首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   423003篇
  免费   47732篇
  国内免费   153篇
  2018年   3907篇
  2016年   5284篇
  2015年   6906篇
  2014年   8155篇
  2013年   11245篇
  2012年   12802篇
  2011年   13250篇
  2010年   9089篇
  2009年   8444篇
  2008年   12126篇
  2007年   12625篇
  2006年   11846篇
  2005年   11322篇
  2004年   11357篇
  2003年   10641篇
  2002年   10454篇
  2001年   17412篇
  2000年   17422篇
  1999年   13800篇
  1998年   4838篇
  1997年   5078篇
  1996年   4744篇
  1995年   4617篇
  1994年   4485篇
  1993年   4543篇
  1992年   11605篇
  1991年   11535篇
  1990年   11295篇
  1989年   10905篇
  1988年   10486篇
  1987年   10076篇
  1986年   9360篇
  1985年   9232篇
  1984年   7745篇
  1983年   6702篇
  1982年   5162篇
  1981年   4614篇
  1980年   4458篇
  1979年   7417篇
  1978年   5859篇
  1977年   5392篇
  1976年   5199篇
  1975年   5594篇
  1974年   6301篇
  1973年   6165篇
  1972年   5762篇
  1971年   5224篇
  1970年   4635篇
  1969年   4574篇
  1968年   4419篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The paper gives comparative analysis of the species composition of fish in the upper Tom’ from the zone of discharge of warm water of electrical power station and background parts of the riverbed. The areas subjected to thermal pollution feature an increase in the number and biomass of introduced species: the share of invaders makes up 19% of the total ichthyomass, 41% of the total number, and 25% of the species diversity of fish. Fish productivity of the mountain taiga part of the upper Tom’ with the increased water temperature exceeds this value of the main riverbed 2–4 times and correlates with the level of productivity of the plain rivers in the south of West Siberia, which is related to eutrophication.  相似文献   
992.
Objective: Lipoprotein lipase (LPL) and hepatic lipase (HL) are essential in hydrolysis of triglyceride‐rich lipoproteins. LPL activity is negatively, whereas HL activity is positively, associated with total body fat. We determined the associations of trunk and leg fat mass with plasma LPL and HL activities in a cross‐sectional study. Research Methods and Procedures: LPL and HL activities were determined in post‐heparin plasma in a sample of 197 men and 209 women, 60 to 87 years of age. A total body DXA scan was performed to determine trunk and leg fat mass. Results: In women, but not in men, trunk fat mass was negatively associated with LPL activity, whereas leg fat mass was positively associated, after mutual adjustment and adjustment for age. Standardized βs (95% confidence interval) for trunk and leg fat mass were ?0.24 (?0.41; ?0.08) and 0.14 (?0.02; 0.31), respectively (interaction by sex, p = 0.03). Larger trunk fat mass was associated with higher HL activity in men [0.48 (0.28; 0.68)] and women [0.40 (0.24; 0.56)]. A negative association of leg fat mass and HL activity was observed in men, although not statistically significant [?0.13 (?0.33; 0.06)], and in women [?0.28 (?0.38; ?0.18)]. Discussion: Abdominal fat is associated with unfavorable and femoral fat with favorable LPL and HL activities in plasma.  相似文献   
993.
A genetic screen for Dictyostelium mutants that phenotypically resemble cells lacking the G-protein beta-subunit yielded the protein kinase YakA. Like gbeta-null cells, yakA-null cells fail to enter development and display slow growth on bacterial lawns. We created a temperature-sensitive yakA mutant and showed that YakA activity is required not only at the onset but also during development. The yakA-null cells have strong defects in folic acid-induced responses, such as actin polymerization and cGMP accumulation, indicating that they play a role in G-protein-mediated signaling responses. We propose that YakA acts downstream of G-proteins, because cAMP receptors still couple to G-proteins in the yakA mutant. In addition, the previously observed growth arrest induced by overexpression of YakA also occurs in gbeta mutants. We localized YakA-GFP to the cytosol suggesting that YakA may be a functional homolog of its mammalian counterparts Dyrk2 and Dyrk3, a subclass of dual-specificity Yak-related kinases (Dyrk) with unknown function.  相似文献   
994.
Schistosoma mansoni: IL-4 is necessary for concomitant immunity in mice.   总被引:4,自引:0,他引:4  
To ask whether type-2 immune responses serve an essential role in concomitant immunity, that is the prevention of superinfection with Schistosoma mansoni, we compared resistance to a challenge infection in infected wild-type (WT) mice and in infected IL-4-/- mice, which are unable to mount Th2 responses during schistosomiasis. Although WT mice are protected from superinfection, resistance is abrogated in the absence of interleukin (IL)-4. We conclude that IL-4 or IL-4-dependent responses, or both, are necessary for resistance to S. mansoni superinfection in mice.  相似文献   
995.
996.
997.
Zooplankton data collected during September 1995 in the NorthWest Atlantic at 4139'N, 4958'W (the location of the siteof the ‘Titanic’ wreck) were analysed. The regioninvestigated was characterized by a very sharp frontal zonebetween the Gulf Stream and the main stream of the LabradorCurrent. The total plankton biomass in the water column wasvery high. The macroplankton biomass values below the 600 mlayer were significantly higher as compared with the similarvalues measured before in other productive boreal regions ofthe Atlantic and Pacific oceans. A lot of dead mesoplanktonanimals occurred in the deep layers. The reason was that thecold-water mesoplankton advected by the Labrador Current diedoff intensively within the deep layers of the frontal zone andwere used as a food resource by the macroplankton carnivoresand scavengers that were very abundant there.  相似文献   
998.
The enterochromaffin-like (ECL) cells of the oxyntic mucosa (fundus) of the stomach produce, store and secrete histamine, chromogranin A-derived peptides such as pancreastatin, and an unanticipated but as yet unidentified peptide hormone. The cells are stimulated by gastrin and pituitary adenylate cyclase activating peptide and suppressed by somatostatin and galanin. Choline esters and histamine seem to be without effect on ECL cell secretion. The existence of a gastrin-ECL cell axis not only explains how gastrin stimulates acid secretion but also may help to explore the functional significance of the ECL cells with respect to the nature and bioactivity of its peptide hormone. From the results of studies of gastrectomized/fundectomized and gastrin-treated rats, it has been speculated that the anticipated ECL-cell peptide hormone acts on bone metabolism.  相似文献   
999.
1000.
This study evaluated manual caliper measurement of fetal BPD in Saimiri through the abdomen of the dam (TBPD) for correlation with paired ultrasound measurements, prediction of delivery date, subspecies variation, prediction of pregnancy outcome, and correlation between postpartum BPD and TBPD. Regression analysis revealed a close relationship between TBPD measurements and those obtained by ultrasound (P < 0.001). TBPD for Saimiri boliviensis boliviensis increased from a mean of 14.8 ± 0.3 mm for 11 weeks prepartum to a delivery week measurement of 33.2 ± 0.2mm. Delivery-week TBPD of Saimiri boliviensis peruviensis were significantly smaller than Saimiri sciureus sciureus (P < .05). Delivery week TBPD had a correlation coefficient of 0.64 with paired post-patrum measurements. No complications were associated with the technique. Delivery date could be predicted within two weeks. TBPD is an inexpensive, safe, rapid method of approximating fetal growth during the last half of pregnancy in Saimiri.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号