全文获取类型
收费全文 | 804739篇 |
免费 | 75599篇 |
国内免费 | 230篇 |
专业分类
880568篇 |
出版年
2018年 | 7956篇 |
2017年 | 7583篇 |
2016年 | 10784篇 |
2015年 | 13443篇 |
2014年 | 16021篇 |
2013年 | 23123篇 |
2012年 | 25962篇 |
2011年 | 26746篇 |
2010年 | 18265篇 |
2009年 | 16885篇 |
2008年 | 23908篇 |
2007年 | 24877篇 |
2006年 | 23252篇 |
2005年 | 22361篇 |
2004年 | 22138篇 |
2003年 | 21298篇 |
2002年 | 20808篇 |
2001年 | 34734篇 |
2000年 | 34194篇 |
1999年 | 27568篇 |
1998年 | 10176篇 |
1997年 | 10263篇 |
1996年 | 9838篇 |
1995年 | 9057篇 |
1994年 | 8720篇 |
1993年 | 8751篇 |
1992年 | 22333篇 |
1991年 | 21916篇 |
1990年 | 21368篇 |
1989年 | 20812篇 |
1988年 | 19079篇 |
1987年 | 18321篇 |
1986年 | 17091篇 |
1985年 | 16952篇 |
1984年 | 13933篇 |
1983年 | 12181篇 |
1982年 | 9230篇 |
1981年 | 8355篇 |
1980年 | 7753篇 |
1979年 | 12938篇 |
1978年 | 10201篇 |
1977年 | 9191篇 |
1976年 | 8802篇 |
1975年 | 9816篇 |
1974年 | 10484篇 |
1973年 | 10350篇 |
1972年 | 9477篇 |
1971年 | 8456篇 |
1970年 | 7384篇 |
1969年 | 7255篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
801.
W.R.M. Aengevaeren G.J. Laarman M.J. Suttorp J.M. ten Berg A.J. van Boven M.J. de Boer J.J. Piek G.V.A. van Ommen J.G.F. Bronzwaer P. Smits J.W. Deckers 《Netherlands heart journal》2005,13(11):416-422
Interventional cardiology is an expanding field within cardiovascular medicine and today it is generally accepted that cardiologists require specific training, knowledge and skills. Hospitals where coronary interventions are performed must be properly equipped and able to provide specialised care. Percutaneous coronary interventions are frequently used for coronary revascularisation. The public should have confidence in the uniformity of high quality care. Therefore, such quality of care should be maintained by certification of the individual operators, general guidelines for institutional requirements and formal audits. The Netherlands Society of Cardiology (NVVC) will be implementing a new registration system for cardiologists with a subspecialisation that will include registration for interventional cardiology. The NVVC asked the Working Group of Interventional Cardiology (WIC) to update the 1994 Dutch guidelines on operator and institutional competence, and requirements for training in interventional cardiology in order to incorporate them into the official directives. The present guidelines represent the expert opinion of the Dutch interventional cardiology community and are in accordance with international regulations.After two rounds of discussion, the NVVC approved the guidelines in November 2004 during the autumn meeting. 相似文献
802.
803.
RNA isolation from yeast is complicated by the need to initially break the cell wall. While this can be accomplished by glass bead disruption or enzyme treatment, these approaches result in DNA contamination and/or the need for incubation periods. We have developed a protocol for the isolation of RNA samples from yeast that minimizes degradation by RNases and incorporates two purification steps: acid phenol extraction and binding to a silica matrix. The procedure requires no precipitation steps, facilitating automation, and can be completed in less than 90 min. The RNA quality is ideal for microarray analysis. 相似文献
804.
Integrins are a family of adhesion molecules performing a major role in multiple cellular functions. Their contribution in carcinogenesis and metastatic process are the object of intense research activity worldwide during the last decades. This review focuses on the existing knowledge about integrin expression in head and neck squamous cell carcinoma (HNSCC) as it has been acquired mainly by immunohistochemical methods and by in vitro assays. The elucidation of the exact role of integrins and the study of expressive alterations of these molecules in cancer cells, may result in novel therapeutical approaches for useful applications in the clinical routine of HNSCC in the future. 相似文献
805.
