全文获取类型
收费全文 | 855120篇 |
免费 | 93387篇 |
国内免费 | 573篇 |
出版年
2021年 | 7017篇 |
2018年 | 8408篇 |
2017年 | 7843篇 |
2016年 | 11492篇 |
2015年 | 16133篇 |
2014年 | 18748篇 |
2013年 | 26114篇 |
2012年 | 29690篇 |
2011年 | 29977篇 |
2010年 | 20311篇 |
2009年 | 18503篇 |
2008年 | 26419篇 |
2007年 | 27003篇 |
2006年 | 25498篇 |
2005年 | 24574篇 |
2004年 | 24430篇 |
2003年 | 23333篇 |
2002年 | 22547篇 |
2001年 | 39028篇 |
2000年 | 39111篇 |
1999年 | 31247篇 |
1998年 | 11417篇 |
1997年 | 11687篇 |
1996年 | 10938篇 |
1995年 | 10190篇 |
1994年 | 9970篇 |
1993年 | 9745篇 |
1992年 | 25261篇 |
1991年 | 24410篇 |
1990年 | 23809篇 |
1989年 | 23166篇 |
1988年 | 21505篇 |
1987年 | 20093篇 |
1986年 | 18635篇 |
1985年 | 18459篇 |
1984年 | 15415篇 |
1983年 | 12883篇 |
1982年 | 9865篇 |
1981年 | 8794篇 |
1980年 | 8296篇 |
1979年 | 13770篇 |
1978年 | 10740篇 |
1977年 | 9655篇 |
1976年 | 8763篇 |
1975年 | 9680篇 |
1974年 | 10334篇 |
1973年 | 10226篇 |
1972年 | 9098篇 |
1971年 | 8305篇 |
1970年 | 7102篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Six very premature babies (born at 26–28 weeks gestational age) have been studied in hospital for 11–17 weeks, while in intensive care and in an incubator. Apart from suffering occasionally from the neonatal disorders of haemolytic jaundice and ‘respiratory distress of the newborn’, the babies were healthy and developed normally. Initially, the babies were continuously fed intravenously, and the lighting in the ward was on continuously. Routine care was given round the clock. When their medical condition permitted it, the babies were moved in their incubator to an adjacent ward, where they took frequent (2–4 hourly) small meals by mouth, the lighting was dimmed at night, and routine care tended to be given more in the daytime. Hourly recordings of insulated skin temperature were taken throughout the study, and it is the detection of rhythmicity in these measurements that has been the subject of the present study. The methods used were Phase-weighted Stacks, Phasor Walkout and Power Spectral Analysis. These methods have previously been used mainly in geophysical studies, and their value is that they can detect weak signals in noisy data and do not assume a particular waveform of any signal. Circadian rhythmicity was found in all babies for much of the time that were in the constant environment provided by the incubator. Ultradian rhythms were sometimes present also, but they were shorter-lived, and showed a wide range of changing periods, generally in excess of 8 h. When the babies were being treated for jaundice or respiratory distress, there was a tendency for the circadian rhythms to become weaker and for a broader spectrum of ultradian periods to appear. Placing babies in the 12 h : 12 h light : dark environment provided by the ward, and instituting feeding by mouth, had, in most cases, only modest effects upon either circadian or ultradian rhythms. Thus, circadian rhythms continued (but generally with a period not exactly equal to 24 h), and ultradian rhythms, when present, often did not show periods that could be related easily to feeding or care-giving. These results are discussed in terms of evidence for endogenous and exogenous origins of the observed rhythms, and of theories that have postulated the relationship between circadian and ultradian rhythms. It is concluded that the results from the present analyses are difficult to reconcile with the view that circadian rhythms develop from interactions between ultradian oscillators. We suggest that they indicate a matu-ration of the circadian system as a consequence of increasing associations between the circadian elements that are present in the suprachiasmatic nuclei and in other oscillators of the circadian system. The new analytical methods used here also indicate that the results obtained from time-frequency analysis depend to some extent upon the method used. 相似文献
992.
