- 行业: Printing & publishing
- Number of terms: 178089
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McGraw Hill Financial, Inc. is an American publicly traded corporation headquartered in Rockefeller Center in New York City. Its primary areas of business are financial, publishing, and business services.
A chimera is an organism made up of cells from two or more genetically distinct sources. While some chimeras (very rarely) arise naturally through the fertilization of an egg by more than one sperm cell, or from the very early fusion of fraternal twin embryos shortly after fertilization, the avian brain chimeras considered here are different because they are produced surgically by the substitution of presumptive nervous system cells between the early embryos of different bird species. The word “presumptive” is used because the transplanted cells have not yet definitively become nervous system cells at the time of transplantation. This is because the cell substitution takes place before major organ systems or blood vessels are formed in the embryo. The surgeries are guided by “fate maps” that identify which groups of embryonic cells give rise to particular parts of the developed brain. Avian brain chimeras are used in basic research examining the mechanisms responsible for the development of, and evolutionary changes in, neural circuits: complex groups of interconnected nerve cells located in many different parts of the brain that regulate particular perceptual, cognitive, and behavioral functions of organisms.
Industry:Science
A choice flavoring obtained from a climbing orchid, <i>Vanilla fragrans</i>, a native of tropical American forests. The vanilla plant belongs to the orchid family and is indigenous to southeastern Mexico, where it was used by the Aztecs to flavor their cocoa. In 1510 <i>vagnuila</i> first appeared in Spain. Its fruits are pods called vanilla beans (see <b>illus.</b>). These are picked at the proper time before they have fully matured.
Industry:Science
A chromosome is a single deoxyribonucleic acid (DNA) molecule with its associated protein components. In the case of many viruses, most bacteria, and eukaryotic organelles, this DNA molecule is circular; but in the case of most eukaryotic organisms with cell nuclei and large genomes, the genome is partitioned between a number of linear DNA molecules. In simple eukaryotes such as <i>Saccharomyces cerevisiae</i> (baker's yeast), the relatively small size of the genome (more than 12 million nucleotide base pairs) has made the analysis of these chromosomes rather easy at the DNA level. Much progress is being made in understanding the proteins involved in chromosomal processes such as DNA replication, recombination, localization within the nucleus, and segregation of the replicated chromosomes to the daughter cells with high accuracy. This last function is arguably the most fundamental role of the chromosome.
Industry:Science
A chronic condition characterized by widespread pain in muscles, tendons, and ligaments, and the presence of multiple tender points on the body. The illness occurs in up to 5% of the general population, and approximately 90% of affected individuals are women.
Industry:Science
A chronic infectious disease caused by <i>Mycobacterium leprae</i> that primarily affects the skin and peripheral nerves and, to a lesser extent, involves the eyes and mucous membranes. Leprosy, or Hansen's disease, has been known for more than 2000 years. It afflicts at least 3 million people worldwide and is most common in developing countries. There are about 200 new cases yearly in the United States, 85% of them among aliens. The pathogen, <i>M. leprae</i>, has never been cultured in artificial media, but it can be grown in the mouse (in the footpad) and the nine-banded armadillo, among other animals. About 2–3% of armadillos in the wild in the southern United States harbor the infection. Its epidemiology is not fully understood, but transmission probably takes place by the respiratory route. The bacillus is very slow-growing, and the incubation period is usually 3–5 years. Less than 5% of any population is susceptible, and these individuals have a deficient cell-mediated immune response specifically to <i>M. leprae</i>, which may be genetic in origin. Epidemics have occurred, but are rare. Historically, considerable stigma has been attached to the disease and remains a real, though diminishing, problem.
Industry:Science
A circuit composed of a source and four impedances that is used in the measurement of a wide range of physical quantities. The bridge circuit is useful in measuring impedances (resistors, capacitors, and inductors) and in converting signals from transducers to voltage or current signals.
Industry:Science
A circuit having the primary function of changing the phase of a signal by 180°. The phase inverter is most commonly employed as the input stage for a push-pull amplifier. Therefore, the phase inverter must supply two voltages of equal magnitude and 180° phase difference. A variety of circuits are available for the phase inversion. The circuit used in any given case depends upon such factors as the overall gain of the phase inverter and push-pull amplifier, the possibility that the input to the push-pull amplifier may require power, space requirements, and cost.
Industry:Science
A circuit that returns a portion of the output signal of an electronic circuit or control system to the input of the circuit or system. When the signal returned (the feedback signal) is at the same phase as the input signal, the feedback is called positive or regenerative. When the feedback signal is of opposite phase to that of the input signal, the feedback is negative or degenerative.
Industry:Science
A circuit that uses a common-collector transistor amplifier stage with unity voltage gain, large input resistance <i>R<sub>i</sub></i>, and small output resistance <i>R<sub>o</sub></i> (see <b>illus.</b>). In its behavior, the emitter follower is analogous and very similar to the source follower in metal-oxide-semiconductor (MOS) circuits. Many electronic circuits have a relatively high output resistances and cannot deliver adequate power to a low-resistance load, or do suffer unacceptable voltage attenuation. In these cases, an emitter follower acts as a very simple buffer. Widely used, it is often found as the last stage of a multistage amplifier so that the circuit is better able to drive a low-resistance load.
Industry:Science
A circuit which produces a dc output voltage that is a fixed multiple of the input dc voltage or the peak voltage of an ac input waveform. This fixed multiple is approximately an integer (±2, ±3, ±4, …). Voltage multipliers are used to produce a high dc voltage where modest load current is required. The circuit can be implemented with diodes and capacitors or with switched capacitors.
Industry:Science
