vacuum tube: see electron tube.

tube, in electronics: see electron tube.

test-tube baby: see in vitro fertilization.

gas tube: see electron tube.

fallopian tube, either of a pair of tubes extending from the uterus to the paired ovaries in the human female, also called oviducts, technically known as the uterine tube. At one end the long, slender fallopian tube opens into the uterus; the other end expands into a funnel shape near the ovary. The epithelium that lines the tube is covered with cilia that beat continuously toward the uterus. When an ovum is expelled into the peritoneal cavity from the ovary during ovulation, it is propelled into the wide-mouthed opening of the fallopian tube, through the tube, and into the uterus by the wavelike motion of the cilia. If the ovum is fertilized, an event that normally takes place in the fallopian tube, and the embryo (fertilized ovum) implants in the tube, or another area outside the uterus, an ectopic pregnancy occurs. About 98% of ectopic implantations occur in the tubes, but other sites include the abdomen, ovary, and cervix. Immediate surgical removal of the products of conception is necessary to prevent hemorrhage and other complications resulting from ectopic pregnancy. The fallopian tubes are also the site of the most common surgical procedures used to prevent conception or cause infertility in women. Usually the tubes are tied off in a procedure known as tubal ligation, although they are also sometimes excised or occluded by other methods. See reproductive system; uterus.

electron tube, device consisting of a sealed enclosure in which electrons flow between electrodes separated either by a vacuum (in a vacuum tube) or by an ionized gas at low pressure (in a gas tube). The two principal electrodes of an electron tube are the cathode and the anode or plate. The simplest vacuum tube, the diode, has only those two electrodes. When the cathode is heated, it emits a cloud of electrons, which are attracted by the positive electric polarity of the anode and constitute the current through the tube. If the cathode is charged positively with respect to the anode, the electrons are drawn back to the cathode. However, the anode is not capable of emitting electrons, so no current can exist; thus the diode acts as a rectifier, i.e., it allows current to flow in only one direction. In the vacuum triode a third electrode, the grid, usually made of a fine wire mesh or similar material, is placed between the cathode and anode. Small voltage fluctuations, or signals, applied to the grid can result in large fluctuations in the current between the cathode and the anode. Thus the triode can act as a signal amplifier, producing output signals some 20 times greater than input. For even greater amplification, additional grids can be added. Tetrodes, with 2 grids, produce output signals about 600 times greater than input, and pentodes, with 3 grids, 1,500 times. X-ray tubes maintain a high voltage between a cathode and an anode. This enables electrons from the cathode to strike the anode at velocities high enough to produce X rays. A cathode-ray tube can produce electron beams that strike a screen to produce pictures as in oscilloscopes and video displays. Gas tubes behave similarly to vacuum tubes but are designed to handle larger currents or to produce luminous discharges. In some gas tubes the cathode is not designed as an electron emitter; conduction occurs when a voltage sufficient to ionize the gas exists between the anode and the cathode. In these cases the ions and electrons formed from the gas molecules constitute the current. Electron tubes have been replaced by solid-state devices, such as transistors, for most applications. However they are still widely used in high-power transmitters, some television cameras, specialty audio equipment, and as oscilloscope and video displays. A klystron is a special kind of vacuum tube that is a powerful microwave amplifier; it is used to generate signals for radar and television stations.

See also magnetron; photoelectric cell.

cathode-ray tube, special-purpose electron tube in which electrons are accelerated by high-voltage anodes, formed into a beam by focusing electrodes, and projected toward a phosphorescent screen that forms one face of the tube. The beam of electrons leaves a bright spot wherever it strikes the phosphor screen. To form a display, or image, on the screen, the electron beam is deflected in the vertical and horizontal directions either by the electrostatic effect of electrodes within the tube or by magnetic fields produced by coils located around the neck of the tube. Some cathode-ray tubes can produce multiple beams of electrons and have phosphor screens that are capable of displaying more than one color. Cathode-ray tubes are used in television sets, computer monitors, automated teller machines, oscilloscopes, and radar displays.

Geissler tube, gas discharge tube in which light is produced when an electric discharge passes through the rarefied gas in the tube. The color of the glow depends on the gas used. The tubes are made in a variety of shapes and are especially useful in spectroscopy. One later form of the tube is widely used in luminous signs ("neon" signs) and another in fluorescent lamps.

