the ­ simple observation that patients who developed sudden deafness, such as occurred ­ after being shot in the ear with an arrow, also had severe dizziness and disequilibrium. He concluded that parts of the inner ear must be impor­ tant for orientation and balance, in addition to hearing. Although detailed anatomy of the inner ear had been described as early as the mid-­ sixteenth­ century, the vestibular part of the inner ear was thought to be involved in hearing, not balance. For example, the semicircular canals ­ were thought to be impor­ tant for identifying the location of sounds in space, not for sensing motion of the head in space. Based on his experience, Ménière pointed out that patients with acute hearing loss and dizziness usually had a benign course and that aggressive treatments such as bleeding ­ were more dangerous than the under­ lying disorder. Knowing what we know...


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