全文获取类型
收费全文 | 342312篇 |
免费 | 39013篇 |
国内免费 | 196篇 |
出版年
2018年 | 3105篇 |
2017年 | 2866篇 |
2016年 | 4116篇 |
2015年 | 5835篇 |
2014年 | 6904篇 |
2013年 | 9290篇 |
2012年 | 10939篇 |
2011年 | 11167篇 |
2010年 | 7347篇 |
2009年 | 6899篇 |
2008年 | 9825篇 |
2007年 | 10082篇 |
2006年 | 9915篇 |
2005年 | 9504篇 |
2004年 | 9258篇 |
2003年 | 8940篇 |
2002年 | 8679篇 |
2001年 | 17831篇 |
2000年 | 17916篇 |
1999年 | 13880篇 |
1998年 | 4583篇 |
1997年 | 4659篇 |
1996年 | 4327篇 |
1995年 | 3898篇 |
1994年 | 3871篇 |
1993年 | 3969篇 |
1992年 | 10861篇 |
1991年 | 10722篇 |
1990年 | 10219篇 |
1989年 | 10030篇 |
1988年 | 9353篇 |
1987年 | 8699篇 |
1986年 | 7906篇 |
1985年 | 7787篇 |
1984年 | 6287篇 |
1983年 | 5439篇 |
1982年 | 4086篇 |
1981年 | 3643篇 |
1980年 | 3398篇 |
1979年 | 5658篇 |
1978年 | 4431篇 |
1977年 | 4034篇 |
1976年 | 3627篇 |
1975年 | 4013篇 |
1974年 | 4231篇 |
1973年 | 4168篇 |
1972年 | 3606篇 |
1971年 | 3322篇 |
1970年 | 2962篇 |
1969年 | 2876篇 |
排序方式: 共有10000条查询结果,搜索用时 437 毫秒
991.
James A. Simmons Andrea Megela Simmons 《Journal of comparative physiology. A, Neuroethology, sensory, neural, and behavioral physiology》2011,197(5):585-594
Widely divergent vertebrates share a common central temporal mechanism for representing periodicities of acoustic waveform
events. In the auditory nerve, periodicities corresponding to frequencies or rates from about 10 Hz to over 1,000 Hz are extracted
from pure tones, from low-frequency complex sounds (e.g., 1st harmonic in bullfrog calls), from mid-frequency sounds with
low-frequency modulations (e.g., amplitude modulation rates in cat vocalizations), and from time intervals between high-frequency
transients (e.g., pulse-echo delay in bat sonar). Time locking of neuronal responses to periodicities from about 50 ms down
to 4 ms or less (about 20–300 Hz) is preserved in the auditory midbrain, where responses are dispersed across many neurons
with different onset latencies from 4–5 to 20–50 ms. Midbrain latency distributions are wide enough to encompass two or more
repetitions of successive acoustic events, so that responses to multiple, successive periods are ongoing simultaneously in
different midbrain neurons. These latencies have a previously unnoticed periodic temporal pattern that determines the specific
times for the dispersed on-responses. 相似文献
992.
Complex (epidemiological and bacteriological) investigations of the level and structure of meningococcal carriership among the members of organized collective bodies differing in the epidemiological situation with respect to meningococcal infection have been carried out. The absence of differences between the total level of meningococcal carriership and the morbidity rate with respect to the generalized forms of meningococcal infection has been shown. The presence of cases of meningococcal meningitis in the groups under study has been found to depend on the intensity of the circulation of certain meningococcal serogroups. The possibility of the ecological reservation of the causative agents of meningococcal infection as polyagglutinable forms has been suggested. 相似文献
993.
We are investigating the mechanisms for producing or avoiding errors during DNA synthesis catalyzed by DNA replication and repair proteins purified from eukaryotic sources. Using assays that monitor the fidelity of a single round of DNA synthesis in vitro, we have defined the error frequency and mutational specificity of the four classes of animal cell DNA polymerases (alpha, beta, delta, gamma), and the fidelity of an SV40 origin-dependent DNA replication complex in extracts of HeLa cells. 相似文献
994.
Effects of fat content in the diet on rat liver peroxisomes was examined. In the livers of rats fed for one week on the high-fat diet containing 30% fat, the cyanide-insensitive palmitoyl-CoA oxidation was accelerated to eight times that of control and the enzymic activities of catalase, carnitine acetyltransferase and carnitine palmitoyltransferase were elevated by the factors of 1.3, 5 and 2, respectively. In contrast, the activities of D-amino acid oxidase in addition to the three enzymes mentioned above were all lowered by 20% when the animals were maintained on a fat-free diet for the same period of time. It appears that the high-fat diet-induced increase in the activity of carnitine palmitoyltransferase is a result of the raised activity of this enzyme in mitochondria only while the apparent high activity reflects stimulation of carnitine acetyltransferase in all the subcellular fractions. Another notable effect of the high-fat diet was a remarkable increase in the quantity of a peroxisome-associated polypeptide which was separable by sodium dodecyl sulfate polyacrylamide gel electrophoresis. It is noteworthy that this effect of the high-fat diet resemble that of clofibrate. If the diet was deprived of fat, however, this polypeptide species, with an estimated molecular weight of 80 000, decreased to a level slightly lower than normal. On the basis of the electron micrographic criteria, the high-fat diet provoked a marked proliferation of hepatic peroxisomes. 相似文献
995.
996.
The biodistribution and pharmacokinetics of vanadium following i.p. administration of vanadocene dichloride (VDC), a representative of a new class of organometallic anticancer agents, is reported for Strain A mice. A convenient flameless atomic absorption spectroscopic assay is described and is used to determine kinetic profiles for vanadium in blood, kidney, liver, small intestine and brain tissue for times up to 24 h after administration. For a VDC dose of 80 mg/kg, vanadium concentration decreases rapidly from both the blood and small intestine, and the data can be fit to a phenomenological exponential function (blood: t1/2 = 118 +/- 43 min; small intestine: t1/2(alpha) = 18.10 +/- 0.14 min, t1/2(beta) = 341 +/- 45 min). In contrast, vanadium accumulates in both the kidney and liver up to a maximal concentration (1.12 +/- 0.06 mM and 0.56 +/- 0.06 mM after 12 and 8 h, respectively), and is then excreted with estimated half-lives of 7.9 +/- 0.7 and 12.1 +/- 0.1 h, respectively. No detectable levels of vanadium are found in the brain tissue over the temporal course of the experiment. These results are compared to previous mammalian studies with cis-dichlorodiammineplatinum(II) (CDDP) and related 'second generation' platinum derivatives; there are both qualitative similarities between the vanadium and platinum systems as well as important quantitative differences. 相似文献
997.
998.
999.
1000.