diffusion Process by which there is a net flow of matter from a region of high concentration to one of low concentration. It occurs fastest in liq¬ uids and slowest in solids. Diffusion can be observed by adding a few drops of food colouring to a glass of water. The scent from an open bottle of perfume quickly permeates a room because of random motion of the vapour molecules. A spoonful of salt placed in a bowl of water will even¬ tually spread throughout the water.
Marlene Dietrich.
PICTORIAL PARADE
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546 I digestion ► dik-dik
digestion Process of dissolving and chemically converting food for absorption by cells. In the mouth, food is chewed, mixed with saliva, which begins to break down starches, and kneaded by the tongue into a ball for swallowing. Peristalsis propels it through the esophagus and the rest of the alimentary canal. In the stomach, food mixes with acid and enzymes, which further break it down. The mixture, called chyme, enters the duodenum, the first part of the small intestine. Bile from the liver breaks up fat globules. Enzymes from the pancreas and intestinal glands act on specific molecules, breaking carbohydrates down into simple sugars, pro¬ teins into amino acids, and fats into glycerol and fatty acids. These prod¬ ucts are absorbed by the bloodstream. Indigestible substances, such as fibre, pass into the large intestine, where water and ions are reabsorbed and feces held for excretion. See illustration opposite.
digestive tract See alimentary canal
digital camera Camera that captures images electronically rather than on film. The image is captured by an array of charge-coupled devices (CCDs), stored in the camera’s random access memory or a special dis¬ kette, and transferred to a computer for modification, long-term storage, or printing out. Since the technology produces a graphics file, the image can be readily edited using suitable software. Models designed and priced for the mass consumer market—as opposed to costly models designed for photojournalism and industrial photography—first became available in 1996. They appeal particularly to users who want to send pictures over the Internet or to crop, combine, enhance, or otherwise modify their pho¬ tographs.
digital certificate Electronic credit card intended for on-line business transactions and authentications on the Internet. Digital certificates are issued by certification authorities (e.g., VeriSign). They typically contain identification information about the holder, including the person’s public key (used for encrypting and decrypting messages), along with the authority’s digital signature, so that the recipient can verify with the authority that the certificate is authentic. Web sites may also have digital certificates, to enable a person intending to buy its products to confirm that it is an authenticated e-commerce site.
digital computer Computer capable of solving problems by process¬ ing information expressed in discrete form. By manipulating combina¬ tions of binary digits (see binary code), it can perform mathematical calculations, organize and analyze data, control industrial and other pro¬ cesses, and simulate dynamic systems such as global weather patterns. See also analog computer.
Digital Subscriber Line See DSL
digital-to-analog conversion (DAC) Process by which digital sig¬ nals (which have a binary state) are converted to analog signals (which theoretically have an infinite number of states). For example, a modem converts computer digital data to analog audio-frequency signals that can be transmitted over telephone lines.
digital video disk See DVD
digitalis \,di-j3-'ta-l3s\ Drug derived from leaves of the common fox¬ glove and used as a drug that strengthens heart muscle contraction. It was first prescribed in the 18th century. Its active principles belong to a class of steroids called cardiac glycosides. Their dosage must be carefully moni¬ tored because the lethal dose may be only three times the effective dose. Digitoxin and digoxin are among the most commonly prescribed forms of digitalis.
diglossia \dI-'glos-e-3\ Coexistence of two varieties of the same lan¬ guage in a speech community, with each variety being more or less stan¬ dardized and occupying a distinct sociolinguistic niche. Typically, one variety is more formal or prestigious while the other is more suited to informal conversation or is taken as a mark of lower social status or less education. Classic diglossic situations can be found in ARABic-speaking communities, where Modern Standard Arabic coexists with dozens of regional Arabic dialects, and among speakers of Dravidian languages such as Tamil, where different words for basic concepts such as “house” or “water” are chosen depending on the speaker’s caste or religion.
Dijon \de-'zho n \ City (pop., 2004 est.: 149,000), east-central France. The site has been occupied from pre-Roman times. It became the capital of the duchy of Burgundy in 1015 and prospered under the Valois dynasty (1364-1477). Louis XI annexed the city in the late 15th century. Its his¬ toric buildings include a 13th-century church and 14th-century town hall
palate — tongue —
teeth-
epiglottis
salivary glands -pharynx
esophagus
stomach pancreas
colon cecum- sigmoid colon rectum -
small
intestine
___l_
duodenum jejunum — ileum
appendix
anus
Major organs of the human digestive system. Food taken in by the mouth is guided by the tongue as it is sheared and ground by the teeth. The bolus of chewed food is moistened and lubricated with saliva secreted by the salivary glands. Enzymes in the saliva begin the breakdown of starches. The epiglottis, a flap of tissue, prevents food from the pharynx from entering the larynx during swallowing. Muscles in the esophagus wall contract in waves to move the food to the stomach. Gastric juices secreted by the stomach contain a mixture of substances (including enzymes and hydrochloric acid) that break down the food into a semiliquid mass called chyme. The chyme passes into the small intestine, where food molecules are broken down into sugars, amino acids, and fatty acids. These useful substances are absorbed into the bloodstream as the food passes through the duodenum, jejunum, and ileum. The pancreas secretes digestive enzymes into the duodenum. The liver secretes bile salts that make insoluble fats entering the small intestine water-soluble and vulnerable to enzymatic action. Excess bile salts are stored in the gallbladder. The large intestine (colon) serves to remove water and electrolytes from the digested chyme and to compact and store undigestible material, called feces. Fecal matter is moved by muscular contractions into the rectum and expelled from the anus.
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(the former palace of the dukes of Burgundy). Its university was founded in 1722. A market and tourist centre, it is still known for its food prod¬ ucts (e.g., mustard, vinegar, and gingerbread).
dik-dik Any of four species of delicate African antelope (genus Madoqua ), named for the sound it makes when alarmed. It stands 12-16 in. (30-40 cm) high at the shoulder and weighs 7-11 lbs (3-5 kg). It has an elongated snout and a soft coat that is gray or brownish above, white below. The hair on the crown forms an upright tuft and may partially con¬ ceal the short, ringed horns of the male. Dik-diks live in dry areas of dense brush in southern and eastern Africa, and feed chiefly on acacia and other shrubs. See photograph on opposite page.
© 2006 Encyclopaedia Britannica, Inc.
dike ► diminishing returns I 547
dike Bank, usually of earth, constructed to control or confine water. Dikes were purely defensive at first but later became a means to acquire polders (tracts of land reclaimed from a body of water through the con¬ struction of offshore dikes roughly parallel to the shoreline). After a dike is built, the polder is drained by pumping out the water. Where the land surface is above low-tide level, tide gates discharge water into the sea at low tide and automatically close to prevent reentry of seawater at high tide. To reclaim lands that are below low-tide level, the water must be pumped over the dikes. The most notable example of polder construction is the system adjacent to Holland’s IJsselmeer (Zuider Zee) barrier dam. If The Netherlands were to lose the protection of its dikes, its most densely populated portion would be inundated by the sea and rivers.