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Medical Dictionary - Dictionary of Medicine and Human Biology

Medical Dictionary


An outer covering. cytotrophoblastic s. the external layer of fetally derived trophoblastic cells on the maternal surface of the placenta. diffusion s. a small vessel made of a semipermeable membrane through which peptone, but not serum albumin, can pass; used in performing the Abderhalden test. K s. the innermost electron orbit or s.; it can hold two electrons. L s. the next lowest energy level of electrons in the atom, after the K s. (q.v.). M s. the lowest energy level at which electron transitions give rise to x-rays. O s. the outermost s. of electrons, so called because displacement of electrons causes an emission in the visible or optical range.

shellac (she-lak′)
A resinous excretion of an insect, Laccifer (Tachardia) lacca (family Coccidae). The insects suck the juice of various resiniferous Asiatic (chiefly Indian) trees and excrete and deposit “stick-lac.” S. softens at a low temperature. It has many nonmedicinal uses and is also used to coat confections and tablets and in dental materials, e.g., impression compound and denture base plates. SYN: lacca.

David, U.S. biochemist, *1911. See S. cycle.

Edward W.H., English radiologist, 1872–1955. See S. line.

Francis J., Canadian surgeon, 1851–1929. See S. fracture.

Henry C., U.S. biochemist, 1875–1955. See S. unit, S.-Bourquin unit of vitamin B2, S.-Munsell unit.

Sir Charles S., English physiologist and Nobel laureate, 1857–1952. See S. phenomenon, S. law, Schiff-S. phenomenon, Liddell-S. reflex.

shield (sheld)
A protecting screen; lead sheet for protecting the operator and patient from x-rays. [A.S. scild] embryonic s. a thickened area of the embryonic blastoderm from which the embryo develops. nipple s. a cap or dome placed over the nipple to protect it during nursing. oral shields removable appliances used in orthodontic treatment, usually placed between the labial and buccal mucosa and the teeth.

SYN: change. SEE ALSO: deviation. antigenic s. mutation, i.e., sudden change in molecular structure of RNA/DNA in microorganisms, especially viruses, which produces new strains; hosts previously exposed to other strains have little or no acquired immunity to the new strain; antigenic s. is believed to be the explanation for the occurrence of new strains of influenza virus, which occur by recombination or genetic reassortment of 2 different viral strains in a given host, and is associated with large-scale epidemics. axis s. SYN: axis deviation. chemical s. dependence of the resonance frequency of a nucleus on the chemical binding of the atom or molecule in which it is contained. See chemical s. artifact. chloride s. when CO2 enters the blood from the tissues, it passes into the red blood cell and is converted by carbonate dehydratase to bicarbonate (HCO3−); HCO3− ion passes out into the plasma while Cl− migrates into the red blood cell. Reverse changes occur in the lungs when CO2 is eliminated from the blood. SYN: Hamburger phenomenon. Doppler s. the magnitude of the frequency change in hertz when sound and observer are in relative motion away from or toward each other. SEE ALSO: Doppler effect. s. to the left 1. a marked increase in the percentage of immature cells in the circulating blood, based on the premise in hematology that the bone marrow with its immature myeloid cells is on the left, while the circulating blood with its mature neutrophils is on the right; SYN: deviation to the left. 2. See maturation index. luteoplacental s. the change in site of production of the estrogen and progesterone essential for human pregnancy from the corpus luteum to the placenta; ovariectomy always terminates pregnancy in most mammals because their placentas never produce enough estrogen and progesterone, but, after the sixth week of pregnancy, a human placenta can produce enough of these hormones to prevent abortion despite ovariectomy. permanent threshold s. the irreversible hearing loss that results from exposure to intense impulse or continuous sound, as opposed to the reversible temporary threshold s. that also results from such exposure. phase s. in nuclear magnetic resonance, the change in phase caused by movement of the spins, which can be used to show fluid flow. Purkinje s. SYN: Purkinje phenomenon. s. to the right 1. in a differential count of white blood cells in the peripheral blood, the absence of young and immature forms; SYN: deviation to the right. 2. See maturation index. temporary threshold s. the reversible hearing loss that results from exposure to intense impulse or continuous sound, as opposed to the irreversible permanent threshold s. that may result from such exposure. threshold s. the degree of hearing loss or impairment in terms of a decibel s. from an individual's previous audiogram. After exposure to intense sound, there may be temporary threshold s. with recovery in hours or days or permanent threshold s. (noise-induced hearing loss).

