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   20709篇
  免费   681篇
  国内免费   1108篇
系统科学   842篇
丛书文集   664篇
教育与普及   89篇
理论与方法论   42篇
现状及发展   409篇
综合类   20432篇
自然研究   20篇
  2024年   37篇
  2023年   71篇
  2022年   149篇
  2021年   200篇
  2020年   231篇
  2019年   209篇
  2018年   223篇
  2017年   270篇
  2016年   290篇
  2015年   402篇
  2014年   710篇
  2013年   564篇
  2012年   923篇
  2011年   942篇
  2010年   810篇
  2009年   870篇
  2008年   912篇
  2007年   1239篇
  2006年   1143篇
  2005年   1081篇
  2004年   932篇
  2003年   907篇
  2002年   936篇
  2001年   785篇
  2000年   741篇
  1999年   680篇
  1998年   542篇
  1997年   562篇
  1996年   533篇
  1995年   779篇
  1994年   619篇
  1993年   609篇
  1992年   607篇
  1991年   550篇
  1990年   553篇
  1989年   399篇
  1988年   222篇
  1987年   137篇
  1986年   62篇
  1985年   43篇
  1984年   13篇
  1982年   1篇
  1981年   2篇
  1955年   8篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
Summary A technique to assay erythrocyte pyrimidine 5-nucleotidase activity in situ using31P nuclear magnetic resonance spectroscopy is presented. The assay is chemically specific, simple and applicable to untreated lysates. A comparison of enzyme levels in normal controls and in multiple sclerosis patients employing the assay yielded no significant differences between both groups. Difficulties encountered in the quantitative analysis of the assay using1H-NMR spectroscopy are briefly discussed.  相似文献   
142.
Summary Adenomatous prolactin cells lose 39% of their cytoplasm volume within 7 days after the beginning of bromocriptine treatment. A simultaneous reduction of the rough-surfaced endoplasmic reticulum and the Golgi apparatus occurs. Their membranes are removed by rapid transport along the secretory pathway to the cell surface and to lysosomal destruction.We thank Prof. E. del Pozo, Sandoz Ltd, Basle, for the generous gift of injectable bromocriptine, Prof. E.R. Weibel, Dept. of Anatomy, University of Bern, for his technical advice, Dr U. Helfenstein, Institute of Biostatistics, University of Zürich, for his help in the statistical evaluation of the results and the EMDO-Foundation, Zürich, for financial support.  相似文献   
143.
144.
Summary In mixed culture of Chinese hamster fibroblatst, clone 431, and transformed murine L fibroblasts, clone B-82, isoproterenol was found to protect only 431 cells against ionizing radiation. It was shown that 431 cells, in contrast to B-82 cells, possess-adrenoreceptors, and the radioprotective effect of isoproterenol can be realized only if this agent interacts with-adrenoreceptors coupled with the cAMP system. Since malignization often causes the disappearance of-adrenergic and other hormone receptors, the combined culturing and irradiation of the cells studied can be regarded as a model of the growth of malignant cells (B-82) among normal tissue cells (431 cells) under conditions of radiation therapy. A possibility of selective protection against radiation damage of normal tissue cells, with retention of the former radiosensitivity of tumor cells, is discussed.  相似文献   
145.
Summary Visual determination of MSH-induced pigment migration in melanophores of small pieces ofAnolis carolinensis skin is standardized by first measuring photoelectrometrically the change in reflection/transmission of the whole dorsal skin in response to different hormone concentrations. This method allows the rapid and precise recording of time-response curves after photoaffinity labeling of MSH receptors or of dose-response curves of large series of synthetic compounds.Acknowledgments. We wish to thank Ms V. Jäggin, Ms C. Schulthess and Ms G. van Hees for excellent technical assistance, Dr. R. Andreatta, Ciba-Geigy AG, Basel, for his generous gift of -MSH and Prof. A Pletscher for his continuous interest. This work was supported by the Swiss National Science Foundation.  相似文献   
146.
Summary Using isolated, internally perfused bullfrog dorsal root ganglion cells we have studied the dose-response curves for -aminobutyric acid (GABA) in the presence of internally or externally applied GABA antagonists. With external application of antagonists the inhibition of the GABA current by bicuculline was competitive and that by picrotoxin was noncompetitive. Picrotoxin but not bicuculline blocked when internally perfused.To whom reprint requests should be addressed.Acknowledgments. We thank Drs S. Minakami and S. Yasui for helpful discussions and comments.  相似文献   
147.
Summary Glucose-6-phosphate dehydrogenase and N-acetyl--glucosaminidase activities were both elevated after eccentric exercise indicating that this type of exercise causes muscle damage. Muscle damage as measured by glucose-6-phosphate dehydrogenase activity in the vastus intermedius was greater and occurred later in larger rats indicating that the susceptibility to muscle damage is increased and the repair process delayed in older and larger animals.  相似文献   
148.
149.
Summary Quantitative genetic models of sexual selection have disporven some of the central tenets of both the handicap mechanism and the sexy son hypothesis. These results suggest that the good genes approach to sexual selection may generally lead to erroneous results.Runaway sexual selection seems possible under a wide variety of circumstances. Quantittive genetic models have revealed runaway processes for sexually selected attributes expressed in both sexes and for attributes of parental care. Furthermore, the runaway could occur simultaneously in a series of populations that straddle an environmental gradient. While the models support the feasibility of runaway processes, empirical studies are needed to evaluate whether runaways actually happen. Estimates of critical genetic parameters are particularly needed, as well as measures of natural and sexual selection acting on the same population.The models also show that sexual selection has tremendous potential to produce population differentiation, particularly in epigamic traits. Differentiation is promoted by indeterminancy of evolutionary outcome, transient differences among populations during the final slow approach to equilibrium, sampling drift among equilibrium populations, and the tendency of sexual selection to amplify geographic variation arising from spatial differences in natural selection. Recent work with two- and three-locus models of sexual selection has produced results that parallel the results of the polygenic models36–38,58. Thus the feature of indeterminate equilibria (outcome dependent on initial conditions) is common to both types of model.  相似文献   
150.
Summary We report the synthesis, stereochemistry and preliminary pharmacological evaluation of DCN 203-922, a novel ergot alkaloid of the cyclol type, which contains in its peptide moiety the uncommon amino acid L-allo-isoleucine.Part of this paper was reported by this author at the Herbstversammlung der Schweizerischen Chemischen Gesellschaft, Bern, in October 1986.  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