全文获取类型
收费全文 | 454941篇 |
免费 | 50320篇 |
国内免费 | 206篇 |
出版年
2018年 | 4578篇 |
2017年 | 4332篇 |
2016年 | 6175篇 |
2015年 | 8685篇 |
2014年 | 9970篇 |
2013年 | 14007篇 |
2012年 | 16207篇 |
2011年 | 16136篇 |
2010年 | 10670篇 |
2009年 | 9851篇 |
2008年 | 14274篇 |
2007年 | 14179篇 |
2006年 | 13469篇 |
2005年 | 12645篇 |
2004年 | 12479篇 |
2003年 | 11919篇 |
2002年 | 11352篇 |
2001年 | 20254篇 |
2000年 | 20322篇 |
1999年 | 16411篇 |
1998年 | 5734篇 |
1997年 | 5910篇 |
1996年 | 5676篇 |
1995年 | 5329篇 |
1994年 | 5379篇 |
1993年 | 5228篇 |
1992年 | 13428篇 |
1991年 | 12751篇 |
1990年 | 12643篇 |
1989年 | 12613篇 |
1988年 | 11446篇 |
1987年 | 10896篇 |
1986年 | 10069篇 |
1985年 | 10129篇 |
1984年 | 8215篇 |
1983年 | 7139篇 |
1982年 | 5444篇 |
1981年 | 4801篇 |
1980年 | 4664篇 |
1979年 | 7835篇 |
1978年 | 6099篇 |
1977年 | 5445篇 |
1976年 | 5267篇 |
1975年 | 5651篇 |
1974年 | 5945篇 |
1973年 | 5811篇 |
1972年 | 5291篇 |
1971年 | 4663篇 |
1970年 | 4118篇 |
1969年 | 3821篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
2.
3.
András Szilágyi István Scheuring David P. Edwards Jerome Orivel Douglas W. Yu 《Ecology letters》2009,12(12):1306-1316
Theory suggests that spatial structuring should select for intermediate levels of virulence in parasites, but empirical tests are rare and have never been conducted with castration (sterilizing) parasites. To test this theory in a natural landscape, we construct a spatially explicit model of the symbiosis between the ant-plant Cordia nodosa and its two, protecting ant symbionts, Allomerus and Azteca . Allomerus is also a castration parasite, preventing fruiting to increase colony fecundity. Limiting the dispersal of Allomerus and host plant selects for intermediate castration virulence. Increasing the frequency of the mutualist, Azteca , selects for higher castration virulence in Allomerus , because seeds from Azteca -inhabited plants are a public good that Allomerus exploits. These results are consistent with field observations and, to our knowledge, provide the first empirical evidence supporting the hypothesis that spatial structure can reduce castration virulence and the first such evidence in a natural landscape for either mortality or castration virulence. 相似文献
4.
5.
6.
7.
8.
Charles A. Kunos Jeffrey M. Fabien John P. Shanahan Christine Collen Thierry Gevaert Kenneth Poels Robbe Van den Begin Benedikt Engels Mark De Ridder 《Journal of visualized experiments : JoVE》2015,(100)
Physicians considering stereotactic ablative body radiation therapy (SBRT) for the treatment of extracranial cancer targets must be aware of the sizeable risks for normal tissue injury and the hazards of physical tumor miss. A first-of-its-kind SBRT platform achieves high-precision ablative radiation treatment through a combination of versatile real-time imaging solutions and sophisticated tumor tracking capabilities. It uses dual-diagnostic kV x-ray units for stereoscopic open-loop feedback of cancer target intrafraction movement occurring as a consequence of respiratory motions and heartbeat. Image-guided feedback drives a gimbaled radiation accelerator (maximum 15 x 15 cm field size) capable of real-time ±4 cm pan-and-tilt action. Robot-driven ±60° pivots of an integrated ±185° rotational gantry allow for coplanar and non-coplanar accelerator beam set-up angles, ultimately permitting unique treatment degrees of freedom. State-of-the-art software aids real-time six dimensional positioning, ensuring irradiation of cancer targets with sub-millimeter accuracy (0.4 mm at isocenter). Use of these features enables treating physicians to steer radiation dose to cancer tumor targets while simultaneously reducing radiation dose to normal tissues. By adding respiration correlated computed tomography (CT) and 2-[18F] fluoro-2-deoxy-ᴅ-glucose (18F-FDG) positron emission tomography (PET) images into the planning system for enhanced tumor target contouring, the likelihood of physical tumor miss becomes substantially less1. In this article, we describe new radiation plans for the treatment of moving lung tumors. 相似文献
9.
10.
Joyce S. F. D. de Almeida Teobaldo R. Cuya Guizado Ana P. Guimarães Teodorico C. Ramalho Arlan S. Gonçalves Martijn C. de Koning 《Journal of biomolecular structure & dynamics》2016,34(12):2632-2642
In the present work, we performed docking and molecular dynamics simulations studies on two groups of long-tailored oximes designed as peripheral site binders of acetylcholinesterase (AChE) and potential penetrators on the blood brain barrier. Our studies permitted to determine how the tails anchor in the peripheral site of sarin-inhibited human AChE, and which aminoacids are important to their stabilization. Also the energy values obtained in the docking studies corroborated quite well with the experimental results obtained before for these oximes. 相似文献