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  © Luchschen/Dreamstime.com Fluorine  (F) is a naturally occurring  element . Fluorine  compounds , or fluorides, are compounds that contain the element fluorine. Fluoride is the fluorine  ion  (F - ), which is an  anion . Water- soluble  fluorine compounds release fluoride ions into the water they’re dissolved in. Chemical fluoride is added to public water sources in order to reduce dental  caries  (cavities).. Fluoride in toothpaste and in other dental health products has also contributed to the declining incidence of caries. Water fluoridation has an advantage over topical applications of fluoride (for example, by toothpaste) in that it can potentially reduce caries among disadvantaged populations who may not have access to topical means. Before governments began adding fluoride to community water systems, fluoride was already present naturally in water and soil at varying levels. The U.S. began adding fluoride to community water sys...

   use of   drugs   for psychotropic rather than medical purposes. Among the most common psychotropic drugs are opiates ( opium ,   morphine , and   heroin ),   hallucinogens   ( LSD ,   mescaline , and psilocybin),   barbiturates ,   cocaine ,   amphetamines ,   tranquilizers , and   cannabis .   Alcohol   and   tobacco   are also sometimes classified as drugs. The term   drug abuse   is normally applied to excessive and addictive use of drugs. Because such drugs can have severe physiological and psychological, as well as social, effects, many governments regulate their use. Characteristics of drug use and abuse  The functions of  psychotropic drugs To consider drugs only as medicinal agents or to insist that drugs be confined to prescribed  medical practice  is to fail to understand  human nature . The remarks of American sociologist  Bernard Barber are...

  Anatomy of the human ear Outer ear The most-striking differences between the human ear and the ears of other mammals are in the structure of the outermost part, the  auricle . In humans the auricle is an almost  rudimentary , usually immobile shell that lies close to the side of the  head . It consists of a thin plate of yellow elastic  cartilage  covered by closely adherent  skin . The cartilage is molded into clearly defined hollows, ridges, and furrows that form an irregular shallow funnel. The deepest depression, which leads directly to the  external auditory canal , or acoustic meatus, is called the  concha . It is partly covered by two small projections, the tonguelike  tragus  in front and the  antitragus behind. Above the tragus a prominent ridge, the  helix , arises from the floor of the concha and continues as the incurved rim of the upper portion of the auricle. An inner, concentric ridge, the  antihelix,...

 process by which substances in   food   are transformed into body tissues and provide energy for the full range of physical and mental activities that make up   human   life. The study of human nutrition is interdisciplinary in character, involving not only  physiology , biochemistry, and  molecular biology  but also fields such as  psychology  and  anthropology , which explore the influence of attitudes, beliefs, preferences, and cultural traditions on food choices. Human nutrition further touches on  economics  and  political science  as the world  community  recognizes and responds to the suffering and  death  caused by  malnutrition . The ultimate goal of nutritional science is to promote optimal  health  and reduce the risk of chronic diseases such as  cardiovascular disease  and  cancer  as well as to prevent classic nutritional deficiency diseases suc...