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hypnosis State that resembles sleep but is induced by a person (the hyp¬ notist) whose suggestions are readily accepted by the subject. The hyp¬ notized individual seems to respond in an uncritical, automatic fashion, ignoring aspects of the environment (e.g., sights, sounds) not pointed out by the hypnotist. Even the subject’s memory and awareness of self may be altered by suggestion, and the effects of the suggestions may be extended (posthypnotically) into the subject’s subsequent waking activ¬ ity. The history of hypnotism is as old as that of sorcery and magic. It was popularized in the 18th century by Franz Anton Mesmer (as “mesmerism”) and was studied in the 19th century by the Scottish surgeon James Braid (1795-1860). Sigmund Freud relied on it in exploring the unconscious, and it eventually came to be recognized in medicine and psychology as useful in helping to calm or anesthetize patients, modify unwanted behaviours, and uncover repressed memories. There remains no generally acceptable explanation for hypnosis, though one prominent theory focuses on the possibility of discrete dissociative states affecting portions of conscious¬ ness.
hypochondriasis N.hl-ps-kon-'dri-o-sosV Mental disorder in which an individual is excessively preoccupied with his own health and inclined to treat insignificant physical signs or symptoms as evidence of a serious disease. The hypochondriac may become convinced that he is ill even though he has no symptoms at all, or may exaggerate the importance of minor aches and pains, becoming obsessed with the fear of a life- threatening illness. A doctor’s reassurances often have only a slight or temporary effect on the hypochondriac’s anxieties. Hypochondriasis usu¬ ally first manifests itself in early adulthood and is equally common among males and females. In some cases it may represent a psychological cop¬ ing mechanism that the individual resorts to in order to deal with stress¬ ful life situations.
hypoglycemia X.hl-pd-gll-'se-me-oX Below-normal levels of blood glucose, quickly reversed by administration of oral or intravenous glu¬ cose. Even brief episodes can produce severe brain dysfunction. Fasting hypoglycemia can be life-threatening; it occurs most often in patients with diabetes mellitus who mistime insulin therapy or miss meals. It also results from insulin-producing tumours, starvation, or metabolic disorders. Reac¬ tive hypoglycemia occurs when the body produces too much insulin in response to sugar intake. Symptoms range from irritability to confusion and seizures, leading to coma and death in severe cases.
hypophosphatemia V.hl-po-.fas-fa-'te-me-oX Low phosphate levels in blood. It usually occurs in conjunction with other metabolic disturbances, disrupting energy metabolism and impairing delivery of oxygen to tis¬ sues. Acute hypophosphatemia causes neurological symptoms (weakness, tremors, and confusion). Chronic hypophosphatemia, from long-term deficiency, causes general weakness and appetite loss. Treatment involves correcting the metabolic problem and giving phosphate supplements. Familial hypophosphatemia, an inherited disorder, is a major cause of rickets in developed nations. See also ATP.
hyposensitization See desensitization
hypostyle Vhl-po-.stlL hall Imposing interior space with a flat roof that rests on many rows of columns. The design allows for the construc¬ tion of large spaces without arches. It was used extensively in ancient Egypt (e.g.. Temple of Amon at Karnak) and Persia. The elaborately carved pillars consumed much of the floor space and therefore assumed great importance. Hypostyles are rarely seen in more recent architecture because of more effective means of roof support.
hypotension or low blood pressure Condition in which blood pressure is abnormally low. It may result from reduced blood volume (e.g., from heavy bleeding or plasma loss after severe burns) or increased blood¬ vessel capacity (e.g., in syncope). Orthostatic hypotension—drop in blood pressure on standing—results from failure of the reflexes that contract muscles and constrict blood vessels in the legs to offset gravity as one rises. Hypotension is also a factor in poliomyelitis, shock, and barbiturate poisoning.
hypothalamus V.hl-po-'tha-lo-mosX Region of the brain containing a control centre for many autonomic-nervous-system functions. Its complex interaction with the pituitary gland makes it an important part of the endo¬ crine system. As a critical link between the body’s two control systems, the
hypothalamus regulates homeostasis. Nervous and hormonal pathways connect it with the pituitary, which it stimulates to release various hor¬ mones. The hypothalamus influences food intake, weight regulation, fluid intake and balance, thirst, body heat, and the sleep cycle. Disorders can produce pituitary dysfunction, diabetes insipidus, insomnia, and temperature fluctuations.
hypothermia Abnormally low body temperature, with slowing of physiological activity. It is artificially induced (usually with ice baths) for certain surgical procedures and cancer treatments. Accidental hypother¬ mia can result from falling into cold water or overexposure in cold weather. Underlying conditions such as cerebrovascular disease or intoxi¬ cation increase the risk from exposure. Hypothermia is serious when body temperature is below 95 °F (35 °C) and an emergency below 90 °F (32.2 °C), at which point shivering stops. Pulse, respiration, and blood pressure are depressed. Even when the victim appears dead, revival may be pos¬ sible with very gradual passive rewarming (e.g., with blankets). See also FROSTBITE.
hypothesis testing In statistics, a method for testing how accurately a mathematical model based on one set of data predicts the nature of other data sets generated by the same process. Hypothesis testing grew out of quality control, in which whole batches of manufactured items are accepted or rejected based on testing relatively small samples. An initial hypothesis (null hypothesis) might predict, for example, that the widths of a precision part manufactured in batches will conform to a normal dis¬ tribution with a given mean (see mean, median, and mode). Samples from new batches either confirm or disprove this hypothesis, which is refined based on these results.
hypoxia \,hip-'ak-se-3, hI-'pak-se-9\ Condition in which tissues are starved of oxygen. The extreme is anoxia (absence of oxygen). There are four types: hypoxemic, from low blood oxygen content (e.g., in altitude sickness); anemic, from low blood oxygen-carrying capacity (e.g., in car¬ bon monoxide poisoning); distributive, from low blood flow (e.g., gen¬ erally in shock or locally in atherosclerosis); and histotoxic, from poisoning (e.g., with cyanide) that keeps cells from using oxygen. If not reversed quickly, hypoxia can lead to necrosis (tissue death), as in heart
ATTACK.
hyrax \'hl-,raks\ Any member of three genera of small, hoofed, quad¬ ruped, rodentlike mammals (order Hyracoidea) native to Africa and extreme South Asia. Hyraxes have a squat body, a short neck and tail, and short, slender legs. Adults are 12-20 in. (30-50 cm) long and weigh 8-11 lbs (4-5 kg). They are primarily her¬ bivores. They are agile and climb well with the aid of special pads on their feet. Their relationship to ungu¬ lates is unclear. See also cony.
hyssop Vhi-S9p\ Small perennial garden herb {Hyssopus officinalis ) of the mint family, native to the area from southern Europe east to Central Asia and naturalized in North America. Its flowers and evergreen leaves have long been used as a fla¬ vouring for foods and beverages and as a folk medicine for nose, throat, and lung afflictions. The plant has a sweet scent and a warm, bitter taste.