Kiyoshi, Japanese bacteriologist, 1870–1957. See Shigella, S. bacillus, S.-Kruse bacillus.

Shigella (she-gel′la)
A genus of nonmotile, aerobic to facultatively anaerobic bacteria (family Enterobacteriaceae) containing Gram-negative nonencapsulated rods. These organisms cannot use citrate as a sole source of carbon; their growth is inhibited by potassium cyanide and their metabolism is fermentative; they ferment glucose and other carbohydrates with the production of acid but not gas; lactose is ordinarily not fermented, although it is sometimes slowly attacked; the normal habitat is the intestinal tract of humans and of higher apes; all of the species produce dysentery. The type species is S. dysenteriae. [Kiyoshi Shiga] S. boydii a species found only in feces of symptomatic individuals; occurs in a low proportion of cases of bacillary dysentery. S. dysenteriae a species causing severe necrotizing dysentery in humans induced by a virulent shiga toxin found only in feces of symptomatic individuals; the type species of the genus S.. SYN: Shiga bacillus, Shiga-Kruse bacillus. S. flexneri a species found in the feces of symptomatic individuals and of convalescents or carriers; a common cause of dysentery epidemics, especially in Asia and the Middle East. Now known sometimes to be sexually transmitted through anal intercourse. SYN: Flexner bacillus, paradysentery bacillus. S. sonnei a species causing dysentery, sometimes milder than that caused by other species. The most common S. species causing disease in the U.S.

shigellosis (shig-e-lo′sis)
Bacillary dysentery caused by bacteria of the genus Shigella, often occurring in epidemic patterns; an opportunistic infection of persons with AIDS.

shikimate dehydrogenase (shi-kim′at)
An oxidoreductase reversibly reacting 3-dehydroshikimic acid with NADPH acid to produce shikimic acid and NADP+ in l-phenylalanine and l-tyrosine biosynthesis.

D. B., 20th century U.S. engineer. See Björk-S. valve.

shim (shim)
In magnetic resonance imaging, fine adjustment of the magnetic field to improve uniformity.

SYN: anterior border of tibia. [A.S. scina] saber s. the sharp-edged, anteriorly convex tibia in congenital syphilis. toasted shins SYN: erythema ab igne.

J., contemporary Australian molecular biologist.

shingles (shing′glz)
SYN: herpes zoster. [L. cingulum, girdle]

Tenderness and pain with induration and swelling of pretibial muscles, following athletic overexertion by the untrained; it may be a mild form of anterior tibial compartment syndrome.

A structure resembling the hull of a s.. Fabricius s. the outlines of the sphenoid, occipital, and frontal bones, from their fancied resemblance to the hull of a s..

Walter C., U.S. psychiatrist, *1903. See S.-Hartford scale.

N.V., Indian obstetrician and gynecologist, 1900–1971. See S. operation.

1. To shake or tremble, especially from cold. 2. A tremor; a slight chill.

Trembling from cold or fear.