Eustachian tube [for Bartolomeo Eustachi], a hollow structure of bone and cartilage extending from the middle ear to the rear of the throat, or pharynx, technically known as the pharyngotympanic or auditory tube. By permitting air to leave or enter the middle ear, the tube equalizes air pressure on either side of the eardrum. The tube can become blocked, as by enlarged adenoids or the mucous secretions of a cold, so that external and internal pressure become imbalanced. Earache and diminution of hearing may result. The tube may also serve as a pathway to the ear for infections of the throat. A common ear disease known as Otitis Media, usually appearing in early childhood, is thought to be related to the Eustachian tube. The tube tends to be shorter and more horizontal among children, factors which facilitate the spread of infections from upper respiratory diseases to the middle ear, as well as the accumulation of fluids in the region.

Crookes tube, device invented by Sir William Crookes (c.1875) consisting essentially of a sealed glass tube from which nearly all the air has been removed and through the walls of which are passed two electrodes. When a high voltage is applied between the two electrodes, electrons are emitted from the cathode and are accelerated toward the anode. Many of these electrons, or cathode rays (as they are usually called), miss the anode and strike instead the glass wall of the tube, causing it to exhibit fluorescence. The behavior of the rays indicates that they travel in straight lines and exert a pressure on any object placed in their path. The Crookes tube was used by Crookes in a number of experiments and was later used in experiments leading to the discovery of X rays by W. C. Roentgen (1895) and of the electron by J. J. Thomson (1897).

Short pipe with a constricted inner surface, used to measure fluid flows and as a pump. The effects of constricted channels on fluid flow were first investigated by Giovanni Battista Venturi (1746–1822), but it was Clemens Herschel (1842–1930) who devised the instrument in 1888. Fluid passing through the tube speeds up as it enters the tube's narrow throat, and the pressure drops. There are countless applications for the principle, including the carburetor, in which air flows through a venturi channel at whose throat gasoline vapour enters through an opening, drawn in by the low pressure. The pressure differential can also be used to measure fluid flow (see flow meter).

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Any of numerous species of sedentary, solitary or colonial, marine worms that spend their entire life in a tube made from special secretions or from sand grains glued together. Found worldwide, tube worms range from less than an inch (25 mm) to more than 20 ft (6 m) long. The bottom of the tube is attached to the seafloor; the mouth and tentacles are at the upper, open end. The worm breathes through gills, the tentacles, or the body wall. The tentacles, variously arranged, are used to filter-feed aquatic plants and animals. Tube worms occur in the annelid class Polychaeta and in the phyla Phoronida and Pogonophora. Many, mostly unnamed, forms live in deep-ocean vent communities.

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Underground railway system used to transport passengers within urban and suburban areas. The first subway line, 3.75 mi (6 km) long, opened in London in 1863 and carried 912 million passengers in its first year. The first electrified subway opened in 1890 in London (where it is called the underground or tube). Subways opened in Budapest in 1896 (the first on the European continent), Boston in 1897, Paris in 1900 (where it is called the métro), Berlin in 1902, New York in 1904, and later in Madrid (1919), Tokyo (1927), and Moscow (1935). Improvements in systems built from the 1970s on (including San Francisco, Washington, D.C., and Los Angeles) include computer technology to run subway trains by remote control, and refinements in track and car construction for faster, smoother, and quieter rides.

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Congenital defect of the brain or spinal cord from abnormal growth of their precursor, the neural tube (see embryology), usually with spine or skull defects. The tube may fail to close properly, have parts missing, or have a blockage (see hydrocephalus). In spina bifida, vertebrae are open over the back of the spinal cord, usually at the base. This may not affect function if no further defects (local absence of skin or meninges, protrusion of tissue, defect opening into the spinal cord) exist. The more serious forms can cause paralysis and impair bladder and bowel function. In encephalocele, a meningeal sac containing brain tissue protrudes from the skull. The effects depend on the amount of tissue involved. Adequate folic-acid intake by women of childbearing age reduces the risk of neural tube defects. Early surgery can prevent or minimize disability.

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