Lisa A. Schulte David J. Mladenoff Sean N. Burrows Theodore A. Sickley Erik V. Nordheim 《Ecosystems》2005,8(1):73-94
We elucidate spatial controls of wind and fire disturbance across northern Wisconsin (USA), where climatic and topographic gradients are not strong, using data from the original US Public Land Survey (PLS) notes. These records contain information on the location and extent of heavy windthrows and stand-replacing fires prior to Euro-American settlement. The spatial patterns of windthrow and fire were spatially clustered at all scales in this historical environment, with stronger associations at local than regional scales. Logistic regression shows environmental variables to have a strong influence on this pattern. In the case of heavy windthrow, environmental drivers of disturbance pattern are fairly consistent across the region. The effects of climate and vegetation are predominant at all scales, but effects are often indirect, with strong interactions between them. Interactions between these two drivers and soil characteristics are also sometimes present. In contrast, models of stand-replacing fire show simple and direct control within and across fire-prone landscapes of historical northern Wisconsin, with climate and physiography as the main factors explaining the distribution of fire disturbance. This simple and direct control is lost at the regional scale, where climate, physiographic, soil, and vegetation variables, along with interactions between them, are significant factors. Contrary to other regions, the topographic effects are generally not important in predicting either wind or fire disturbance. Our work suggests that, in landscapes that lack strong environmental patterning, climate maintains its role as a primary driver of these natural disturbances, but topography is replaced by interactions and feedbacks with other forms of environmental heterogeneity. 相似文献
806.
Nelson G. Hairston Jr Stephen P. Ellner Monica A. Geber Takehito Yoshida Jennifer A. Fox 《Ecology letters》2005,8(10):1114-1127
Recent studies have documented rates of evolution of ecologically important phenotypes sufficiently fast that they have the potential to impact the outcome of ecological interactions while they are underway. Observations of this type go against accepted wisdom that ecological and evolutionary dynamics occur at very different time scales. While some authors have evaluated the rapidity of a measured evolutionary rate by comparing it to the overall distribution of measured evolutionary rates, we believe that ecologists are mainly interested in rapid evolution because of its potential to impinge on ecological processes. We therefore propose that rapid evolution be defined as a genetic change occurring rapidly enough to have a measurable impact on simultaneous ecological change. Using this definition we propose a framework for decomposing rates of ecological change into components driven by simultaneous evolutionary change and by change in a non‐evolutionary factor (e.g. density dependent population dynamics, abiotic environmental change). Evolution is judged to be rapid in this ecological context if its contribution to ecological change is large relative to the contribution of other factors. We provide a worked example of this approach based on a theoretical predator–prey interaction [ Abrams, P. & Matsuda, H. (1997) . Evolution, 51, 1740], and find that in this system the impact of prey evolution on predator per capita growth rate is 63% that of internal ecological dynamics. We then propose analytical methods for measuring these contributions in field situations, and apply them to two long‐term data sets for which suitable ecological and evolutionary data exist. For both data sets relatively high rates of evolutionary change have been found when measured as character change in standard deviations per generation (haldanes). For Darwin's finches evolving in response to fluctuating rainfall [ Grant, P.R. & Grant, B.R. (2002) . Science, 296, 707], we estimate that evolutionary change has been more rapid than ecological change by a factor of 2.2. For a population of freshwater copepods whose life history evolves in response to fluctuating fish predation [ Hairston, N.G. Jr & Dillon, T.A. (1990) . Evolution, 44, 1796], we find that evolutionary change has been about one quarter the rate of ecological change – less than in the finch example, but nevertheless substantial. These analyses support the view that in order to understand temporal dynamics in ecological processes it is critical to consider the extent to which the attributes of the system under investigation are simultaneously changing as a result of rapid evolution. 相似文献
807.
808.
809.
810.
V. L. Sviderskii A. E. Khovanskikh E. V. Rozengart 《Doklady. Biochemistry and biophysics》2005,402(1-6):248-250