shock (shok)
1. The condition in which the cells of the body receive inadequate amounts of oxygen secondary to changes in perfusion; most commonly secondary to blood loss or sepsis. 2. A sudden physical or biochemical disturbance that results in inadequate blood flow and oxygenation of an animal's vital organs. 3. A state of profound mental and physical depression consequent upon severe physical injury or an emotional disturbance. 4. A state characterized by inadequacy of blood flow throughout the body to the extent that damage occurs to the cells of the tissues; if the s. is prolonged, the cardiovascular system itself becomes damaged and begins to deteriorate, resulting in a vicious cycle that leads to death. See diastolic s., systolic s.. [Fr. choc, fr. Germanic] anaphylactic s. a severe, often fatal form of s. characterized by smooth muscle contraction and capillary dilation initiated by cytotropic (IgE class) antibodies; typically an antibody-associated phenomenon (type I allergic reaction). SEE ALSO: anaphylaxis, serum sickness. anaphylactoid s. a reaction that is similar to anaphylactic s., but which does not require the incubation period characteristic of induced sensitivity (anaphylaxis); it is unrelated to antigen-antibody reactions. SYN: anaphylactoid crisis (1) , pseudoanaphylactic s.. anesthetic s. s. produced by the administration of anesthetic drug(s), usually in relative overdosage. break s. the s. produced by breaking a constant current passing through the body. cardiac s. SYN: cardiogenic s.. cardiogenic s. s. resulting from decline in cardiac output secondary to serious heart disease, usually myocardial infarction. SYN: cardiac s.. chronic s. the state of peripheral circulatory insufficiency developing in elderly patients with a debilitating disease, e.g., carcinoma; a subnormal blood volume makes the patient susceptible to hemorrhagic s. as a result of even a moderate blood loss such as may occur during an operation. counter-s. countershock. cultural s. a form of stress associated with the beginning of a person's assimilation into a new culture vastly different from that in which he or she was raised. declamping s. SYN: declamping phenomenon. deferred s., delayed s. a state of s. coming on at a considerable interval after the receipt of the injury. diastolic s. the abnormally palpable impact, appreciated by a hand on the chest wall, of an accentuated third heart sound. electric s. a sudden violent impression caused by the passage of a current of electricity through any portion of the body. endotoxin s. s. induced by release of endotoxin from Gram-negative bacteria, especially by Escherichia coli. hemorrhagic s. hypovolemic s. resulting from acute hemorrhage, characterized by hypotension, tachycardia, pale, cold, and clammy skin, and oliguria. histamine s. the s. state produced in animals by the injection of histamine; characterized by bronchiolar spasm in the guinea pig and constriction of hepatic veins in the dog. hypovolemic s. s. caused by a reduction in volume of blood, as from hemorrhage or dehydration. insulin s. severe hypoglycemia produced by administration of insulin, manifested by sweating, tremor, anxiety, vertigo, and diplopia, followed by delirium, convulsions, and collapse. SYN: wet s.. irreversible s. s. that has progressed because of cell injury beyond the stage where resuscitation is possible. nitroid s. a syndrome resembling that produced by the administration of a large dose of a nitrite, sometimes caused by a too rapid intravenous injection of arsphenamine or some other drug; See nitritoid reaction. oligemic s. s. associated with pronounced fall in blood volume, sometimes resulting from increased permeability of blood vessels. osmotic s. a sudden change in the osmotic pressure to which a cell is subjected, usually in order to cause it to lyse. primary s. s. mainly nervous in nature, from pain, anxiety, etc., which ensues almost immediately upon the receipt of a severe injury. protein s. the systemic reaction following the parenteral administration of a protein. pseudoanaphylactic s. SYN: anaphylactoid s.. reversible s. s. that will respond to treatment and from which recovery is possible. septic s. 1. s. associated with infection that has released large enough quantities of toxins or vasoactive substances including, cytokines, to be associated with hypotension; 2. s. associated with septicemia caused by Gram-negative bacteria. serum s. anaphylactic or anaphylactoid s. caused by the injection of antitoxic or other foreign serum. shell s. SYN: battle fatigue. spinal s. transient depression or abolition of reflex activity below the level of an acute spinal cord injury or transection. systolic s. the abnormally palpable impact, appreciated by a hand on the chest wall, of an accentuated first heart sound. toxic s. toxic s. syndrome. vasogenic s. s. resulting from depressed activity of the higher vasomotor centers in the brain stem and the medulla, producing vasodilation without loss of fluid so that the container is disproportionately large. In oligemic s., blood volume is reduced; in both, return of venous blood is inadequate. wet s. SYN: insulin s..

John D., 20th century English cardiologist. See S. anomaly, S. complex, S. syndrome.

shook jong (shuk-yong′)
SYN: koro.

Richard E., U.S. pathologist, 1902–1966. See S. fibroma, S. fibroma virus, S. papilloma, S. papilloma virus.

short-chain acyl-CoA dehydrogenase
See acyl-CoA dehydrogenase (NADPH).

shortsightedness (short′sit-ed-nes)
SYN: myopia.


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