993.
Shells of the bivalve Arctica islandica are used to reconstruct paleo-environmental conditions (e.g. temperature) via biogeochemical proxies, i.e. biogenic components that are related closely to environmental parameters at the time of shell formation. Several studies have shown that proxies like element and isotope-ratios can be affected by shell growth and microstructure. Thus it is essential to evaluate the impact of changing environmental parameters such as high pCO2 and consequent changes in carbonate chemistry on shell properties to validate these biogeochemical proxies for a wider range of environmental conditions. Growth experiments with Arctica islandica from the Western Baltic Sea kept under different pCO2 levels (from 380 to 1120 µatm) indicate no affect of elevated pCO2 on shell growth or crystal microstructure, indicating that A. islandica shows an adaptation to a wider range of pCO2 levels than reported for other species. Accordingly, proxy information derived from A. islandica shells of this region contains no pCO2 related bias. 相似文献
994.
995.
Caitlin M. Cossaboom Laura Córdoba Dianjun Cao Yan-Yan Ni Xiang-Jin Meng 《Journal of virology》2012,86(23):13124-13125
Hepatitis E virus (HEV) is a single-strand positive-sense RNA virus in the family Hepeviridae. The disease caused by HEV, hepatitis E, is an important public health problem in developing countries of Asia and Africa and is also endemic in many industrialized countries, including the United States. HEV has been identified from several other animal species in addition to humans, including the pig, chicken, mongoose, deer, rabbit, ferret, bat, and fish. Here we report the complete genome sequence of the first strain of HEV from rabbits in the United States. Sequence and phylogenetic analyses revealed that the U.S. rabbit HEV is a distant member of the zoonotic genotype 3 HEV, thus raising a concern for potential zoonotic human infection. A unique 90-nucleotide insertion within the X domain of the ORF1 was identified in the rabbit HEV, and this insertion may play a role in the species tropism of HEV. 相似文献
996.
Fusarium oxysporum f. sp. lycopersici and Rhizoctonal solani were grown in a complete 1.0 mM nutrient solution, and in solutions where Ca, Fe, K, Mg, N, P, and S were either excluded (0.0 mM) or included at depleted levels (0.1 mM) while all other constituents were maintained at 1.0 mM levels. Dry weight of both fungi were determined. For both fungi some of the lowest dry weights were recorded for samples grown in the complete solution. Exclusion of K, Mg, and S significantly increased dry weight of Fusarium. Inclusion at the 0.1 mM level of most components significantly increased Fusarium dry weight over values for both the complete and corresponding excluded nutrient solutions. The exception was S where there was no difference between excluded and 0.1 mM solutions. For Rhizoctonia dry weights in Fe excluded solutions were less than the complete solution, while dry weights in S excluded solutions were greater than the complete solution. At the 1.0 mM level Fe, K, and Mg dry weights were significantly increased over the dry weights produced in both the complete and deficient solutions. 相似文献
997.
998.
999.
M C Peterson 《The Western journal of medicine》1994,161(2):148
Campylobacter jejuni is an almost ubiquitous, microaerophilic, gram-negative rod. Outbreaks have been associated with drinking raw milk or contaminated water and eating poultry. Campylobacter jejuni accounts for 3.2% to 6.1% of cases of diarrheal illness in the general population of the United States, and infected patients frequently present with abdominal pain and fever. Less frequently, C jejuni is responsible for bacteremia, septic arthritis, septic abortion, and other extraintestinal infections. Reactive arthritis, Reiter''s syndrome, the Guillain-Barré syndrome, and pancreatitis may accompany or follow C jejuni enterocolitis. Campylobacter jejuni is an important cause of diarrheal illness and is a more commonly identified stool organism than Salmonella or Shigella species. Recurrent and chronic infection is generally reported in immunocompromised hosts. 相似文献
1000.